Chapter 7: Chemical Coordination In Plants
In the summer of 1880, in a house in Kent, an elderly man and his son sat watching grass seedlings by lamplight. Charles Darwin, then in his seventies, had already spent years on barnacles, orchids and earthworms. With his son Francis he had turned to a question that sounds almost too simple to ask: why does a young shoot bend towards a window?
They sowed canary grass, Phalaris canariensis, and let the seedlings come up in darkness. Then they placed a single small lamp to one side. Within hours the pale shoots had curved over towards it. The bending, they noticed, did not happen at the very tip; it happened in the region a little below the tip.
That was the puzzle. On some seedlings the Darwins slipped a tiny cap over the tip, one that no light could pass through, leaving the rest of the shoot bare. Those seedlings stood straight. On others they shaded the lower part instead and left the tip uncovered. Those bent as before. Fig. 7.0-1 sets the three cases side by side.
So the part of the seedling that received the light was not the part that moved. Something, the Darwins concluded, must pass down from the tip to the shoot below and make one side of it grow longer than the other. What that something was, they could not say. Naming it would take another fifty years of patient work by other people.
Diagram
Fig. 7.0-1 — The Darwins’ Test On Grass Seedlings Line drawing of three young canary grass seedlings standing in soil in a row, with a single beam of light arriving from the left shown as an arrow; seedling A uncovered and curving towards the light, seedling B with a small opaque cap fitted over its tip and standing straight, seedling C with an opaque sleeve around its lower part and its tip left uncovered, curving towards the light; label direction of light, shoot tip, opaque cap, opaque sleeve, region of bending, and mark the three seedlings A, B and C.
7.1 Movement And Response In Plants
A plant spends its whole life in one place. Yet it is not still. Over a few days a shoot leans towards a window, a root turns aside to reach damp soil, and a tendril winds itself around a stick.
These are movements, but movements of a special kind. To see what makes them special, it helps to set a plant beside an animal.
7.1.1 How Plant Movement Differs From Animal Movement
An animal moves its whole body from place to place, and moves parts of its body as well. A plant, anchored in the soil by its roots, does neither. Its movement shows up only as the bending, twisting or elongation of certain parts — a stem, a root, a tendril, a flower stalk.
The difference goes back to how the two feed themselves. A green plant makes its own food from carbon dioxide, water and sunlight, and all three come to it where it stands. An animal cannot make food. It must go in search of food, escape from being eaten, and find a mate. Bodily movement is essential to an animal and unnecessary to a plant.
Why Plant Movements Are Slow
A plant part moves by growing. When the cells along one side of a shoot take in water and stretch their walls, that side becomes longer than the other, and the shoot curves. Nothing is pulled and nothing contracts; a cellulose wall is stretched, cell by cell, over hours and days.
This has two consequences. Plant movements are slow, and they are usually permanent — a stem that has bent towards a window stays bent even after the window is shuttered. Animal movements, worked by muscles, are quick and can be undone in a moment. Table 7.1 sets the two kinds of movement side by side.
7.1.2 Stimulus And Response
Something must set a movement off. A stimulus is any change in the surroundings of an organism, or inside it, that the organism can detect. The action the organism then takes is its response.
A plant is sensitive to a wide range of stimuli. It responds to light — to the direction light comes from, to how bright it is, and to how many hours a day it falls. It also responds to water, to gravity, to touch, to chemicals, to temperature and to the turn of the seasons. Fig. 7.1-1 gathers some of these around a single plant.
Diagram
Fig. 7.1-1 — Stimuli To Which A Plant Responds Line drawing of a young plant growing in soil, seen from the side, with four labelled arrows marking the stimuli acting on it: a beam of light arriving from one side, a downward arrow marked force of gravity, a patch of moist soil to one side of the root system, and a thin stick touched by a coiling tendril; label shoot, root, tendril, direction of light, force of gravity, moist soil and support.
Positive And Negative Responses
A response may be made towards the stimulus or away from it. A movement towards the stimulus is called a positive response, and a movement away from it a negative response.
Different parts of the same plant may answer the same stimulus in opposite ways. A shoot grows towards light while the root of that very plant grows away from light. The two organs have different jobs to do, so the same signal is useful to them in opposite directions.
