how does water travel through xylem

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Water Transport in Plants: Xylem

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Learning Objectives

• Explain water potential and predict movement of water in plants by applying the principles of water potential
• Describe the effects of different environmental or soil conditions on the typical water potential gradient in plants
• Identify and differentiate between the three pathways water and minerals can take from the root hair to the vascular tissue
• Explain the three hypotheses explaining water movement in plant xylem, and recognize which hypothesis explains the heights of plants beyond a few meters
• Define transpiration and identify the source of energy that drives transpiration

Water Potential and Water Transport from Roots to Shoots

The information below was adapted from OpenStax Biology 30.5

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and products of photosynthesis throughout the plant. The phloem is the tissue primarily responsible for movement of nutrients and photosynthetic produces, and xylem is the tissue primarily responsible for movement of water). Plants are able to transport water from their roots up to the tips of their tallest shoot through the combination of water potential, evapotranspiration, and stomatal regulation – all without using any cellular energy!

Water potential is a measure of the potential energy in water based on potential water movement between two systems. Water potential can be defined as the difference in potential energy between any given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ pure H2O ) is defined as zero (even though pure water contains plenty of potential energy, this energy is ignored in this context).

Water potential can be positive or negative, and water potential is calculated from the combined effects of  solute concentration   (s) and  pressure (p) . The equation for this calculation is Ψ

An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered. Water is lost from the leaves via transpiration (approaching Ψ p  = 0 MPa at the wilting point) and restored by uptake via the roots.

This video provides an overview of water potential, including solute and pressure potential (stop after 5:05):

And this video describes how plants manipulate water potential to absorb water and how water and minerals move through the root tissues:

Impact of Soil and Environmental Conditions on the Plant Water Potential Gradient

As noted above, Ψ soil  must be > Ψ root  > Ψ stem  > Ψ leaf  > Ψ atmosphere in order for transpiration to occur (continuous movement of water through the plant from the soil to the air without equilibrating. This continuous movement of water relies on a water potential gradient , where water potential decreases at each point from soil to atmosphere as it passes through the plant tissues. However, this gradient can become disrupted if the soil becomes too dry, which can result in both decreased solute potential (due to the same amount of solutes dissolved in a smaller quantity of water) as well as decreased pressure potential in severe droughts (resulting from negative pressure or a “vacuum” in the soil due to loss of water volume). If water potential becomes sufficiently lower in the soil than in the plant’s roots, then water will move out of the plant root and into the soil.

Pathways of Water and Mineral Movement in the Roots

Once water has been absorbed by a root hair, it moves through the ground tissue and along its water potential gradient through one of three possible routes before entering the plant’s xylem:

• the  symplast : “sym” means “same” or “shared,” so symplast is “shared cytoplasm”.  In this pathway, water and minerals move from the cytoplasm of one cell in to the next, via plasmodesmata that physically join different plant cells, until eventually reaching the xylem.
• the  transmembrane  pathway: in this pathway, water moves through water channels present in the plant cell plasma membranes, from one cell to the next, until eventually reaching the xylem.
• the  apoplast : “a” means “outside of,” so apoplast is “outside of the cell”. In this pathway, water and dissolved minerals never move through a cell’s plasma membrane but instead travel through the porous cell walls that surround plant cells.

Water and minerals that move into a cell through the plasma membrane has been “filtered” as it passes through water or other channels within the plasma membrane; however water and minerals that move via the apoplast do not encounter a filtering step until they reach a layer of cells known as the endodermis which separate the vascular tissue (called the stele in the root) from the ground tissue in the outer portion of the root. The endodermis is present only in roots, and serves as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip , forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This process ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded.

Movement of Water Up the Xylem Against Gravity

How is water transported up a plant against gravity, when there is no “pump” or input of cellular energy to move water through a plant’s vascular tissue? There are three hypotheses that explain the movement of water up a plant against gravity. These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees:

• Root pressure  pushes water up
• Capillary action draws water up within the xylem
• Cohesion-tension pulls water up the xylem

We’ll consider each of these in turn.

Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil. Water moves into the roots from the soil by osmosis, due to the low solute potential in the roots (lower Ψs in roots than in soil). This intake o f water in the roots increases Ψp in the root xylem, “pushing” water up. In extreme circumstances, or when stomata are closed at night preventing water from evaporating from the leaves, root pressure results in guttation , or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not sufficient to move water up the height of a tall tree. 

Capillary action  (or capillarity) is the tendency of a liquid to move up against gravity when confined within a narrow tube (capillary). You can directly observe the effects of capillary action when water forms a meniscus when confined in a narrow tube. Capillarity occurs due to three properties of water:

• Surface tension , which occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water.
• Adhesion , which is molecular attraction between “unlike” molecules. In the case of xylem, adhesion occurs between water molecules and the molecules of the xylem cell walls.
• Cohesion , which is molecular attraction between “like” molecules. In water, cohesion occurs due to hydrogen bonding between water molecules.

On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree.

This video provides an overview of the important properties of water that facilitate this movement:

The cohesion-tension  hypothesis is the most widely-accepted model for movement of water in vascular plants. Cohesion-tension combines the process of capillary action with transpiration or the evaporation of water from the plant stomata. Transpiration is ultimately the main driver of water movement in xylem, combined with the effects of capillary action. The cohesion-tension model works like this:

• Transpiration (evaporation) occurs because stomata in the leaves are open to allow gas exchange for photosynthesis. As transpiration occurs, evaporation of water deepens the meniscus of water in the leaf, creating negative pressure (also called tension or suction).
• The tension created by transpiration “pulls” water in the plant xylem, drawing the water upward in much the same way that you draw water upward when you suck on a straw.
• Cohesion (water molecules sticking to other water molecules) causes more water molecules to fill the gap in the xylem as the top-most water is pulled toward end of the meniscus within the stomata.

Transpiration results in a phenomenal amount of negative pressure within the xylem vessels and tracheids, which are structurally reinforced with lignin to cope with large changes in pressure. The taller the tree, the greater the tension forces (and thus negative pressure) needed to pull water up from roots to shoots.

Follow this link to watch this video on YouTube for an overview of the different processes that cause water to move throughout a plant (this video is linked because it cannot be directly embedded within the textbook; if needed, the video url is https://www.youtube.com/watch?v=8YlGyb0WqUw )

Transpiration Energy Source

The term “ transpiration ” has been used throughout this reading in the context of water movement in plants. Here we will define it as: evaporation of water from the plant stomata resulting in the continuous movement of water through a plant via the xylem, from soil to air, without equilibrating.

Transpiration is a passive process with respect to the plant, meaning that ATP is not required to move water up the plant’s shoots. The energy source that drives the process of transpiration is the extreme difference in water potential between the water in the soil and the water in the atmosphere. Factors that alter this extreme difference in water potential can also alter the rate of transpiration in the plant.

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Water Uptake and Transport in Vascular Plants

Why Do Plants Need So Much Water?

If water is so important to plant growth and survival, then why would plants waste so much of it? The answer to this question lies in another process vital to plants — photosynthesis. To make sugars, plants must absorb carbon dioxide (CO 2 ) from the atmosphere through small pores in their leaves called stomata (Figure 1). However, when stomata open, water is lost to the atmosphere at a prolific rate relative to the small amount of CO 2 absorbed; across plant species an average of 400 water molecules are lost for each CO 2 molecule gained. The balance between transpiration and photosynthesis forms an essential compromise in the existence of plants; stomata must remain open to build sugars but risk dehydration in the process.

From the Soil into the Plant

Essentially all of the water used by land plants is absorbed from the soil by roots. A root system consists of a complex network of individual roots that vary in age along their length. Roots grow from their tips and initially produce thin and non-woody fine roots. Fine roots are the most permeable portion of a root system, and are thought to have the greatest ability to absorb water, particularly in herbaceous (i.e., non-woody) plants (McCully 1999). Fine roots can be covered by root hairs that significantly increase the absorptive surface area and improve contact between roots and the soil (Figure 2). Some plants also improve water uptake by establishing symbiotic relationships with mycorrhizal fungi, which functionally increase the total absorptive surface area of the root system.

Roots of woody plants form bark as they age, much like the trunks of large trees. While bark formation decreases the permeability of older roots they can still absorb considerable amounts of water (MacFall et al . 1990, Chung & Kramer 1975). This is important for trees and shrubs since woody roots can constitute ~99% of the root surface in some forests (Kramer & Bullock 1966).

Roots have the amazing ability to grow away from dry sites toward wetter patches in the soil — a phenomenon called hydrotropism. Positive hydrotropism occurs when cell elongation is inhibited on the humid side of a root, while elongation on the dry side is unaffected or slightly stimulated resulting in a curvature of the root and growth toward a moist patch (Takahashi 1994). The root cap is most likely the site of hydrosensing; while the exact mechanism of hydrotropism is not known, recent work with the plant model Arabidopsis has shed some light on the mechanism at the molecular level (see Eapen et al . 2005 for more details).

Roots of many woody species have the ability to grow extensively to explore large volumes of soil. Deep roots (>5 m) are found in most environments (Canadell et al . 1996, Schenk & Jackson 2002) allowing plants to access water from permanent water sources at substantial depth (Figure 3). Roots from the Shepard's tree ( Boscia albitrunca ) have been found growing at depths 68 m in the central Kalahari, while those of other woody species can spread laterally up to 50 m on one side of the plant (Schenk & Jackson 2002). Surprisingly, most arid-land plants have very shallow root systems, and the deepest roots consistently occur in climates with strong seasonal precipitation (i.e., Mediterranean and monsoonal climates).

Through the Plant into the Atmosphere

Flow = Δψ / R ,

which is analogous to electron flow in an electrical circuit described by Ohm's law equation:

i = V / R ,

where R is the resistance, i is the current or flow of electrons, and V is the voltage. In the plant system, V is equivalent to the water potential difference driving flow (Δψ) and i is equivalent to the flow of water through/across a plant segment. Using these plant equivalents, the Ohm's law analogy can be used to quantify the hydraulic conductance (i.e., the inverse of hydraulic R ) of individual segments (i.e., roots, stems, leaves) or the whole plant (from soil to atmosphere).

