Plant Structure and Function

This is Unit Eleven of Biology, Part Two: transport, tropisms, and hormones. Water moves down a gradient of water potential, and water potential sums four components — solute, pressure, gravity and matric. Water responds to the total, never to any one of them considered alone. Getting water a hundred metres up a redwood takes no pump. Evaporation at the leaf surface puts the whole water column under tension, and cohesion holds it together as it is pulled. Which means the column has to stay unbroken — one air bubble blocks it. The two transport systems differ in a way worth fixing early. Xylem flow runs only upward. Phloem flow runs toward whichever sink needs sugar, which can be downward, upward, or both directions in one plant at the same time. Then the tropisms, where two light systems get conflated. Phototropins sense blue light and handle direction. Phytochrome senses red and far-red and handles timing — germination and flowering. Separate systems, separate jobs. And the part closes on apical dominance, which looks structural and turns out to be a live ratio between two hormones pulling opposite ways. By the end you will be able to do five things. One: say what pulls water up. Two: say how the two flows differ. Three: tell the two light systems apart. Four: say what the mica barrier showed. Five: say why dominance is a balance.

Three entries. The quantity that governs every movement of water in a plant, the theory explaining how water reaches the top of a tall tree, and the process driving it — which turns out to waste most of what the roots take up.

Four components.

Bio 2541 Water Potential: Solute, Pressure, Gravity, Matric Components

A plant's water potential sums four components — solute, pressure, gravity and matric — and water always moves from higher total potential to lower. Dissolved solute lowers a cell's potential below the surrounding soil's, drawing water in by osmosis. Water responds to the total only, never to one component alone.

Pulled from above.

Bio 2543 Cohesion-Tension Theory of Sap Ascent

Cohesion-tension theory explains sap ascent: evaporation at the leaf creates tension pulling water up through the xylem, held continuous by cohesion between water molecules and adhesion to vessel walls. A redwood lifts water some hundred and sixteen metres this way with no pump expending energy. The column must stay unbroken — an air bubble blocks the flow.

The leak that drives it.

Bio 2545 Transpiration and Stomatal Regulation

Transpiration, water vapour lost at the leaf surface, drives that ascent, and guard cells regulate it by opening and closing stomata — closing under water stress, which cuts loss but also blocks the carbon dioxide photosynthesis needs. Up to ninety percent of the water a root absorbs leaves this way; most never becomes plant tissue.

Two entries. Sugar transport works on pressure rather than tension, and it runs whichever way the demand is — which the walkthrough makes concrete by tracing both systems working simultaneously in one leaf.

Source to sink.

Bio 2547 Translocation: Pressure-Flow in Phloem

Translocation moves sugar from source tissues, where photosynthate forms, to sink tissues where it is used or stored, driven by pressure — sucrose loading at a source lowers phloem water potential, drawing xylem water in and pushing sap toward the sink. Unlike xylem, which flows only upward, phloem flows either direction depending on where the sink is.

Both systems at once.

Bio 2549 Tracing Water and Photosynthate Transport, Worked

A leaf on a sunny afternoon. Water enters the root because root cytoplasm holds lower water potential than the soil, then ascends the xylem under tension created by evaporation at the leaf, held unbroken by cohesion. Meanwhile that same leaf's mesophyll makes sucrose, so it becomes a source: sucrose loads into the phloem, pulls xylem water in, and the resulting pressure pushes sap toward a sink such as a root tip. One leaf, pulling water up and pushing sugar out simultaneously.

Four entries. Growth responses to light, gravity and touch — and the first entry separates two light-sensing systems that are easy to run together because both involve light and both change what a plant does.

Bending to light.

Bio 2551 Phototropism: the Phytochrome and Blue-Light Systems

Phototropism, directional bending toward or away from light, is mediated by blue-light receptors called phototropins — a windowsill seedling bending as its shaded side elongates faster. These are distinct from the red and far-red phytochrome system: one tracks blue light for direction, the other red for timing.

Counting the dark.

Bio 2553 Photoperiodism

Photoperiodism is a plant's use of the phytochrome system to track night length, timing flowering and dormancy to season rather than to temperature alone — a short-day plant flowering once nights pass a critical length, judged from the ratio of active to inactive phytochrome at dawn. Day-neutral plants exist; not every species uses photoperiod at all.

Up and down.

Bio 2555 Gravitropism

Gravitropism orients growth to gravity: shoots grow upward in negative gravitropism, roots downward in positive, both mediated by starch-filled amyloplasts settling within cells — a plant laid on its side redirecting shoot growth within hours. Positive and negative describe direction relative to gravity, not a judgement about usefulness.

Responding to touch.