7.1.3 Coordination By Chemical Control
An animal detects a stimulus with its sense organs and carries the message along nerves to the part that must act. A plant has no sense organs, no nerves, no brain and no muscles. Yet the tip of a shoot senses the light while a region lower down does the bending, so a message of some kind must pass from the one to the other.
That message is carried by chemicals. Certain cells make a chemical substance, the substance travels through the plant, and wherever it arrives it alters how fast the cells there grow. Control of the body by chemical messengers of this kind is called chemical control.
What it achieves is coordination — the different parts of an organism working together, in the right amount and at the right time, so that the organism as a whole responds properly. In a plant, coordination is entirely chemical.
Chemical control is slow compared with a nerve impulse, and a plant can afford that. It does not need to answer a small change in its surroundings within a second. What it does need is to answer light, gravity and the seasons over days and weeks, and to keep the growth of its own parts in step with one another. FC 7.1 sets out the path from a stimulus to the response it produces.
Flowchart
FC 7.1 — From Stimulus To Response In A Plant Stimulus (light, gravity, water, touch or a chemical) → stimulus perceived by a particular region of the plant, often a growing tip → chemical messenger made there → messenger transported to another region → cells of that region grow faster or slower than the cells opposite them → that part bends, elongates or coils (response)
The chemicals that carry out this control are the plant hormones. The next section takes them one at a time.
7.2 Plant Hormones
A plant hormone is a chemical substance made in one part of a plant, carried to another part, and active there in extremely small amounts. Because almost everything they regulate has to do with growth, they are also called plant growth regulators.
Such a chemical is called a phytohormone (Greek phyton, plant) when we wish to keep it apart from the hormones of animals. The two are alike in the way they work — made in one place, effective in another — but they are different substances.
The main way in which a plant answers a hormone is by growing. Growth may be speeded up or slowed down, and the same hormone may speed it up in one organ and slow it down in another. When one side of a shoot is made to grow more than the other, the shoot bends; that is how a chemical message turns into a movement.
Growth begins in the meristems — regions of small, actively dividing cells that lie just behind the tip of every shoot and every root, and along the sides of stems. Meristematic cells are sensitive to hormones. A hormone can loosen their cellulose walls so that the cells stretch and go on dividing.
The word hormone came into biology from animal studies. In 1902 William Bayliss and Ernest Starling, working in London, found a chemical made by the lining of the small intestine that travelled in the blood and set the pancreas working. In 1905 Starling gave messengers of this kind the name hormone, from a Greek word meaning to set in motion.
Five plant hormones are dealt with here: auxins, gibberellins, cytokinins, ethylene and abscisic acid.
7.2.1 Auxins
The question left open at the start of this chapter — what passes down from the tip of a shoot to the region that bends? — was settled nearly fifty years later, by an experiment simple enough to picture at once. It is set out in Fig. 7.2-1.
Diagram
Fig. 7.2-1 — Went’s Agar Block Experiment Three-stage line drawing of an oat seedling in a row of panels: panel 1, the tip of the coleoptile being cut off and stood on a small block of agar; panel 2, the agar block placed on one side of the cut end of a tipless seedling, kept in darkness; panel 3, the same seedling some hours later, curved away from the side bearing the block; include a fourth small panel showing a plain agar block on a tipless seedling that has stayed straight. Label cut tip, agar block, plain agar block, tipless seedling, side that elongates and direction of curving.
The substance came to be called auxin, from the Greek auxein, meaning to grow. It was the first plant hormone to be identified.
Auxins are powerful growth stimulants and are effective at extremely low concentrations. They are found throughout the higher plants and in lower plants such as algae and fungi as well. In a higher plant the largest amounts occur in the actively growing regions — the shoot apex, the root apex and the lateral meristems. The chief auxin made naturally by plants is indole-3-acetic acid (IAA).
Functions Of Auxins
- Auxins promote the growth of stems, roots and fruits by cell elongation. The hormone loosens the cell wall so that it relaxes and can be stretched, rather as rubber stretches when pulled; the cell then takes in water and lengthens. The rate of elongation is proportional to the concentration of auxin, and a stem needs a far higher concentration than a root does.
- Auxins delay senescence — the ageing of a plant part that ends in its yellowing and falling. Leaves supplied with auxin stay green and attached for longer.
- Auxins promote the growth of the apical bud, the bud at the very tip of a shoot, and hold back the growth of the lateral buds lower down. This suppression is called apical dominance.