Upon absorption by the root, water first crosses the epidermis and then makes its way toward the center of the root crossing the cortex and endodermis before arriving at the xylem (Figure 4). Along the way, water travels in cell walls (apoplastic pathway) and/or through the inside of cells (cell to cell pathway, C-C) (Steudle 2001). At the endodermis, the apoplastic pathway is blocked by a gasket-like band of suberin — a waterproof substance that seals off the route of water in the apoplast forcing water to cross via the C-C pathway. Because water must cross cell membranes (e.g., in the cortex and at apoplastic barriers), transport efficiency of the C-C pathway is affected by the activity, density, and location of water-specific protein channels embedded in cell membranes (i.e., aquaporins). Much work over the last two decades has demonstrated how aquaporins alter root hydraulic resistance and respond to abiotic stress, but their exact role in bulk water transport is yet unresolved.

Once in the xylem tissue, water moves easily over long distances in these open tubes (Figure 5). There are two kinds of conducting elements (i.e., transport tubes) found in the xylem: 1) tracheids and 2) vessels (Figure 6). Tracheids are smaller than vessels in both diameter and length, and taper at each end. Vessels consist of individual cells, or "vessel elements", stacked end-to-end to form continuous open tubes, which are also called xylem conduits. Vessels have diameters approximately that of a human hair and lengths typically measuring about 5 cm although some plant species contain vessels as long as 10 m. Xylem conduits begin as a series of living cells but as they mature the cells commit suicide (referred to as programmed cell death), undergoing an ordered deconstruction where they lose their cellular contents and form hollow tubes. Along with the water conducting tubes, xylem tissue contains fibers which provide structural support, and living metabolically-active parenchyma cells that are important for storage of carbohydrates, maintenance of flow within a conduit (see details about embolism repair below), and radial transport of water and solutes.

When water reaches the end of a conduit or passes laterally to an adjacent one, it must cross through pits in the conduit cell walls (Figure 6). Bordered pits are cavities in the thick secondary cell walls of both vessels and tracheids that are essential components in the water-transport system of higher plants. The pit membrane, consisting of a modified primary cell wall and middle lamella, lies at the center of each pit, and allows water to pass between xylem conduits while limiting the spread of air bubbles (i.e., embolism) and xylem-dwelling pathogens. Thus, pit membranes function as safety valves in the plant water transport system. Averaged across a wide range of species, pits account for >50% of total xylem hydraulic resistance. The structure of pits varies dramatically across species, with large differences evident in the amount of conduit wall area covered by pits, and in the porosity and thickness of pit membranes (Figure 6).

After traveling from the roots to stems through the xylem, water enters leaves via petiole (i.e., the leaf stalk) xylem that branches off from that in the stem. Petiole xylem leads into the mid-rib (the main thick vein in leaves), which then branch into progressively smaller veins that contain tracheids (Figure 7) and are embedded in the leaf mesophyll. In dicots, minor veins account for the vast majority of total vein length, and the bulk of transpired water is drawn out of minor veins (Sack & Holbrook 2006, Sack & Tyree 2005). Vein arrangement, density, and redundancy are important for distributing water evenly across a leaf, and may buffer the delivery system against damage (i.e., disease lesions, herbivory, air bubble spread). Once water leaves the xylem, it moves across the bundle sheath cells surrounding the veins. It is still unclear the exact path water follows once it passes out of the xylem through the bundle sheath cells and into the mesophyll cells, but is likely dominated by the apoplastic pathway during transpiration (Sack & Holbrook 2005).

Mechanism Driving Water Movement in Plants

Stephen Hales was the first to suggest that water flow in plants is governed by the C-T mechanism; in his 1727 book Hales states "for without perspiration the [water] must stagnate, notwithstanding the sap-vessels are so curiously adapted by their exceeding fineness, to raise [water] to great heights, in a reciprocal proportion to their very minute diameters." More recently, an evaporative flow system based on negative pressure has been reproduced in the lab for the first time by a ‘synthetic tree' (Wheeler & Stroock 2008).

When solute movement is restricted relative to the movement of water (i.e., across semipermeable cell membranes) water moves according to its chemical potential (i.e., the energy state of water) by osmosis — the diffusion of water. Osmosis plays a central role in the movement of water between cells and various compartments within plants. In the absence of transpiration, osmotic forces dominate the movement of water into roots. This manifests as root pressure and guttation — a process commonly seen in lawn grass, where water droplets form at leaf margins in the morning after conditions of low evaporation. Root pressure results when solutes accumulate to a greater concentration in root xylem than other root tissues. The resultant chemical potential gradient drives water influx across the root and into the xylem. No root pressure exists in rapidly transpiring plants, but it has been suggested that in some species root pressure can play a central role in the refilling of non-functional xylem conduits particularly after winter (see an alternative method of refilling described below).

Disruption of Water Movement

Water transport can be disrupted at many points along the SPAC resulting from both biotic and abiotic factors (Figure 8). Root pathogens (both bacteria and fungi) can destroy the absorptive surface area in the soil, and similarly foliar pathogens can eliminate evaporative leaf surfaces, alter stomatal function, or disrupt the integrity of the cuticle. Other organisms (i.e., insects and nematodes) can cause similar disruption of above and below ground plant parts involved in water transport. Biotic factors responsible for ceasing flow in xylem conduits include: pathogenic organisms and their by-products that plug conduits (Figure 8); plant-derived gels and gums produced in response to pathogen invasion; and tyloses, which are outgrowths produced by living plant cells surrounding a vessel to seal it off after wounding or pathogen invasion (Figure 8).

Abiotic factors can be equally disruptive to flow at various points along the water transport pathway. During drought, roots shrink and lose contact with water adhering to soil particles — a process that can also be beneficial by limiting water loss by roots to drying soils (i.e., water can flow in reverse and leak out of roots being pulled by drying soil). Under severe plant dehydration, some pine needle conduits can actually collapse as the xylem tensions increase (Figure 8).

Water moving through plants is considered meta-stable because at a certain point the water column breaks when tension becomes excessive — a phenomenon referred to as cavitation. After cavitation occurs, a gas bubble (i.e., embolism) can form and fill the conduit, effectively blocking water movement. Both sub-zero temperatures and drought can cause embolisms. Freezing can induce embolism because air is forced out of solution when liquid water turns to ice. Drought also induces embolism because as plants become drier tension in the water column increases. There is a critical point where the tension exceeds the pressure required to pull air from an empty conduit to a filled conduit across a pit membrane — this aspiration is known as air seeding (Figure 9). An air seed creates a void in the water, and the tension causes the void to expand and break the continuous column. Air seeding thresholds are set by the maximum pore diameter found in the pit membranes of a given conduit.

Fixing the Problem

Failure to re-establish flow in embolized conduits reduces hydraulic capacity, limits photosynthesis, and results in plant death in extreme cases. Plants can cope with emboli by diverting water around blockages via pits connecting adjacent functional conduits, and by growing new xylem to replace lost hydraulic capacity. Some plants possess the ability to repair breaks in the water columns, but the details of this process in xylem under tension have remained unclear for decades. Brodersen et al . (2010) recently visualized and quantified the refilling process in live grapevines ( Vitis vinifera L.) using high resolution x-ray computed tomography (a type of CAT scan) (Figure 10). Successful vessel refilling was dependent on water influx from living cells surrounding the xylem conduits, where individual water droplets expanded over time, filled vessels, and forced the dissolution of entrapped gas. The capacity of different plants to repair compromised xylem vessels and the mechanisms controlling these repairs are currently being investigated.

References and Recommended Reading

Agrios, G. N. Plant Pathology . New York, NY: Academic Press, 1997.

Beerling, D. J. & Franks, P. J. Plant science: The hidden cost of transpiration. Nature 464, 495-496 (2010).

Brodersen, C. R. et al . The dynamics of embolism repair in xylem: In vivo visualizations using high-resolution computed tomography Plant Physiology 154 , 1088-1095 (2010).

Brodribb, T. J. & Holbrook, N. M. Water stress deforms tracheids peripheral to the leaf vein of a tropical conifer. Plant Physiology 137 , 1139-1146 (2005)

Canadell, J. et al . Maximum rooting depth of vegetation types at the global scale. Oecologia 108, 583-595 (1996).

Choat, B., Cobb, A. R. & Jansen, S. Structure and function of bordered pits: New discoveries and impacts on whole-plant hydraulic function. New Phytologist 177, 608-626 (2008).

Chung, H. H. & Kramer, P. J. Absorption of water and "P through suberized and unsuberized roots of loblolly pine. Canadian Journal of Forest Research 5, 229-235 (1975).

Eapen, D. et al . Hydrotropism: Root growth responses to water. Trends in Plant Science 10, 44-50 (2005).

Hetherington, A. M. & Woodward, F. I. The role of stomata in sensing and driving environmental change. Nature 424, 901-908 (2003).

Holbrook, N. M. & Zwieniecki, M. A. Vascular Transport in Plants . San Diego, CA: Elsevier Academic Press, 2005.

Javot, H. & Maurel, C. The role of aquaporins in root water uptake. Annals of Botany 90, 1-13 (2002).

Kramer, P. J. & Boyer, J. S. Water Relations of Plants and Soils . New York, NY: Academic Press, 1995.

Kramer, P. J. & Bullock, H. C. Seasonal variations in the proportions of suberized and unsuberized roots of trees in relation to the absorption of water. American Journal of Botany 53, 200-204 (1966).

MacFall, J. S., Johnson, G. A. & Kramer, P. J. Observation of a water-depletion region surrounding loblolly pine roots by magnetic resonance imaging. Proceedings of the National Academy of Sciences of the United States of America 87 , 1203-1207 (1990).

McCully, M. E. Roots in Soil: Unearthing the complexities of roots and their rhizospheres. Annual Review of Plant Physiology and Plant Molecular Biology 50, 695-718 (1999).

McDowell, N. G. et al . Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought? New Phytologist 178, 719-739 (2008).