Bio 2557 Thigmotropism and Thigmonasty

Thigmotropism is directional growth toward touch, as in a coiling tendril; thigmonasty is a touch response independent of the stimulus's direction, as in a flytrap snapping shut whichever side is brushed. Both differ from thigmomorphogenesis, the slower thickening triggered by sustained stress such as wind.

Two entries. The hormone behind both phototropism and gravitropism, and the pair of classic experiments that located its signal and then proved what kind of thing it was — the second by ruling something out.

Elongation.

Bio 2559 Auxin

Auxin drives the cell elongation behind phototropism and gravitropism, controls apical dominance and promotes root formation, moving to the side away from a stimulus — accumulating on a stem's shaded side so those cells elongate faster and the stem bends toward light. Synthetic auxins act as herbicides at high concentration, though natural auxin at ordinary levels drives normal growth.

Two experiments.

Bio 2561 The Darwin and Boysen-Jensen Phototropism Experiments, Worked

The Darwins observed in 1880 that a grass seedling bends toward light while the tip, not the bending region, does the sensing — so something had to travel down. Boysen-Jensen tested it in 1913: cut the tip off, insert permeable gelatin, replace the tip, and the seedling still bends, so the signal crosses a gap. Insert impermeable mica instead and bending stops. That control is what shows the signal is a diffusible chemical rather than a mechanical or electrical connection.

Four entries. Cell division, elongation, ripening and stress response — and each carries a limit worth keeping, from what a gas does in a sealed bag to what actually ends seed dormancy.

Division.

Bio 2563 Cytokinins

Cytokinins promote cell division, delay leaf senescence, and counter auxin's suppression of axillary buds, working most strongly in actively dividing tissue such as roots and embryos — applying cytokinin produces bushier branching. Apical dominance results from a balance between auxin and cytokinin, not from either acting alone.

Elongation and dormancy.

Bio 2565 Gibberellins

Gibberellins stimulate shoot elongation, break seed dormancy in species needing cold or light, and promote fruit maturation — treating seedless grapes with gibberellin increases size and loosens clustering, since absent seeds would normally supply it. Ethylene acts independently, triggering ripening rather than further elongation.

A gas.

Bio 2567 Ethylene

Ethylene, a volatile gas, triggers fruit ripening, leaf and flower abscission and senescence, released more heavily from aging or wounded tissue — sealing unripe avocados in a bag traps what one ripening fruit gives off and speeds the rest. That works precisely because it is a gas: sealing concentrates it where ventilation would dilute it.

Under stress.

Bio 2569 Abscisic Acid

Abscisic acid accumulates under stress such as drought or cold, inducing seed and bud dormancy and closing stomata to conserve water — low soil moisture raising its level and cutting transpiration loss. What releases a seed from dormancy is the acid's breakdown over winter rather than any fixed internal timer.

One capstone taking something that looks like a fixed feature of a plant's architecture and showing it is a ratio actively maintained — with an experiment that proves it by restoring the effect without the structure.

The capstone. Two hormones, opposed.

Bio 2571 Tracing Apical Dominance Through Auxin-Cytokinin Balance, Worked Capstone

An intact conifer holds its conical shape because the apical meristem continuously makes auxin, which travels down and suppresses axillary buds, while cytokinin made in root tips travels up and counters it. Auxin dominates near the tip, so dominance holds. Remove the apical bud and the auxin source goes with it: buds near the cut branch out, which is what pruning exploits. But apply synthetic auxin to the cut stem and suppression returns without any bud present — proving the signal, not the structure, maintains dominance.

Five things to carry out of Part Two. Water rises by tension, not pressure. Evaporation at the leaf surface pulls the whole column upward, and cohesion between water molecules plus adhesion to the vessel walls holds that column together. No organ expends energy on the lift. What stops it is a break — an air bubble in the column blocks the flow entirely. The two flows differ in direction. Xylem runs only upward. Phloem runs from source to sink, which means it can run downward to a root, upward to a growing fruit, or both at once in the same plant, depending on where sugar is being demanded. Two light systems, two jobs. Phototropins sense blue light and control direction — the bend toward a window. Phytochrome senses red and far-red and controls timing — germination, and flowering keyed to night length. Conflating them makes both harder to reason about. The mica barrier showed the phototropism signal is a diffusible chemical. Permeable gelatin between tip and base still allowed bending; impermeable mica stopped it. That contrast rules out a mechanical or electrical connection, which the gelatin result alone could not. And apical dominance is a balance, not a lock. Auxin from the tip travels down and suppresses; cytokinin from the roots travels up and releases. Removing the bud removes the auxin source and lateral buds grow — but painting synthetic auxin onto the cut restores suppression with no bud there at all. Part Three turns to plant nutrition and soil.