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A redwood draws water from soil to a leaf one hundred metres above its roots — without a pump, without any expenditure of metabolic energy beyond the maintenance of living parenchyma alongside the conducting tissue. The physical mechanism by which this happens is one of the most beautiful applications of physical chemistry to whole-organism biology: the cohesion-tension theory, in which evaporation of water from leaves generates a negative pressure that is transmitted, via a continuous column of hydrogen-bonded water molecules anchored to the walls of dead lignified conducting cells, all the way to the root surface. Loss of water vapour to the atmosphere — transpiration — is the cost of doing business for a terrestrial plant that must keep stomata open for CO₂ uptake; the consequent tension is what powers the ascent of sap and the simultaneous distribution of dissolved mineral ions throughout the plant. This lesson examines the structure of xylem, the three radial pathways for water across the root, the cohesion-tension mechanism for water ascent, the factors that govern transpiration rate, the AQA required practical for measuring transpiration with a potometer, and the adaptations of xerophytes and hydrophytes to extreme water environments.
This lesson maps to AQA 7402 Section 3.3.4 — Mass transport in plants (xylem and water transport) (refer to the official AQA specification document for exact wording). The specification requires that students understand the structure of xylem in flowering plants, the cohesion-tension theory of water transport, the factors affecting transpiration, and the use of a potometer in an investigation of water uptake (required practical). Xerophyte adaptations are an explicit specification topic.
Xylem is the principal water-conducting tissue of vascular plants. It also provides structural support and transports dissolved mineral ions absorbed by the roots. The conducting cells of xylem in flowering plants (angiosperms) are of two types — tracheids and xylem vessels — both of which are dead at functional maturity.
Tracheids are elongated cells (up to a few mm long) with tapered overlapping ends. Their lateral walls are heavily lignified with characteristic pit pairs through which water passes laterally between adjacent tracheids. Tracheids occur in all vascular plants and are the only conducting elements in conifers and ferns. Water passing from one tracheid to the next must cross a thin pit membrane of primary cell wall — a friction-generating arrangement that nevertheless prevents the spread of cavitations (air bubbles) from one cell to a whole vascular bundle.
Vessel elements are an angiosperm innovation. They are wider than tracheids (up to ~500 μm in some climbing vines) and shorter; their end walls are heavily perforated or, in many species, dissolved away entirely (perforation plates). Vessel elements stack end-to-end to form continuous xylem vessels — long open tubes that offer little resistance to flow. A typical vessel is a few centimetres long, but in extreme cases (oak ring-porous vessels) tens of metres. Wider lumen reduces frictional resistance (which scales as the inverse fourth power of the radius — Hagen-Poiseuille framework, paraphrase only) but predisposes to embolism, so vessel diameter is a key evolutionary trade-off.
Both tracheids and vessel elements have lignified secondary cell walls. Lignin is a complex aromatic polymer deposited between cellulose microfibrils after the secondary wall is laid down. Lignification serves two roles: it stiffens the wall against the negative pressures of the transpiration stream (preventing the conduit from collapsing inward like a soft straw), and it provides the compressive strength on which trees rely for upright growth. Lignin patterns vary — annular, spiral, reticulate, scalariform, pitted — and are diagnostic of vessel age and function. The protoplasm of the cell is digested away during xylem maturation; xylem is dead at functional maturity, meaning the conducting column is unobstructed by cytoplasm, nucleus or vacuole and has no metabolic demand of its own.
Lateral water movement between adjacent vessels passes through pits — circular gaps in the secondary wall where only the primary wall remains. Bordered pits in conifers have a thickened central torus that can deflect to seal the pit when one side cavitates, an elegant passive valve mechanism. Pit fields are arranged in characteristic patterns that taxonomists use to identify wood.
Water is absorbed from soil into the root by a combination of bulk flow (when the soil is wet enough) and osmosis (the dominant mechanism in most circumstances).
The epidermis of the root is studded with root hair cells — single-cell extensions ~1 mm long and a few micrometres wide. Root hairs collectively multiply the absorbing surface area by ~30 × in young roots. Their cytoplasm contains a high concentration of dissolved solutes (sucrose, amino acids, mineral ions), giving them a more negative water potential than the surrounding soil solution, so water enters by osmosis down the water potential gradient.
Water entering the root must cross the cortex to reach the central stele, where the xylem lies. AQA recognises three radial pathways.
flowchart LR
SOIL[Soil water<br/>high water potential] --> RH[Root hair cell]
RH -->|apoplastic<br/>through cell walls<br/>fastest| AP[Apoplast pathway]
RH -->|symplastic<br/>through cytoplasm<br/>via plasmodesmata| SY[Symplast pathway]
RH -->|vacuolar<br/>through vacuoles<br/>slowest| VA[Vacuolar pathway]
AP --> EN[Endodermis<br/>Casparian strip blocks apoplast]
SY --> EN
VA --> EN
EN -->|all water forced<br/>symplastic across endodermis| XY[Xylem in stele<br/>low water potential]
The apoplastic pathway is through the cell-wall continuum and intercellular spaces. It is fastest because cell walls are highly porous and pose little resistance. The symplastic pathway is through the cytoplasm of cells linked by plasmodesmata (cytoplasmic strands through holes in adjacent cell walls). The vacuolar pathway is through the vacuoles as well as cytoplasm — the slowest route, requiring crossing of the tonoplast (vacuolar membrane) at each cell.