Nardini, A., Lo Gullo, M. A. & Salleo, S. Refilling embolized xylem conduits: Is it a matter of phloem unloading? Plant Science 180, 604-611 (2011).

Pittermann, J. et al . Torus-margo pits help conifers compete with angiosperms. Science 310, 1924 (2005).

Sack, L. & Holbrook, N. M. Leaf hydraulics. Annual Review of Plant Biology 57, 361-381 (2006).

Sack, L. & Tyree, M. T. "Leaf hydraulics and its implications in plant structure and function," in Vascular Transport in Plants , eds. N. M. Holbrook & M. A. Zwieniecki. (San Diego, CA: Elsevier Academic Press, 2005) 93-114.

Schenk, H. J. & Jackson, R. B. Rooting depths, lateral root spreads, and belowground/aboveground allometries of plants in water-limited environments. Journal of Ecology 90, 480-494 (2002).

Sperry, J. S. & Tyree, M. T. Mechanism of water-stress induced xylem embolism. Plant Physiology 88, 581-587 (1988).

Steudle, E. The cohesion-tension mechanism and the acquisition of water by plants roots. Annual Review of Plant Physiological and Molecular Biology 52, 847-875 (2001).

Steudle, E. Transport of water in plants. Environmental Control in Biology 40, 29-37 (2002).

Takahashi, H. Hydrotropism and its interaction with gravitropism in roots. Plant Soil 165 , 301-308 (1994).

Tyree, M. T. & Ewers, F. W. The hydraulic architecture of trees and other woody plants. New Phytologist 119, 345-360 (1991).

Tyree, M. T. & Sperry, J. S. Vulnerability of xylem to cavitation and embolism. Annual Review of Plant Physiology and Molecular Biology 40, 19-38 (1989).

Tyree, M. T. & Zimmerman, M. H. Xylem Structure and the Ascent of Sap . 2nd ed. New York, NY: Springer-Verlag, 2002.

Tyree, M. T. & Ewers, F. The hydraulic architecture of trees and other woody plants. New Phytologist 119, 345-360 (1991).

Wheeler, T. D. & Stroock, A. D. The transpiration of water at negative pressures in a synthetic tree. Nature 455, 208-212 (2008).

Wullschleger, S. D., Meinzer, F. C. & Vertessy, R. A. A review of whole-plant water use studies in trees. Tree Physiology 18, 499-512 (1998).

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Xylem and Phloem – Plant Vascular System

The vascular system of plants consists of the xylem and phloem. They are somewhat like blood vessels in animals, but plants transport materials using two tissues rather than one. Here is a look at what xylem and phloem are, what they transport, and how they work.

What are Xylem and Phloem?

Xylem and phloem are the two types of transport tissue found in vascular plants. They form a complex network running throughout the plant, carrying resources to different parts and disposing of waste products.

• Xylem primarily transports water and mineral nutrients from the roots to the rest of the plant, and it also plays a role in physical support.
• Phloem transports organic substances, such as sugars produced during photosynthesis, from the leaves to other parts of the plant.

Importance of the Vascular System in Plants

The vascular system allows plants to grow taller and larger, enabling them to inhabit a wide variety of environments. Without these conduits, plants only grow to a small size. Non-vascular plants, such as mosses and liverworts, lack xylem and phloem and rely on diffusion and osmosis for the distribution of nutrients. Vascular plants, including trees, flowering plants, and ferns, use xylem and phloem to efficiently transport nutrients, even against gravity.

The term “xylem” comes from the Greek word “xylon,” which means “wood.” This reflects the role of xylem tissue in contributing to the structural strength of plants, particularly woody ones.

Function and Structure of Xylem

Xylem transports water and dissolved minerals absorbed from the soil by the roots to the above-ground parts of the plant. The plant uses the water transported by the xylem photosynthesis and transpiration. Additionally, the xylem also provides structural support to the plant.

The xylem tissue consists of four main types of cells: tracheids, vessel elements, xylem parenchyma, and xylem fibers. The vessel elements and tracheids are the water-conducting cells. Vessel elements are wider and shorter than tracheids and connect together at the ends. The ends have perforation plates that permit water transfer between cells. Tracheids are long, thin, and tapered at the ends. The secondary cell walls of the tracheids contain lignin. The parenchyma stores food and helps in the repair and growth of xylem, while xylem fibers provide support.

In most plants, the xylem is in the center of the stem, forming a core of rigid, woody material. Mature xylem consists of dead vessel element and tracheid cells connected by hollow ends.

Transportation in Xylem

The mechanism of water transport in xylem primarily involves a process known as cohesion-tension theory. Here, the evaporative pull of transpiration from the leaves creates a tension or negative pressure that pulls water upward from the roots through the xylem tissue. Also, root pressure also plays a role. Here, water enters roots from the soil via osmosis, generating a positive pressure that forces water upward into the plant.

The term “phloem” comes from the Greek word “phloios,” meaning “bark.” This name is fitting, as phloem is often found just beneath the bark in trees.

Function and Structure of Phloem

Phloem transports organic nutrients, particularly sugars synthesized during photosynthesis, from the leaves to all other cells of the plant, including the roots.

Phloem tissue is composed of sieve-tube elements, companion cells, phloem fibers, and phloem parenchyma. The sieve-tube elements, along with their companion cells, primarily control the transportation of food. Phloem fibers provide support, and phloem parenchyma assists with food storage and the secretion of plant resins.

In most plants, the phloem is towards the exterior of the plant, just below the bark in stems and roots. The sieve-tube cells are alive, but they lack a nucleus and have less cytoplasm than other plant cells . The companion cells are living cells with a normal composition.

Transportation in Phloem

The transport mechanism in phloem is known as translocation. It involves an active process where sugars load into sieve tubes in the leaves (source) and unload where they are needed (sink), such as roots or developing shoots. This differential in sugar concentration results in water moving from xylem to phloem, building a pressure that drives the sap down the plant.

Differences in Xylem and Phloem in Monocots and Dicots

Monocots and dicots differ in the arrangement and structure of their xylem and phloem.

In dicot plants, the vascular system is organized in a ring, with the xylem typically inside, surrounded by phloem. There is often a region of meristematic cambium cells, which divide to produce more xylem or phloem cells, allowing the stem or root to increase in diameter.

In monocot plants, the xylem and phloem are paired into bundles scattered throughout the stem. Monocots do not have a vascular cambium, meaning they typically do not increase in diameter after growth.

Girdling and Its Impacts

Girdling is a practice that removes a ring of bark (the phloem layer) from around the entire circumference of a tree or plant stem. This disrupts the downward transportation of sugars and other metabolites from the leaves through the phloem. Girdling can cause the death of a tree because it interrupts the supply of food from leaves to the roots, essentially starving the plant.

However, girdling also has a deliberate use in horticulture. It encourages the plant to produce larger fruits or to direct the plant’s energy towards certain branches. By disrupting the flow of nutrients, the plant overcompensates in the remaining portions, often leading to increased yield or size of the produce.

• Lucas, William; et al. (2013). “”The Plant Vascular System ” Evolution, Development and Functions”. Journal of Integrative Plant Biology . 55 (4): 294–388. doi: 10.1111/jipb.12041
• McCulloh, Katherine A.; John S. Sperry; Frederick R. Adler (2003). “Water transport in plants obeys Murray’s law”. Nature . 421 (6926): 939–942. doi: 10.1038/nature01444
• Raven, Peter A.; Evert, Ray F.; Eichhorn, Susan E. (1999). Biology of Plants . W.H. Freeman and Company. ISBN 978-1-57259-611-5.
• Roberts, Keith (ed.) (2007). Handbook of Plant Science . Vol. 1 (Illustrated ed.). John Wiley & Sons. ISBN 9780470057230.
• Slewinski, Thomas L.; Zhang, Cankui; Turgeon, Robert (2013-07-05). “Structural and functional heterogeneity in phloem loading and transport”. Frontiers in Plant Science . 4: 244. doi: 10.3389/fpls.2013.00244

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Learning Goals

By the end of this reading you should be able to:

• Describe the structure of xylem and differentiate between tracheids and vessel elements
• Explain how water is moved from roots to leaves of plants
• Relate the movement of water in xylem to the properties of water
• Explain how xylem structure helps to prevent collapse and cavitation

Introduction

On a summer day, a tree can transport many hundreds of liters of water from the soil to its leaves. This is an impressive feat given that it is accomplished without any moving parts. Even more remarkable, trees and other plants transport water without any direct expenditure of energy. The upward transport of water is possible because the structure of vascular plants allows them to use the evaporation of water from leaves to pull water from the soil.

This figure shows the cross-section of a sunflower stem. Like a leaf, it has a surface layer of epidermal cells. This layer encloses thin-walled parenchyma cells in the interior. Notice that the stem also contains differentiated tissues that lie in a ring near the outside of the stem. These are the vascular tissues, which form a continuous pathway that extends from near the tips of the roots, through the stem, and into the network of veins within leaves. The outer tissue, called phloem, transports carbohydrates from leaves to the rest of the plant body. The inner tissue, called xylem, transports water and nutrients from the roots to the leaves.

Show Responses

Multiple answers: Multiple answers are accepted for this question What are some of the key functions of xylem? A

Xylem Structure: A Low Resistant Pathway for Water

Water travels with relative ease through xylem because of the structure of the water-transporting cells within this tissue. As they develop, these cells become greatly elongated. When they complete their growth, they secrete an additional wall layer that is very thick and which contains lignin, a chemical compound that increases mechanical strength. Most remarkable is the final stage of development when the nucleus and cytoplasm are lost, leaving behind only the cell walls.

These thick walls form a hollow conduit through which water can flow. Water enters and exits xylem conduits through pits, circular or ovoid regions in the walls where the lignified cell wall layer is not produced. Instead, pits contain only the thin, water­permeable cell wall that surrounded each cell as it grew. As we will see, pits play an important role in xylem because they allow the passage of water, but not air, from one conduit to another.