- Auxins make cuttings form roots. A cut stem of rose or bougainvillea, treated with auxin, produces roots where none would otherwise appear.
- Auxins can make a fruit form without fertilisation. The development of fruit without fertilisation is called parthenocarpy, and fruits formed in this way are parthenocarpic fruits. Since no fertilisation has taken place, no seed is formed, so such fruits are seedless. Apples, tomatoes and bananas can all be produced this way.
Apical Dominance
Auxin is made at the shoot tip and moves downwards through the stem. As long as the tip is there, the buds below it receive enough auxin to be held dormant, and the plant grows tall and unbranched. FC 7.2 follows what happens when the tip is cut away.
Flowchart
FC 7.2 — Apical Dominance And What Removing The Tip Does Apical bud intact → auxin made at the shoot tip → auxin moves down the stem → lateral buds kept dormant → plant grows tall with few branches → apical bud cut off → supply of auxin from the tip stops → lateral buds begin to grow → plant becomes short and bushy
Diagram
Fig. 7.2-2 — Apical Dominance Two line drawings of the same kind of plant side by side. On the left, a plant with its apical bud intact, growing tall and unbranched, with the dormant lateral buds marked along the stem and a downward arrow on the stem labelled movement of auxin from the tip. On the right, the same plant with the apical bud removed, showing several lateral buds grown out into branches. Label apical bud, dormant lateral bud, growing lateral branch and cut end.
Gardeners use this every day. Pinching out the growing tip of a basil plant, or clipping the top of a hedge, removes the source of auxin and forces the side buds into growth, so the plant thickens instead of lengthening.
7.2.2 Gibberellins
Gibberellins are a second kind of plant hormone. They take their name from a fungus. Rice seedlings attacked by Gibberella fujikuroi grow abnormally tall and spindly and topple over — a disease Japanese farmers called the foolish seedling disease. In 1926 Eiichi Kurosawa showed that it was not the fungus itself but a substance released by it that caused the overgrowth, and that substance was named gibberellin.
Many forms of gibberellin are known, reported from different plants and named GA₁, GA₂, GA₃ and so on. Gibberellic acid, GA₃, is the most studied of them. In higher plants gibberellins occur mainly in meristematic regions — the stem apex, the root apex, buds and seeds.
Functions Of Gibberellins
- The main function of gibberellins is to promote the growth of internodes by cell elongation. An internode is the length of stem between two nodes, the points at which leaves are attached, so lengthening the internodes is what makes a stem tall.
- Gibberellins break seed dormancy — the resting state in which a seed stays alive but does not grow — and start germination.
- They promote the growth of fruits and can induce parthenocarpy.
- They delay senescence.
- They are widely used in horticulture and in the food industries. They lengthen the internodes of dwarf plants, and are applied commercially to increase the length of grapes and to elongate apples and improve their shape. Gibberellic acid is also used to speed up malting, the controlled sprouting of barley grain used in brewing.
7.2.3 Cytokinins
The cytokinins were discovered comparatively recently, in the 1950s, by Folke Skoog and Carlos Miller, who were trying to find what makes plant tissue kept in culture go on dividing. Their effect is a specific one: they act on cell division.
Cytokinins are widely distributed in plants. They are produced in the root tips and are carried upwards through the xylem, the tissue that conducts water from root to shoot. Comparatively large amounts are found in germinating seeds, developing fruits and the embryo.
Functions Of Cytokinins
- Cytokinins stimulate growth by cell division, whereas auxins and gibberellins stimulate growth by cell elongation. They can make cells divide even in tissues that are not meristematic.
- In seeds they cause the cotyledons, the seed leaves that store or absorb food for the young plant, to expand.
- They break seed dormancy and promote germination.
- They promote the making of chlorophyll in the chloroplasts and delay leaf senescence, so a leaf supplied with cytokinin stays green.
- They inhibit apical dominance, acting against auxins on the buds along a stem.
7.2.4 Ethylene
Ethylene is the only plant hormone that is a gas at ordinary temperature. It is produced in ripening fruits and acts in the very tissue that makes it, so unlike the other hormones its site of production and its site of action are not different. Being a gas, however, it also escapes into the air around the fruit and reaches whatever is stored nearby.