At the endodermis — the innermost cortical layer surrounding the stele — the radial and transverse walls of each cell are impregnated with suberin, a waxy hydrophobic biopolymer forming a continuous band called the Casparian strip. The Casparian strip is impermeable to water; the apoplastic pathway is therefore blocked at the endodermis. All water — and dissolved solutes — must enter the endodermal cytoplasm and continue symplastically across the endodermis into the stele. This forces water through living cell membranes at exactly this single point, where the plant can exercise selective control over which mineral ions reach the xylem (via membrane-bound transporters at the endodermal plasma membrane). The Casparian strip is therefore the principal mineral-selection valve of the root.
Endodermal cells use ATP-dependent ion pumps to actively load mineral ions (e.g. K⁺, NO₃⁻, PO₄³⁻) into the xylem of the stele. The accumulating ions in the xylem lower its water potential, and water follows by osmosis from the surrounding cells, generating a positive root pressure of up to ~0.1 MPa. Root pressure is observable as guttation — droplets of xylem sap exuded at leaf hydathodes at dawn, particularly visible on grass blades or strawberry leaves on a damp humid morning. Root pressure is sufficient to fill a short herbaceous stem but is wholly inadequate to push sap to the top of a tall tree; cohesion-tension is the dominant mechanism in tall plants. In some tree species, root pressure becomes important for refilling embolised vessels overnight when transpiration is paused.
The cohesion-tension theory — proposed by Henry Dixon and John Joly in 1894 (paraphrase the framework — no verbatim quotation) and supported by abundant subsequent experimental evidence — explains how water reaches the top of a tall tree without metabolic pumping.
Liquid water in the mesophyll air spaces of the leaf evaporates into the gaseous phase. The water lost to the atmosphere is replaced by water drawn from the xylem at the leaf vein endings, by capillary action. This continuous evaporation generates a negative water potential — tension — at the top of the xylem column. The tension can reach −2 MPa or more in actively transpiring leaves of tall trees on a hot dry day.
Water molecules cohere through hydrogen bonds between the partial positive hydrogen and the partial negative lone-pair-bearing oxygen of adjacent molecules (synoptic link to course 1 lesson 0 on water properties). In a confined narrow tube — the xylem vessel lumen — the cohesion is so strong that the water column can withstand the negative pressures generated by transpiration without snapping. Tensile strength experiments have measured xylem water-column tensile strength of >25 MPa under ideal conditions, far exceeding any pressure encountered in physiology. The water column behaves as a continuous unbroken thread.
Water also adheres to the polar functional groups (predominantly hydroxyl groups of cellulose and the polar groups of lignin) on the xylem vessel wall. Adhesion anchors the column to the wall and supports the water against gravity, complementing cohesion. Adhesion is also why the column does not detach from the walls when tension increases.
The tension generated at the leaf is transmitted through the continuous water column all the way to the root. At the root, water enters from the soil by osmosis to replace the water leaving the column at the top. The whole xylem column is therefore under negative pressure (tension) along its length when the plant is transpiring; this is in stark contrast to mammalian blood vessels, which are always under positive pressure. The driving force is at the top, the sink for water is the atmosphere outside the leaf, and the molecular cohesion of water against air-water surface tension at the meniscus in each mesophyll cell wall is the immediate site of force generation.
The greatest weakness of the cohesion-tension mechanism is cavitation: when tension exceeds the cohesive strength of the column (e.g. during severe drought, or when an air bubble nucleates at a defect), the water column snaps and an air bubble — an embolism — fills the vessel. The vessel becomes non-conducting and water flow is diverted around it. The bordered pits with tori in conifers, and the pit membranes between angiosperm vessels, prevent the air bubble from spreading from one vessel to its neighbour: cavitation is localised, not catastrophic. Plants vary substantially in cavitation resistance; species with wider vessels tend to be more vulnerable. Recovery from embolism is an area of active research; some species can refill embolised vessels overnight using root pressure or local osmotic loading by adjacent living cells, but the mechanism remains debated.
Transpiration is the loss of water vapour from the aerial parts of a plant, predominantly through stomata on the leaf surfaces but also (minor contribution) directly through the cuticle and through lenticels on stems. Transpiration is largely an unavoidable consequence of having stomata open for CO₂ uptake (the photosynthesis trade-off, synoptic with Section 3.5.1). A typical maize plant transpires ~200 L of water per growing season, ~99 % of total water uptake; only ~1 % is incorporated into tissue or used metabolically.
The rate at which water vapour leaves the leaf depends on the diffusion gradient between the saturated air inside the mesophyll air spaces and the air outside the leaf, and on the resistance of the stomatal pathway. Four factors dominate.
Soil water availability, root system size, and leaf area also matter at the whole-plant scale.
The AQA specification anchors this lesson with an investigation of water uptake using a potometer. A potometer measures the rate at which a leafy shoot takes up water, which (with care) approximates the transpiration rate.
The leaf surface area must be standardised (count leaves and trace silhouettes onto squared paper, or measure with image software). Light intensity, temperature and humidity must be held constant unless they are the independent variable being investigated. Each shoot has its own xylem anatomy, so a single shoot should be used across conditions and replicates.
Volume of water taken up per unit time = (π × r² × d) / t, where r is capillary internal radius (mm), d is bubble distance (mm), t is time (s). Rate of water uptake per unit leaf area = (volume / time) / leaf area. Compare rates between conditions using a t-test or ANOVA depending on the design.
The potometer measures water uptake, not strictly transpiration. The two differ by a small amount used for turgor maintenance and (negligibly) photosynthesis; for short-term measurements, water uptake ≈ transpiration. Sources of error include: air bubbles introduced at the joints or stomatal pore (use Vaseline at joints; cut underwater); incomplete equilibration before measurement (allow at least 15 min); slow capillary refilling at very high transpiration rates (use a wider-bore capillary). Each error can be controlled by a methodological correction; candidates must be able to identify which controls address which errors.
Plants exposed to extreme water environments show characteristic structural and behavioural adaptations.
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