Xylem conduits can be formed from a single cell or multiple cells stacked to form a hollow tube. Unicellular conduits are called tracheids, and multicellular conduits are called vessels. Because tracheids are the product of a single cell, they are typically 4 to 40 µ,m in diameter and no more than 2 to 3 cm long. Vessels, which are made up of individual cells called vessel elements, can be much wider and longer. Vessel diameters range from 5 to 500 µm, and lengths can be up to several meters.

Water enters a tracheid through pits, travels upward through the conduit interior, and then flows outward through other pits into an adjacent, partially overlapping tracheid. Water also enters and exits a vessel through pits. In contrast to tracheids, however, once the water is inside the vessel, little or nothing blocks the flow of water from one cell to the next. That is because during the development of a xylem vessel, the end walls of the vessel elements are digested away, allowing water to flow along the entire length of the vessel without having to cross any pits. At the end of a vessel, however, the water must flow through pits if it is to enter an adjacent vessel and thereby continue its journey from the soil to the leaves.

The rate at which water moves through xylem is a function of both the number of conduits and their size. Conduit length determines how often water must flow across pits, which exert a significant resistance to flow. Conduit width also has a strong effect on the rate of water transport. Like the flow through pipes, water flow through xylem conduits is greater when the conduits are wider. The flow is proportional to the radius of the conduit raised to the fourth power, so doubling the radius increases flow sixteenfold. Because vessels are both longer and wider than tracheids, plants with vessels achieve greater rates of water transport than is possible through tracheids alone. Tracheids are the most common conduits in lycophytes, ferns and horsetails, and gymnosperms, whereas vessels are the principal conduit in angiosperms.

Moving water without energy

Water is pulled through xylem by an evaporative pump. If you cut a plant’s roots off underwater, the leaves continue to transpire for some time. The persistence of transpiration when roots are absent demonstrates that the driving force for water transport is not generated in the roots, but instead comes from the leaves. In essence, water is pulled through the xylem from above rather than being pushed from below.

The forces pulling water upward through the plant are large. Not only must these forces be able to lift water against gravity, but they must also be able to pull water from the soil, which becomes increasingly difficult as the soil dries. In addition, they must be able to overcome the physical resistance associated with moving water through the plant itself. To replace water lost by transpiration with water pulled from the soil, the leaves must exert forces that are many times greater than the suctions that we can generate with a vacuum pump. How do leaves exert this force?

When stomata are open, water evaporates from the walls of cells lining the intercellular air spaces of leaves. The partial dehydration of the cell walls creates a force that pulls water towards the sites of evaporation. One hypothesis for how this force is generated is that water is pulled by capillary action into spaces between the cellulose microfibrils in the cell wall, the same reason that water is drawn into a sponge. A second hypothesis is that the pectin gel in which the cellulose microfibrils are embedded causes water to flow into the partially dehydrated cell walls by osmosis. Osmotic forces can be generated in cell walls because the negatively charged pectin network restricts the diffusion of positively charged ions, much as the plasma membrane maintains a high concentration of solutes in the cytoplasm.

Once generated by the evaporation of water from leaves, this force is transmitted through the xylem, beginning in the leaf veins, then down through the stem, and out through the roots to the soil

Water can be pulled through xylem because of the strong hydrogen bonds that form between water molecules(cohesion). The xylem structure also enhances the ability of water molecules to bind to other surfaces (adhesion). These mechanisms of water transport only work, however, if there is a continuous column of water in the xylem that extends from the roots to the leaves. This video will help you to visualize the process of transpiration.

What is the main force that drives most of the water movement within xylem vessels toward the top of a plant? A

The structure of xylem conduits reduces the risks of collapse and cavitation.

The fact that water is pulled from the soil means that xylem conduits must be structured in such a way as to minimize two distinct risks. The first is the danger of collapse. If you suck too hard on a drinking straw, it will collapse inward, blocking flow. Much the same thing can happen in the xylem. Although in metabolic terms, lignin is more costly to produce than cellulose, lignin makes conduit walls rigid, reducing the risk of collapse.

The second danger associated with pulling water through the xylem is cavitation, which occurs when the water in a conduit is abruptly replaced by water vapor. Because cavitation disrupts the continuity of the water column, cavitated conduits can no longer transport water from the soil. Cavitation in xylem results from the presence of microscopic gas bubbles in the water that are sufficiently large that they expand under the tensile, or pulling, the force exerted by the leaves. The microscopic bubbles that cause cavitation can be formed in either of two ways. The first is if an air bubble is pulled through a pit because of lower pressure in the water compared to the air. The larger the tensile forces exerted by leaves, the more likely it is that air will be pulled across a pit. Thus, higher rates of transpiration increase the risk of cavitation.

The second way that gas bubbles can form within xylem is if gases come out of solution during freezing. Gases are much less soluble in ice than in water, so as a conduit freezes, dissolved gas molecules aggregate into tiny bubbles that can cause cavitation when the conduit thaws. Wide vessels are especially vulnerable to cavitation at freezing temperatures. The susceptibility of wide vessels to cavitation partly explains why boreal (that is, subarctic) forests contain few angiosperms, which commonly have large vessel diameters.

Once cavitation has occurred, the liquid phase will not re-form as long as tension persists within the xylem. Thus, xylem is organized in ways that minimize the likelihood and impact of cavitation. For example, xylem consists of many conduits in parallel, so the loss of any one conduit to cavitation does not result in a major loss of transport capacity. Similarly, as water flows from the soil to the leaves, it passes from one conduit to another, each one of finite length. The likelihood that cavitation will spread is thereby reduced because air can be pulled through pits only when the tensile forces in the xylem are large.

The structure of xylem tracheids and vessel elements in a key component to the movement of water from plant roots to leaves. In addition, the properties of water, adhesion and cohesion, allow the formation of a continuous column of water in the xylem. As the water is pulled up by transpiration cohesion keeps water molecules together, and adhesion of water to the xylem walls helps to keep it from reversing direction. When excessive pressure or the introduction of air bubbles occurs this can impair the capacity of xylem to move water. Xylem tissue, however, is composed of large numbers of cells and thus a large number of pathways within the cells for water movement, thus collapse or cavitation of individual cells has a smaller impact as water moves around these in surrounding cells.

Review Questions

Transpiration in plants requires all of the following except? A

In plants water moves from areas of high water potential to areas of low water potential by osmosis and diffusion. In the xylem water stream, where is the lowest water potential? A

Compared to tracheids, vessel elements are? A

To the extent possible under law, s2jrmoor has waived all copyright and related or neighboring rights to VCU BIOL 152: Introduction to Biological Sciences II , except where otherwise noted.

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How Water Moves Through Plants

Two Environmental Factors That Affect Transpiration

The importance of plants in everyday life cannot be understated. They provide oxygen, food, shelter, shade and countless other functions.

They also contribute to the movement of water through the environment. Plants themselves boast their own unique way of taking in water and releasing it into the atmosphere.

TL;DR (Too Long; Didn't Read)

Plants require water for biological processes. The movement of water through plants involves a pathway from root to stem to leaf, using specialized cells.

Water Transportation in Plants

Water is essential to the life of plants at the most basic levels of metabolism. In order for a plant to access water for biological processes, it needs a system to move water from the ground to different plant parts.

The chief water movement in plants is through osmosis from the roots to the stems to the leaves. How does water transportation in plants occur? Water movement in plants occurs because plants have a special system to draw water in, conduct it through the body of the plant and eventually to release it to the surrounding environment.

In humans, fluids circulate in bodies via the circulatory system of veins, arteries and capillaries. There is also specialized network of tissues that aids the process of nutrient and water movement in plants. These are called xylem and phloem .

What Is Xylem?

Plant roots reach into the soil and seek water and minerals for the plant to grow. Once the roots find water, the water travels up through the plant all the way to its leaves. The plant structure used for this water movement in plants from root to leaf is called xylem.

Xylem is a kind of plant tissue that is made out of dead cells that are stretched out. These cells, named tracheids , possess a tough composition, made of cellulose and the resilient substance lignin . The cells are stacked and form vessels, allowing water to travel with little resistance. Xylem is waterproof and has no cytoplasm in its cells.

Water travels up the plant through the xylem tubes until it reaches mesophyll cells, which are spongy cells that release the water through miniscule pores called stomata . Simultaneously, stomata also allow for carbon dioxide to enter a plant for photosynthesis. Plants possess several stomata on their leaves, particularly on the underside.

Different environmental factors can rapidly trigger stomata to open or close. These include temperature, carbon dioxide concentrate in the leaf, water and light. Stomata close up at night; they also close in response to too much internal carbon dioxide and to prevent too much water loss, depending on the air temperature.

Light triggers them to open. This signals the plant’s guard cells to draw in water. The guard cells’ membranes then pump out hydrogen ions, and potassium ions can enter the cell. Osmotic pressure declines when the potassium builds up, resulting in water attraction to the cell. In hot temperatures, these guard cells do not have as much access to water and can close up.

Air can also fill the xylem’s tracheids. This process, named cavitation , can result in tiny air bubbles that could impede water flow. To avoid this problem, pits in xylem cells allow for water to move while preventing gas bubbles from escaping. The rest of the xylem can continue moving water as usual. At night, when stomata close up, the gas bubble may dissolve into the water again.

Water exits as water vapor from the leaves and evaporates. This process is called transpiration .

What Is Phloem?

In contrast to xylem, phloem cells are living cells. They make up vessels as well, and their main function is to move nutrients throughout the plant. These nutrients include amino acids and sugars.

Over the course of the seasons, for example, sugars may be moved from the roots to the leaves. The process of moving nutrients throughout the plant is called translocation .

Osmosis in Roots

The tips of plant roots contain root hair cells. These are rectangular in shape and have long tails. The root hairs themselves can extend into the soil and absorb water in a process of diffusion called osmosis.

Osmosis in roots leads to water moving into root hair cells. Once water moves into the root hair cells, it can travel throughout the plant. Water first makes its way to the root cortex and passes through the endodermis . Once there, it can access the xylem tubes and allow for water transportation in plants.