Ethylene is produced by higher plants and by fungi. Every living plant cell can make it, but meristematic tissues make the most.
Functions Of Ethylene
Ethylene causes two general changes in a plant — a reduction in stem elongation and an acceleration of senescence. Besides these, it helps in:
- the ripening of fruits;
- initiating germination in peanut seeds;
- the sprouting of the potato tuber;
- promoting root growth and the formation of root hairs;
- inducing flowering in mango.
Ethylene is, for these reasons, the most widely used plant growth hormone in agriculture.
7.2.5 Abscisic Acid
Abscisic acid, usually written ABA, is a growth-retarding hormone. It is found in angiosperms, gymnosperms, pteridophytes and some mosses. Within a leaf it occurs in the chloroplasts, and the highest amounts in the whole plant are found in fruits and seeds.
Functions Of ABA
- Abscisic acid acts as a general growth inhibitor, slowing down the plant’s metabolism.
- It inhibits the germination and development of seeds. By inducing dormancy it allows a seed to withstand desiccation, that is extreme dryness, and other conditions unfavourable for growth. In this it works against the gibberellins.
- It accelerates senescence and abscission — the falling away of leaves, buds, flowers and fruits from the plant.
- It brings about the closing of the stomata in the epidermis and raises the plant’s tolerance of stresses of various kinds. For this reason it is called the stress hormone.
The fourth function is worth pausing on. Stomata are the tiny pores in the leaf surface through which gases enter and leave, and through which water vapour is lost. When the soil dries, abscisic acid accumulates and the pores shut, and the plant stops losing water it cannot replace.
Seeds show the same logic from the other side. A seed that germinated during a dry spell would die as a seedling, so dormancy is not idleness but a way of waiting. FC 7.3 shows how the two hormones between them decide when the waiting ends.
Flowchart
FC 7.3 — How A Seed Is Held Dormant And Then Released Seed ripens on the parent plant → abscisic acid accumulates in the seed → seed becomes dormant and survives drying → seed lies in the soil, not germinating → water and warmth arrive → level of abscisic acid falls and gibberellins increase → dormancy is broken → germination begins → seedling grows
7.2.6 Plant Hormones At A Glance
The five hormones can now be set beside one another. Table 7.2 gives where each is made and what it chiefly does, and Fig. 7.2-3 shows those sites on a plant.
Diagram
Fig. 7.2-3 — Where The Hormones Are Made In A Plant Outline line drawing of a flowering plant with root, stem, leaves, flower and one green fruit, with lead lines from five regions to labels: shoot apex and young leaves — auxins; apical buds and root meristems — gibberellins; root tips — cytokinins; ripening fruit and yellowing leaf — ethylene; green fruit and seeds — abscisic acid. Keep all lead lines straight and non-crossing.
No hormone in this table works alone. Every growing region receives several of them at once, and what the region does depends on the balance between them rather than on any one. Auxin imposes apical dominance and cytokinin lifts it; gibberellin breaks dormancy and abscisic acid imposes it; auxin holds a leaf on the plant and ethylene brings it down.
That is why the same organ can be made to behave in opposite ways at different times of year, with no change in the hormones available to it — only in their proportions.
7.3 Tropic Movements In Plants
When a seed germinates in the soil, the root and the shoot both begin to grow, and each must grow in the right direction if the seedling is to survive. The root must go downwards into the soil, which anchors the plant and supplies it with water and minerals. The shoot must go upwards towards the light, so that its first leaves can photosynthesise and feed the growing plant.
A seed has no way of knowing which way up it came to rest. The direction has to be worked out after germination, from the surroundings, and the plant works it out by growing. In movements of this kind the direction of the response is decided by the direction from which the stimulus arrives.
Such a response is called a tropism, and the growth movements themselves are called tropic movements. The word comes from the Greek tropos, meaning to turn.
Each tropic movement is named after the stimulus that causes it. Table 7.3 names the five that follow, along with the stimulus each answers and the way the two main organs of the plant respond.
7.3.1 Phototropism
Phototropism is the growth movement of a plant part in response to light. A plant lit evenly from all sides grows more or less straight upwards, because no side of it receives more light than another.
Now let the light be brighter on one side than on the other — unilateral light, that is, light arriving from one side only. The shoot bends towards the brighter side. The roots, if they are exposed to the light, grow away from it.