There are multiple paths for water’s journey across roots. One method keeps water between cells so that the water does not enter them. In another method, water does cross cell membranes . It can then move out of the membrane to other cells. Yet another method of water movement from the roots involves water passing through cells via junctions between cells called plasmodesmata .

After passing through the root cortex, water moves through the endodermis, or waxy cellular layer. This is a sort of barrier for water and shunts it through endodermal cells like a filter. Then water can access the xylem and proceed toward the plant’s leaves.

Transpiration Stream Definition

People and animals breathe. Plants possess their own process of breathing, but it is called transpiration .

Once water travels through a plant and reaches its leaves, it can eventually release from the leaves via transpiration. You can see evidence of this method of “breathing” by securing a clear plastic bag around a plant’s leaves. Eventually you'll see water droplets in the bag, demonstrating transpiration from the leaves.

The transpiration stream describes the process of water transported from the xylem in a stream from root to leaf. It also includes the method of moving mineral ions around, keeping plants sturdy via water turgor, making sure leaves have enough water for photosynthesis and allowing the water to evaporate to keep leaves cool in warm temperatures.

Effects on Transpiration

When plant transpiration is combined with evaporation from land, this is called evapotranspiration . The transpiration stream results in approximately 10 percent of moisture release into the atmosphere of the Earth.

Plants can lose a significant amount of water through transpiration. Even though it is not a process that can be seen with the naked eye, the effect of water loss is measurable. Even corn can release as much as 4,000 gallons of water in a day. Large hardwood trees can release as much as 40,000 gallons daily.

Rates of transpiration vary depending on the status of the atmosphere around a plant. Weather conditions play a prominent role, but transpiration is also affected by soils and topography.

Temperature alone greatly affects transpiration. In warm weather, and in strong sun, the stomata are triggered to open and release water vapor. However, in cold weather, the opposite situation occurs, and the stomata will close up.

The dryness of the air directly affects transpiration rates. If the weather is humid and the air full of moisture, a plant is less likely to release as much water via transpiration. However, in dry conditions, plants readily transpire. Even the movement of wind can increase transpiration.

Different plants adapt to different growth environments, including in their rates of transpiration. In arid climates such as deserts, some plants can hold onto water better, such as succulents or cacti.

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• BBC Bitesize: Structure of Plants
• BBC Bitesize: Transport in Plants
• USGS Water Science School: What Is Evapotranspiration?
• University of Nebraska Lincoln: Plant & Soil Sciences eLibrary: Transpiration – Water Movement Through Plants

About the Author

J. Dianne Dotson is a science writer with a degree in zoology/ecology and evolutionary biology. She spent nine years working in laboratory and clinical research. A lifelong writer, Dianne is also a content manager and science fiction and fantasy novelist. Dianne features science as well as writing topics on her website, jdiannedotson.com.

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The Pathway and Movement of Water into the Roots and Xylem (A-level Biology)

The pathway and movement of water into   the roots and xylem, water movement, in the root.

• Water is a transport system . Water is essential in plants as it it used as a transport system for nutrients and minerals across a water potential gradient .
• Water potential is higher within the soil than the root hair cell . Water is taken up by the roots of a plant and through the endodermis , before being moved into xylem tissue, which is in the centre of the root. The water potential inside the soil is higher than that of that root hair cells. This is because of the dissolved substances in the cell sap.
• Root hair cells increase surface area . Root hair cells function to increase the surface area in order to be pumped across against the concentration gradient .

Water can reach the cortex of the xylem vessels via two pathways:

• Symplast , where water moves between the cytoplasm of neighbouring cells.
• Apoplast , where water can moves directly through the permeable cell walls and intercellular spaces of neighbouring cells.

The symplast pathway allows water to enter the cytoplasm via the plasma membrane, where it travels between cells through plasmodesmata .

Plasmodesmata are tiny channels which cross the cell walls of neighbouring plant cells in order to be able to connect their cytoplasm, creating a large multinucleate mass of plant cells.

The apoplast pathway doesn’t need to travel through the plasma membranes in order for the water to move through the spaces between the cellulose molecules. Because of this, it can carry dissolved mineral ions and salts.

Once the water reaches the endodermis of the root, a layer of suberin (known as the Casparian strip ) stops the water’s path. This is because it cannot be penetrated by water.

To be able to cross the endodermis, the moving water can now use the symplast pathway by going down the water potential gradient to reach a pit in the xylem vessel. This is where water enters the vessel.

In the Xylem

Moving down the water potential gradient, water gets removed from the top of xylem vessel into mesophyll cells.

Root pressure, where the endodermis uses active transport to move minerals into the xylem, pushes the water upwards, into the xylem by osmosis.

Cohesion-Tension Theory

The cohesion-tension theory explains how water moves up the xylem.

• Water evaporates from the mesophyll cells within the leaf . This is known as transpiration , which is the evaporation of water from a plant.
• As water molecules are cohesive, tension is created . Water molecules stick together because the molecules can form hydrogen bonds with one another.
• This cohesion and tension causes more water to be drawn up into the xylem . The water is pulled up through the xylem via osmosis so   that the vessel is filled with an uninterrupted column of water. This column will make its way up through the xylem until it evaporates from the mesophyll cell’s walls. The water then diffuses from air sacs in the leaf, through open stoma.

Cohesion is the attraction of same molecules. Hydrogen bonds are formed between water molecules.

Adhesion is the attraction of unlike molecules. Hydrogen bonds are formed between water and surfaces. An example of a surface used in adhesion it the pores in mesophyll cells.

Water moves into plants through the roots and into the xylem, which is responsible for transporting water and dissolved minerals from the roots to the rest of the plant.

Water moves into the roots of a plant through osmosis, which is the movement of water molecules from an area of high concentration to an area of low concentration. In the case of plants, water moves from the soil into the roots due to a lower concentration of water inside the roots.

Transpiration is the process by which water is lost from the leaves of a plant through small pores called stomata. This water loss creates a negative pressure in the xylem that pulls water up from the roots and into the rest of the plant.

Root pressure is a natural force that results from the accumulation of water and minerals in the roots. This pressure helps to push water up into the xylem and into the rest of the plant.

The structure of the xylem plays a critical role in the movement of water into plants. The xylem is composed of long, narrow tubes that run from the roots to the leaves, and the continuous column of water in the xylem is maintained by the cohesive forces of the water molecules.

Capillarity is the process by which water is drawn up into small tubes, such as the xylem in plants. The combination of capillarity and transpiration creates a continuous column of water in the xylem, which provides a pathway for water to move from the roots to the leaves.

The pathway and movement of water into plants is a critical concept for A-Level Biology students to understand because it highlights the interconnectedness of different plant structures and functions. Additionally, this knowledge provides a foundation for further studies in botany, plant physiology, and related fields.

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• Plant Biology
• Human Biology
• Biology Plant Biology Water Transport Experiment

How does water move in plants?

Have you ever wondered how plants are able to pull water out of the ground? It’s not like they have a heart to pump water around or even a digestion system to extract the water from the soil!

In fact, water movement in plants doesn’t rely on energetically expensive biological pumps or even magic. It relies on some pretty basic physical principles operating within unique plant structures, and anyone can understand it. We’ll see how in this home experiment.

3 glass or plastic cups (sturdy enough not to tip over) 300 g room-temperature water Food coloring Metric scale Fan Medium-to-large sealable plastic box (tall enough to fit an upright stalk of celery inside) 2 small squares of plastic wrap 2 stalks celery, leaves attached

• Pick two celery stalks that they have similar amounts of leaves. (Hint: If you can’t find celery with leaves attached in your grocery store, buy a head of celery. The small inner stalks usually still contain leaves.) Cut off the bases of the stalks so that they are roughly the same height.
• Place one glass on the scale and tare it: press the “zero” button so that the cup registers as “0 g”. Fill with 150 g of room-temperature water; gently place two drops of food coloring in the water; stir. Be careful not to spill any colored water! Repeat with the second glass.
• Place one celery stalk in each glass, leaf–end at the top.
• Wrap one square of the plastic wrap around the top of each glass and the celery stalk. This is to prevent any colored water evaporating into the air directly from the glass.
• Fill the bottom of the plastic box with roughly one inch of room-temperature water. Place one of the cups with the celery stalk inside the box and seal the lid to create a humid, closed environment.
• Place both the boxed celery and the naked celery in front of a fan, and turn it on the lowest setting. Record the time: _________
• Wait 24 hours.
• Boxed celery:___________________________________________
• Naked celery:___________________________________________
• Place the third glass cup on the scale, and tare it again so that the scale reads “0 g”.
• Boxed celery:____________
• Naked celery:____________
• Celery cross-section:____________________________

How does water move up the stalk?

Although plants don’t have circulatory systems like animals, they do have something quite similar—a network of small tubes called xylem , used for carrying water.

Xylem is composed of long, hollow tubes formed by overlapping cells. As these cells grow, they stretch out and elongate, die, and leave behind hollow cavities that are all interconnected to form one long tube. Plants contain many xylem vessels stretching from the roots to the tips of the leaves, just like a series of drinking straws. When you sliced the celery in half and saw colored dots in the cross-section of the stalk, you were actually looking at the xylem vessels!

Xylem works within some basic physical principles to bring water from the ground up into the rest of the plant. The whole process starts out in the leaves: when the plant is photosynthesizing, it opens tiny holes in the underside of the leaf called stomata . The plant does this so that carbon dioxide can flow in, but it also has a downside: water also diffuses out of the stomata at the same time, drying out the inside of the leaf ever so slightly.

As the plant dries out from the leaves, it brings more water in from the xylem due to some interesting chemical properties. Water is a polar molecule, meaning that it’s slightly “sticky”—it forms temporary hydrogen bonds with itself. This creates cohesion ; small quantities of water will tend to stick together rather than scattering and spreading everywhere (think of dew drops on grass). Water also sticks to the inside of small tubes due to a property called capillary action . These two properties allow the water to travel in one unbroken column through the xylem from the roots to the leaves.

What factors affect how water moves through the plant?

Water moves through plants thanks to a few basic principles, but none of these can work without the first step in the process: water loss from the leaves. This process, called transpiration , happens faster when humidity is low, such as on a hot, windy day. This causes water to evaporate quickly, so the plant needs to suck up more water from the ground (or from the cup) to catch up!

When you put the celery stalk inside the plastic box with water, it increased the humidity in the box, so the celery didn’t lose very much water from the leaves. On the flip side, when you placed the naked celery stalk in front of the fan, it was losing a lot of water! It needed to catch up, so it sucked up more water, and food coloring with it.

When you measured the amount of water left in the glasses at the end of the experiment, you found that the naked celery actually did suck up more water. And, in case you didn’t believe the numbers, you could actually observe that the naked celery had a lot more food coloring within its leaves.

Normally, we can’t see transpiration and water transport happening within plants, but rest assured: as long as it’s above freezing, this process is always happening on a mass scale all over the world!

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Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

Plant Biology-boo

• Monocots vs Dicots Explained
• Phototropism Experiment
• Plant Diversity
• Plant Growth Hormones
• Plant Memory
• Transport in Plants
• Water Transport Experiment
• Why leaves change color

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February 8, 1999

11 min read

How do large trees, such as redwoods, get water from their roots to the leaves?

Last week we presented a general outline of how trees lift water. Donald J. Merhaut of Monrovia Nursery Company, headquartered in Azusa, Calif., has provided a more detailed reply:

"Water is often the most limiting factor to plant growth. Therefore, plants have developed an effective system to absorb, translocate, store and utilize water. To understand water transport in plants, one first needs to understand the plants' plumbing. Plants contain a vast network of conduits, which consists of xylem and phloem tissues. This pathway of water and nutrient transport can be compared with the vascular system that transports blood throughout the human body. Like the vascular system in people, the xylem and phloem tissues extend throughout the plant. These conducting tissues start in the roots and transect up through the trunks of trees, branching off into the branches and then branching even further into every leaf.

"The phloem tissue is made of living elongated cells that are connected to one another. Phloem tissue is responsible for translocating nutrients and sugars (carbohydrates), which are produced by the leaves, to areas of the plant that are metabolically active (requiring sugars for energy and growth). The xylem is also composed of elongated cells. Once the cells are formed, they die. But the cell walls still remain intact, and serve as an excellent pipeline to transport water from the roots to the leaves. A single tree will have many xylem tissues, or elements, extending up through the tree. Each typical xylem vessel may only be several microns in diameter.

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"The physiology of water uptake and transport is not so complex either. The main driving force of water uptake and transport into a plant is transpiration of water from leaves. Transpiration is the process of water evaporation through specialized openings in the leaves, called stomates. The evaporation creates a negative water vapor pressure develops in the surrounding cells of the leaf. Once this happens, water is pulled into the leaf from the vascular tissue, the xylem, to replace the water that has transpired from the leaf. This pulling of water, or tension, that occurs in the xylem of the leaf, will extend all the way down through the rest of the xylem column of the tree and into the xylem of the roots due to the cohesive forces holding together the water molecules along the sides of the xylem tubing. (Remember, the xylem is a continuous water column that extends from the leaf to the roots.) Finally, the negative water pressure that occurs in the roots will result in an increase of water uptake from the soil.

"Now if transpiration from the leaf decreases, as usually occurs at night or during cloudy weather, the drop in water pressure in the leaf will not be as great, and so there will be a lower demand for water (less tension) placed on the xylem. The loss of water from a leaf (negative water pressure, or a vacuum) is comparable to placing suction to the end of a straw. If the vacuum or suction thus created is great enough, water will rise up through the straw. If you had a very large diameter straw, you would need more suction to lift the water. Likewise, if you had a very narrow straw, less suction would be required. This correlation occurs as a result of the cohesive nature of water along the sides of the straw (the sides of the xylem). Because of the narrow diameter of the xylem tubing, the degree of water tension, (vacuum) required to drive water up through the xylem can be easily attained through normal transpiration rates that often occur in leaves."

Alan Dickman is curriculum director in the biology department at the University of Oregon in Eugene. He offers the following answer to this oft-asked question:

"Once inside the cells of the root, water enters into a system of interconnected cells that make up the wood of the tree and extend from the roots through the stem and branches and into the leaves. The scientific name for wood tissue is xylem; it consists of a few different kinds of cells. The cells that conduct water (along with dissolved mineral nutrients) are long and narrow and are no longer alive when they function in water transport. Some of them have open holes at their tops and bottoms and are stacked more or less like concrete sewer pipes. Other cells taper at their ends and have no complete holes. All have pits in their cell walls, however, through which water can pass. Water moves from one cell to the next when there is a pressure difference between the two.

"Because these cells are dead, they cannot be actively involved in pumping water. It might seem possible that living cells in the roots could generate high pressure in the root cells, and to a limited extent this process does occur. But common experience tells us that water within the wood is not under positive pressure--in fact, it is under negative pressure, or suction. To convince yourself of this, consider what happens when a tree is cut or when a hole is drilled into the stem. If there were positive pressure in the stem, you would expect a stream of water to come out, which rarely happens.

"In reality, the suction that exists within the water-conducting cells arises from the evaporation of water molecules from the leaves. Each water molecule has both positive and negative electrically charged parts. As a result, water molecules tend to stick to one another; that adhesion is why water forms rounded droplets on a smooth surface and does not spread out into a completely flat film. As one water molecule evaporates through a pore in a leaf, it exerts a small pull on adjacent water molecules, reducing the pressure in the water-conducting cells of the leaf and drawing water from adjacent cells. This chain of water molecules extends all the way from the leaves down to the roots and even extends out from the roots into the soil. So the simple answer to the question about what propels water from the roots to the leaves is that the sun's energy does it: heat from the sun causes the water to evaporate, setting the water chain in motion."

Updated on February 8, 1999

Ham Keillor-Faulkner is a professor of forestry at Sir Sandford Fleming College in Lindsay, Ontario. Here is his explanation:

To evolve into tall, self-supporting land plants, trees had to develop the ability to transport water from a supply in the soil to the crown--a vertical distance that is in some cases 100 meters or more (the height of a 30-story building). To understand this evolutionary achievement requires an awareness of wood structure, some of the biological processes occurring within trees and the physical properties of water.

Water and other materials necessary for biological activity in trees are transported throughout the stem and branches in thin, hollow tubes in the xylem, or wood tissue. These tubes are called vessel elements in hardwood or deciduous trees (those that lose their leaves in the fall), and tracheids in softwood or coniferous trees (those that retain the bulk of their most recently produced foliage over the winter). Vessel elements are joined end-to-end through perforation plates to form tubes (called vessels) that vary in size from a few centimeters to many meters in length depending on the species. Their diameters range from 20 to 800 microns. Along the walls of these vessels are very small openings called pits that allow for the movement of materials between adjoining vessels.

Tracheids in conifers are much smaller, seldomly exceeding five millimeters in length and 30 microns in diameter. They do not have perforated ends, and so are not joined end-to-end into other tracheids. As a result, the pits in conifers, also found along the lengths of the tracheids, assume a more important role. They are they only way that water can move from one tracheid to another as it moves up the tree.

To move water through these elements from the roots to the crown, a continuous column must form. It is believed that this column is initiated when the tree is a newly germinated seedling, and is maintained throughout the tree's life span by two forces--one pushing water up from the roots and the other pulling water up to the crown. The push is accomplished by two actions, namely capillary action (the tendency of water to rise in a thin tube because it usually flows along the walls of the tube) and root pressure. Capillary action is a minor component of the push. Root pressure supplies most of the force pushing water at least a small way up the tree. Root pressure is created by water moving from its reservoir in the soil into the root tissue by osmosis (diffusion along a concentration gradient). This action is sufficient to overcome the hydrostatic force of the water column--and the osmotic gradient in cases where soil water levels are low.

Capillary action and root pressure can support a column of water some two to three meters high, but taller trees--all trees, in fact, at maturity--obviously require more force. In some older specimens--including some species such as Sequoia , Pseudotsuga menziesii and many species in tropical rain forests--the canopy is 100 meters or more above the ground! In this case, the additional force that pulls the water column up the vessels or tracheids is evapotranspiration, the loss of water from the leaves through openings called stomata and subsequent evaporation of that water. As water is lost out of the leaf cells through transpiration, a gradient is established whereby the movement of water out of the cell raises its osmotic concentration and, therefore, its suction pressure. This pressure allows these cells to suck water from adjoining cells which, in turn, take water from their adjoining cells, and so on--from leaves to twigs to branches to stems and down to the roots--maintaining a continuous pull.

To maintain a continuous column, the water molecules must also have a strong affinity for one other. This idea is called the cohesion theory. Water does, in fact, exhibit tremendous cohesive strength. Theoretically, this cohesion is estimated to be as much as 15,000 atmospheres (atm). Experimentally, though, it appears to be much less at only 25 to 30 atm. Assuming atmospheric pressure at ground level, nine atm is more than enough to "hang" a water column in a narrow tube (tracheids or vessels) from the top of a 100 meter tree. But a greater force is needed to overcome the resistance to flow and the resistance to uptake by the roots. Even so, many researchers have demonstrated that the cohesive force of water is more than sufficient to do so, especially when it is aided by the capillary action within tracheids and vessels.

In conclusion, trees have placed themselves in the cycle that circulates water from the soil to clouds and back. They are able to maintain water in the liquid phase up to their total height by maintaining a column of water in small hollow tubes using root pressure, capillary action and the cohesive force of water.

Mark Vitosh, a Program Assistant in Extension Forestry at Iowa State University, adds the following information:

There are many different processes occuring within trees that allow them to grow. One is the movement of water and nutrients from the roots to the leaves in the canopy, or upper branches. Water is the building block of living cells; it is a nourishing and cleansing agent, and a transport medium that allows for the distribution of nutrients and carbon compounds (food) throughout the tree. The coastal redwood, or Sequoia sempervirens , can reach heights over 300 feet (or approximately 91 meters), which is a great distance for water, nutrients and carbon compounds to move. To understand how water moves through a tree, we must first describe the path it takes.