Shoots are therefore said to be positively phototropic and roots negatively phototropic. Fig. 7.3-1 shows the response as it can be produced in any classroom.
Diagram
Fig. 7.3-1 — Demonstrating Phototropism Line drawing of a healthy potted plant standing inside a darkened room, close to an open window through which a beam of daylight enters from one side; the shoot of the plant is curved towards the window. Label direction of light, darkened room, open window, and the curved shoot.
To see the demonstration for yourself, keep a healthy potted plant in a dark room, by the side of an open window. After a few days the plant will be found bending towards the window — a positive response to light.
The advantage to the plant is easy to state. A shoot that leans towards the brighter side spreads its leaves where the light is strongest, and the more light the leaves receive, the more photosynthesis they can carry out. A root, which does no photosynthesis and belongs in the dark soil, gains nothing from light and grows away from it.
How Auxin Bends The Shoot
Auxins carry out this bending. Auxin made at the shoot tip does not stay evenly spread when the light is one-sided; it collects in the part of the shoot that is turned away from the light.
The cells of that shaded side, receiving more auxin, elongate faster than the cells on the lit side. One side of the shoot thus becomes longer than the other, and the shoot curves over towards the light. FC 7.4 sets out the sequence, and Fig. 7.3-2 shows what is happening inside the shoot.
Flowchart
FC 7.4 — How A Shoot Bends Towards Light Light falls on one side of the shoot → auxin made at the shoot tip → auxin collects on the shaded side → cells of the shaded side elongate more than those of the lit side → shaded side becomes longer → shoot curves towards the light
Diagram
Fig. 7.3-2 — Distribution Of Auxin In A Shoot Tip Two line drawings of the tip of a shoot placed side by side, each cut lengthwise to show the cells. In the first, the shoot is lit evenly from above, auxin is shown as evenly scattered dots on both sides, the cells on the two sides are of equal length and the shoot is straight. In the second, light arrives from the left as an arrow, the dots are crowded on the right (shaded) side, the cells of that side are drawn longer, and the shoot curves to the left. Label direction of light, shoot tip, auxin, shaded side, lit side, elongated cells.
7.3.2 Geotropism
Geotropism is the growth movement of a plant part in response to gravity. It is also called gravitropism. An organ that grows towards the pull of gravity is positively geotropic, and one that grows away from it is negatively geotropic.
The root tip is positively geotropic and the shoot tip is negatively geotropic. This is exactly what the seedling needs: roots down in the soil for anchorage and water, shoots up in the air for light.
The response is easy to watch. Sow a few seeds of bean or gram in moist soil in a glass beaker, keep them watered, and follow them for eight or nine days. Each seedling’s stem lengthens away from the pull of gravity while its root lengthens towards it, and the glass lets you see both at once.
A plant that has been knocked over shows the same thing more sharply. Fig. 7.3-3 shows a potted plant lying on its side: within a day or two the stem has curved upwards again and the roots have turned downwards, although the plant as a whole has not been moved.
Diagram
Fig. 7.3-3 — Geotropism In A Fallen Plant Two line drawings side by side. The first shows an upright potted plant with its stem vertical and its root system spreading downwards in the soil, labelled before the pot falls over. The second shows the same pot lying on its side, with the root system now curving downwards towards the ground and the stem curving upwards away from it. Label the stem grows upwards, away from gravity; the roots grow downwards, towards gravity.
The Clinostat
The response to gravity can also be tested in the laboratory with a clinostat, an instrument that holds a potted plant and rotates it slowly. Take two clinostats and fix a potted plant horizontally on each. Rotate one and leave the other stationary.
On the stationary clinostat the shoot bends upwards and the root bends downwards, showing negative and positive geotropism as usual. The plant on the rotating clinostat does not bend at all.
The reason lies in the meaning of a tropic movement. On the rotating clinostat every side of the plant is turned towards the pull of gravity in turn, so all parts are exposed equally and no side receives more of the stimulus than another. A stimulus that arrives from every direction gives the plant no direction to answer, and no bending occurs. Fig. 7.3-4 shows the two set-ups together.