Water and mineral nutrients--the so-called sap flow--travel from the roots to the top of the tree within a layer of wood found under the bark. This sapwood consists of conductive tissue called xylem (made up of small pipe-like cells). There are major differences between hardwoods (oak, ash, maple) and conifers (redwood, pine, spruce, fir) in the structure of xylem. In hardwoods, water moves throughout the tree in xylem cells called vessels, which are lined up end-to-end and have large openings in their ends. In contrast, the xylem of conifers consists of enclosed cells called tracheids. These cells are also lined up end-to-end, but part of their adjacent walls have holes that act as a sieve. For this reason, water moves faster through the larger vessels of hardwoods than through the smaller tracheids of conifers.

Both vessel and tracheid cells allow water and nutrients to move up the tree, whereas specialized ray cells pass water and food horizontally across the xylem. All xylem cells that carry water are dead, so they act as a pipe. Xylem tissue is found in all growth rings (wood) of the tree. Not all tree species have the same number of annual growth rings that are active in the movement of water and mineral nutrients. For example, conifer trees and some hardwood species may have several growth rings that are active conductors, whereas in other species, such as the oaks, only the current years' growth ring is functional.

This unique situation comes about because the xylem tissue in oaks has very large vessels; they can carry a lot of water quickly, but can also be easily disrupted by freezing and air pockets. It's amazing that a 200 year-old living oak tree can survive and grow using only the support of a very thin layer of tissue beneath the bark. The rest of the 199 growth rings are mostly inactive. In a coastal redwood, though, the xylem is mostly made up of tracheids that move water slowly to the top of the tree.

Now that we have described the pathway that water follows through the xylem, we can talk about the mechanism involved. Water has two characteristics that make it a unique liquid. First, water adheres to many surfaces with which it comes into contact. Second, water molecules can also cohere, or hold on to each other. These two features allow water to be pulled like a rubber band up small capillary tubes like xylem cells.

Water has energy to do work: it carries chemicals in solution, adheres to surfaces and makes living cells turgid by filling them. This energy is called potential energy. At rest, pure water has 100 percent of its potential energy, which is by convention set at zero. As water begins to move, its potential energy for additional work is reduced and becomes negative. Water moves from areas with the least negative potential energy to areas where the potential energy is more negative. For example, the most negative water potential in a tree is usually found at the leaf-atmosphere interface; the least negative water potential is found in the soil, where water moves into the roots of the tree. As you move up the tree the water potential becomes more negative, and these differences create a pull or tension that brings the water up the tree.

A key factor that helps create the pull of water up the tree is the loss of water out of the leaves through a process called transpiration. During transpiration, water vapor is released from the leaves through small pores or openings called stomates. Stomates are present in the leaf so that carbon dioxide--which the leaves use to make food by way of photosynthesis--can enter. The loss of water during transpiration creates more negative water potential in the leaf, which in turn pulls more water up the tree. So in general, the water loss from the leaf is the engine that pulls water and nutrients up the tree.

How can water withstand the tensions needed to be pulled up a tree? The trick is, as we mentioned earlier, the ability of water molecules to stick to each other and to other surfaces so strongly. Given that strength, the loss of water at the top of tree through transpiration provides the driving force to pull water and mineral nutrients up the trunks of trees as mighty as the redwoods.

Original answer posted on February 1, 1999

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4.5.1.4: Water Absorption

• Last updated
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• Page ID 32038

• Melissa Ha, Maria Morrow, & Kammy Algiers
• Yuba College, College of the Redwoods, & Ventura College via ASCCC Open Educational Resources Initiative

Learning Objectives

• Explain the function of root hairs.
• Define root pressure and explain its mechanism.
• Contrast the three pathways of water movement through the roots and identify each cell type or tissue involved.

Most plants secure the water and minerals they need from their roots. Water moves from the soil to the roots, stems, and ultimately the leaves, where transpiration occurs. The roots absorb enough water to compensate for water lost to transpiration. Rapid absorption is aided by root hairs , which extend from epidermal cells, increasing surface area (Figure \(\PageIndex{1}\)). As discussed earlier in this chapter, roots draw water from the soil because they have lower water potential than the soil. Much of this difference in water potential is an indirect result of transpiration, but roots can also water potential by decreasing solute potential.

Root Pressure

When a tomato plant is carefully severed close to the base of the stem, sap oozes from the stump (Figure \(\PageIndex{2}\)). The fluid comes out because roots are constantly absorbing water, drawing it into the vascular cylinder, and pushing it up the xylem. This is called ro ot pressure , and it is created by the osmotic pressure of solutes trapped in the vascular cylinder by the Casparian strip .

Although root pressure plays a small role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport. As evidence, few plants develop root pressures greater than ~0.2 kPa, and some develop no root pressure at all. Additionally, the volume of fluid transported by root pressure is not enough to account for the measured movement of water in the xylem of most trees and vines. Furthermore, the highest root pressures occur in the spring, but water moves through the xylem most rapidly in the summer (when transpiration is high).

As discussed in the Cohesion-Tension Theory section , transpiration, rather than root pressure, is typically the driving force for upward water movement in a plant. However, when transpiration rates are very low, such as in cool and humid weather, root pressure pushes water up the xylem faster than water is lost through the stomata. As a result water droplets are forced out of openings on the leaf margin, a phenomenon called guttation (Figure \(\PageIndex{3}\)). Droplets resulting from guttation are not to be confused with dew droplets, which result from the condensation of water vapor when the air becomes colder and has less capacity to hold water. In other words, guttation results from water that was inside of the plant, but dew droplets form from water vapor that was in the surrounding air.

Pathways of Water Movement

Water can move through the roots by three separate pathways: apoplast, symplast, and transmembrane (transcellular). In the apoplast pathway (apoplastic route), water moves through the spaces between the cells and in the cells walls themselves. In the symplast pathway (symplastic route), water passes from cytoplasm to cytoplasm through plasmodesmata (Figure \(\PageIndex{4}\)). In the transmembrane pathway , water crosses plasma membranes, entering and exiting each cell. Water moving through the transmembrane pathway thus moves through both the symplast (interconnected cytoplasms) and apoplast (cell walls and spaces in between cells). Water may also cross the tonoplast, entering the central vacuole as part of the transmembrane pathway.

Water from the soil is absorbed by the root hairs of the epidermis and then moves through the cortex through one of the three pathways. However, the inner boundary of the cortex, the endodermis , is impervious to water due to the Casparian strip. Regardless of how the water moved up to this point (apoplast, symplast, transmembrane), it must enter the cytoplasms of the endodermal cells. From here it can pass via plasmodesmata into the cells of the vascular cylinder (stele). Once inside, water is again free to move through the apolast, the symplast, or both (transmembrane).

Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessel elements and tracheids. At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues. At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.

Figure \(\PageIndex{4}\) illustrates minerals moving through the apoplast or symplast, but minerals typically move through the symplast. Minerals enter the root by active transport into the symplast of epidermal cells and move toward and into the vascular cylinder through the plasmodesmata connecting the cells. They enter the conducting cells of the xylem from the pericycle and other parenchyma cells via active transport through specialized transmembrane channels.

Attributions

Curated and authored by Melissa Ha using the following sources:

• 16.2A Xylem from Biology by John. W. Kimball (licensed CC-BY )
• 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY ). Access for free at openstax.org .

Science in School

How water travels up trees teach article.

Author(s): Clare van der Willigen

Why do giant redwoods grow so tall and then stop? It all has to do with how high water can travel up their branches.

The redwoods of northern California, Sequoia sempervirens , are the tallest trees in the world and can grow to heights of more than 110 m. However, what finally limits their height is still debated.

The most popular theory is the ‘hydraulic limitation hypothesis’ ( Ryan & Yoder, 1997 ), which suggests that as trees grow taller, it becomes more difficult to supply water to their leaves. This hydraulic limitation results in reduced transpiration and less photo-synthesis, causing reduced growth.

In tall trees, water supply can be limited by two factors: distance and gravity. Tall trees have a longer path- way of transport tissue – known as xylem – which increases the difficulty of water to travel, something we call hydraulic resistance. In addition, not only is the xylem pathway long, but the trees are tall and the water has to overcome gravity. Increased force is necessary to pull the water up to the highest leaves. This situation differs from a long hosepipe lying along the ground: it would have high resistance due to its length, but not the additional difficulty of being upright.

Fast-growing trees often have shorter life spans. To achieve their rapid growth, pioneer trees have wider xylem vessels, increasing their hydraulic efficiency but also increasing the risk of embolisms (air locks). Air locks in xylem vessels prevent water from being able to travel through them.

In contrast, very tall trees are often very long-lived. It is thought that this is partly because they are more likely to adopt a safe hydraulic design, with multiple narrow xylem vessels instead of a few wider ones.

This increased safety is counteracted by a decreased efficiency of water transport, which consequently limits growth rates. Tree height, therefore, may also be limited by the safety versus efficiency trade-off in xylem function ( Burgess et al, 2006 ).

The following two activities explore the trade-off that plants make between being efficient with water transport and having a safe design. Both activities can be adapted for students aged 15–18 with a wide range of abilities, but you should assess whether the students can perform all of the experiments or whether it is safer for the teacher to do the cutting. Each activity will take about 50 minutes.

Estimating maximum xylem vessel lengths

Comparing the lengths of the xylem vessel will allow students to predict their relative resistance to water flow.

• Selection of recently cut branches from a tree or shrub, including any leaves or side branches, up to 2 m in initial length. If the experiment is to be performed within a few hours of harvesting, keep the plant material in a plastic bag to avoid excessive water loss.
• Rubber/silicon tubing
• Cable ties or jubilee clips
• Sharp pruning shears or scissors
• 60 cm 3  syringes
• Large basin of tap water
• Cut a length of branch over 1 m, making sure the cut is clean and the end of the branch is not crushed. The branch will be much longer than the xylem vessels inside.
• Attach a 60 cm3 syringe, filled with air, to the proximal (wider) end of the branch using silicon tubing and cable ties as required.
• Pressurise the air in the syringe and branch by compressing the volume of air in the syringe by about half (e.g. from 60 cm3 of air to 30 cm3). This pressure must be maintained through steps 4–6.
• Hold the distal end of the branch under water.
• Use a hand lens to see if a steady stream of bubbles can be detected from the distal end of the branch.
• Progressively cut the distal end of the branch back by about 1 to 5 cm at a time, making sure each time that the end of the branch is not crushed and has a clean cut.
• When a stream of bubbles is observed, the length of the branch gives an approximate maximum length of the xylem vessels.

Safety Note

Students should be warned about the safety precautions necessary when using sharp objects. See also the general safety note .

Follow-up activity

Students could compare maximum xylem vessel lengths in a variety of different plants or different parts (roots, main and side branches) of the same plant. It is common for fast-growing plants to have longer xylem vessels and therefore fewer breaks between xylems. Can the students suggest why this might be?

About what happens

A branch contains several xylem vessels linked together. Between the xylem vessels are perforated wall plates. The fewer of these divisions there are, the lower the resistance and the faster water can travel.

A detailed study of vessel length in  Chrysanthemum stems  ( Nijsse et al, 2001 ) and in a wide range of shrubs and trees ( Jacobsen et al, 2012 ) can be used for cross-reference.

Measuring xylem hydraulic conductivity

Measurements of xylem hydraulic properties show how well plants can supply water to their leaves. It is possible to measure the hydraulic conductance of stems, branches and roots in the classroom with some simple, inexpensive equipment. To measure hydraulic conductivity, the branch length should be longer than the mean length of the xylem vessels (see previous activity).

• Selection of recently cut branches from a tree or shrub investigated in the previous experiment. Ensure that the pieces are longer than the longest xylem vessels measured. If the experiment is to be performed within a few hours of harvesting, keep the plant material in a plastic bag to avoid excessive water loss.
• Sharp secateurs, scissors or a large scalpel
• Chopping board
• Large basin of water
• Reservoir of degassed, distilled water in a container with a tap at the bottom. Degas the water by boiling it or using a vacuum pump for approximately 1h until all the gas has been expelled from the water. Air bubbles in water that is not degassed may block the xylem vessels.
• Hydrochloric acid
• 1 cm 3  pipette (a pipette with a 90o bend is most effective. A standard glass pipette can be bent in a very hot flame)
• 50 cm 3  plastic beaker
• Retort stand and clamp
• Balance (precision of at least 0.01 g)
• Stop watch or stop clock

1. Set up the apparatus as illustrated in the diagram above: 

a  Add hydrochloric acid to the degassed, distilled water to give a final concentration of 0.01 M. For example, add 0.5 cm 3  of 0.1 M HCl to 5 dm 3  degassed, distilled water. Hydrochloric acid prevents the long-term decline in conductance by reducing microbial growth in the xylem.

Remember to always add acid to water, not water to acid.

b  Fill the reservoir with the acidified water. Insert a piece of tubing, sealed at one end with a bung, into the top of the reservoir. The open tubing ensures a constant pressure head because even if the water level drops, the effective height of the reservoir will remain the same.

c  To the tap of the reservoir, add some tubing, fill with water from the reservoir, seal the open end and place into the large basin of water.

d  Close the tap.

e  Submerge the proximal end of the branch in the large basin of water. This is the end of the branch that was nearest to the main stem of the plant.

f  Cut approximately 3 cm off the proximal end of the branch under water to ensure that no air pockets remain in the xylem. Shave off the end of the cut using a sharp blade.

g  Connect the newly cut end of the branch to the water-filled tubing attached to the reservoir under water. If the bark is very rough, it can be stripped back prior to connection. A water-tight seal should be achieved using cable ties or jubilee clips if necessary, however do not over-tighten and compress the xylem vessels.

h  Submerge the other end of the branch in the tub of water.

i  Cut approximately 3 cm off the end of the branch under water to ensure that no air pockets remain in the xylem. Shave off the end of the cut using a sharp blade.

j  Measure and record the length of the branch. Ensure it is longer than the maximum xylem vessel length (see previous experience).

k  Connect the bent pipette to more rubber tubing and sub- merge into the basin of water.

l  Connect the newly cut end of the branch to the water-filled tubing attached to the pipette as above.

m  Fill the 50 cm 3  beaker with water and place on the pan balance.

n  Take the branch end and pipette out of the basin of water with the end of the pipette sealed.

o  Place the end of the pipette in the 50 cm 3  beaker on the balance.

p  Use the retort stand and clamp to hold the pipette in place. The tip of the pipette should not lean on the bottom of the beaker, but should be below the water level. This ensures that as the water drips through the branch, there is a smooth increase in the mass of water in the beaker.

2. Open the tap from the reservoir. 

3. Measure the mass of water every 30s for 3 min. 

4. Measure the effective height of the reservoir using the metre rule. This is the height from the bottom of the open tubing in the reservoir to the proximal end of the branch.

Students should be warned about the safety precautions necessary when using sharp objects and acids. See also the general safety note .

Hydraulic conductivity is measured as the mass of water flowing through the system per unit time per unit pressure gradient (Tyree & Ewers, 1991). The hydraulic conductivity of the branch, kh, is calculated using the following formula:

k h  = (flow rate x branch length)/hydrostatic pressure head

where the flow rate is measured in kilograms per second (kg/s); branch length in metres (m); and the pressure head in megaPascals (MPa). To calculate the flow rate, plot the mass of water (in kg) measured in step 3 against time (in s). The flow rate will be the gradient of the line of best fit (in kg/s). See  table 2  and  figure 1  for a worked example.

The hydrostatic pressure head is found by multiplying the effective height of the reservoir, measured in step 4, with the density of liquid and the acceleration due to gravity. The density of the acidified water can be assumed to be 1000 kg/m 3  (at room temperature) and a value of 9.81 m/s 2  can be used for acceleration due to gravity. Thus, with an effective height of the reservoir of 1m, the hydrostatic pressure head would be 1000 x 9.81 x 1 = 9810 Pa or 0.00981 MPa.

Remember, maximum hydraulic conductivity is only achieved if none of the xylem vessels are embolised (filled with air). To try to prevent this, branches can be flushed with water at a pressure of approximately 200 kPa for 20 min before measuring conductivity. Alternatively, ensure that branches are selected from well-watered trees and that the leaves are covered in a large plastic bag prior to measurement.

Follow-up experiments

Investigations on different levels of water stress on the same, or similar, branches would give an indication of plants that are more vulnerable to cavitation, or air bubbles. Hydraulic conductivity can change de- pending on environmental conditions, and the same species of plant that have adapted to different environments could be tested in the laboratory or in the field. Compare branch cross-sections of different diameter or those supporting different leaf areas.

Students could observe the effect on hydraulic conductivity of changing the branch length and relate this to the height of the plant. They could also investigate the effect on the flow rate of changing the height of the reservoir. The reservoir height (pushing force) could be considered as equal, but opposite, to the pulling force created by the low water potential in xylem vessels.

Did you know?

Xylems are essentially porous filters, and scientists think that they could be used to filter water and make it safe to drink. Earlier this year, a group at the Massachusetts Institute of Technology in the USA showed that a 3cm 3  piece of pine branch could act as a filter and remove 99.9 % of bacteria from water, at a rate of several litres a day. The technique isn’t perfect yet: viruses and chemical contamination can’t be stopped by twigs, but the work by  Boutilier et al. (2014)  suggests a cheap way to purify water in developing countries.

• Boutilier M.S.H., Lee J., Chambers V., Venkatesh V., Karnik R. (2014) Water Filtration Using Plant Xylem.  PLoS ONE   9(2) : e89934
• Burgess S.S., Pittermann J., Dawson T.E. (2006)  Hydraulic efficiency and safety of branch xylem increases with height in Sequoia sempervirens (D. Don) crowns .  Plant, Cell and Environment   29(2) : 229-239. doi: 10.1111/j.1365-3040.2005.01415.x
• Jacobsen A.L., Pratt R.B., Tobin M.F., Hacke U.G., Ewers F.W. (2012)  A global analysis of xylem vessel lengths in woody plants .  American Journal of Botany   99 : 1583-1591 doi: 10.3732/ajb.1200140
• Nijsse J., van der Heijden G.W.A.M., van Leperen W., Keijzer C.J., van Meeteren U. (2001)  Xylem hydraulic conductivity to conduit dimensions along chrysanthemum stems.   Journal of Experimental Botany   52 : 319-327 doi: 10.1093/jexbot/52.355.319
• Ryan M.G., and Yoder B.J. (1997)  Hydraulic limits to tree height and tree growth .  BioScience   47(4) : 235-242 doi: 10.2307/1313077
• Tyree M.T., Ewers F.W. (1991) The hydraulic architecture of trees and other woody plants.  New Phytologist   19 : 345-360
• Koch G.W., Sillett S.C., Jennings G.M., Davis S.D. (2004)  The limits to tree height .  Nature  428 : 851–854. doi: 10.1038/nature02417; freely available

Clare van der Willigen has an MSc and a PhD in plant physiology from the University of Cape Town, South Africa. Following postdoctoral research on water stress in plants and aquaporins, she pursued her passion for teaching. She has worked in South Africa, France, The Netherlands and the United Kingdom, and is currently a senior examiner and teacher of many years’ experience.

The article describes two experiments that can easily be conducted in science classrooms or laboratories to study water movement in plants.

Although the procedures are easy to carry out, the concepts and knowledge that are explored aren’t so simple, but are appropriate for upper secondary-school students (aged 15-18). In my experience, there are not very many procedures that consider water movement for this age group, so many science teachers will welcome this article.

There are also relevant opportunities for interdisciplinary teaching involving mathematics in particular. It would be quite interesting to use this experiment as a starting point to introduce students to the development of a database and subsequent statistical analysis (not too complex). For example, students could estimate maximum xylem vessel lengths and measure xylem hydraulic conductivity of different plants and at different times (e.g. winter vs. summer). This database could be extended from year to year with other students. Such a strategy could help students to understand science as a collaborative activity – not only between different disciplines but also between different ‘generations’ of scientists.

Betina Lopes, Portugal

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