Diagram
Fig. 7.3-4 — Demonstration Of Geotropism With A Clinostat Two line drawings, labelled A and B, each showing a potted plant clamped horizontally to the axle of a clinostat. In A the clinostat is stationary and the shoot of the plant is curved upwards while the root is curved downwards. In B the clinostat is rotating, shown by a circular arrow on the axle, and the shoot and root of the plant remain straight along the horizontal axis. Label clinostat stationary, clinostat rotating, direction of rotation, shoot and root.
7.3.3 Hydrotropism
Hydrotropism is the growth movement of a plant part in response to water or moisture. A part that grows towards the source of moisture is positively hydrotropic, and a part that grows away from it is negatively hydrotropic.
Roots are positively hydrotropic and shoots are negatively hydrotropic. Growing towards moisture keeps a root within reach of the water in the soil, which the whole plant depends on.
An Experiment On Roots And Water
Soil holds water unevenly, so a root often has gravity pulling it one way and moisture lying another. Which stimulus does it obey? The arrangement in Fig. 7.3-5 answers the question.
A piece of wire netting or gauze is suspended by wires. Moist sawdust, about an inch deep, is spread on the netting, and germinating bean seeds are embedded in the sawdust. The sawdust is the only source of water in the whole apparatus.
At first the radicles, the young roots, grow straight downwards through the holes of the netting, under the influence of gravity. Soon afterwards they turn and grow upwards again, back towards the moist sawdust — that is, against the force of gravity. The plumules, the young shoots, grow upwards throughout.
So for a root, water is the more effective stimulus of the two. When gravity and moisture disagree, the root follows the moisture.
Diagram
Fig. 7.3-5 — Investigating The Effect Of Water On Roots And Shoots Two line drawings labelled start of experiment and end of experiment. Each shows a flat piece of wire netting suspended horizontally by supporting wires from above, carrying a layer of moist sawdust in which germinating bean seeds are embedded. In the first, the radicles have grown straight downwards through the netting into the air below and the plumules grow upwards. In the second, the radicles have curved back upwards towards the moist sawdust while the plumules still grow upwards. Label supporting wires, wire netting, moist sawdust, germinating bean seeds, radicles growing downwards, radicles growing upwards towards the source of water, and plumules.
7.3.4 Thigmotropism
Thigmotropism is the growth movement of a plant part in response to touch, that is, to contact with a solid object. It is the way a weak-stemmed plant climbs.
Plants such as the sweet pea and many vines bear tendrils — slender, thread-like structures that coil around any support they meet. Sweet peas growing in a field will be found gripping whatever stick or stem stands near them.
A tendril responds to being touched on one side only. The tip of the tendril perceives the contact, and the stimulus is then passed on to the parts below it, so that the whole tendril becomes sensitive. The tendril then twines around the support, and the plant is held up. FC 7.5 follows the sequence.
Flowchart
FC 7.5 — Coiling Of A Tendril Tendril touches a support on one side → tendril tip perceives the contact → stimulus transmitted to the parts below the tip → cells on the side away from the support elongate more than those touching it → tendril curves around the support → tendril coils and grips, holding the weak stem up
Tendrils are of more than one kind, as Fig. 7.3-6 shows. A stem tendril is a modified branch, while a leaf tendril is a modified leaf or part of a leaf; both do the same job of climbing.
Diagram
Fig. 7.3-6 — Tendrils Of Weak-Stemmed Plants Two line drawings side by side, each showing a weak-stemmed plant climbing a bamboo stick. In the first, a coiled stem tendril arising from the stem is wound around the stick; in the second, a coiled leaf tendril arising from the tip of a leaf is wound around the stick. Label stem tendril, leaf tendril, leaf, weak stem and bamboo stick for support.
Not every climber uses tendrils. Cuscuta, the dodder, has none; its whole slender stem winds around the plant it climbs, coiling in response to contact just as a tendril does.
7.3.5 Chemotropism
Chemotropism is the growth of a plant organ in response to a chemical substance. The organ grows towards the chemical, or away from it, according to what the chemical is.
The best-known example occurs in reproduction. When a pollen grain lands on the stigma of a flower, it puts out a pollen tube, which must travel down the style and reach the ovule inside the ovary. Chemicals secreted by the female tissues — sugars and peptones among them — mark the path, and the pollen tube grows towards them. The pollen tubes of angiosperms and gymnosperms both behave in this way. Fig. 7.3-7 shows the course the tube takes.
Diagram
Fig. 7.3-7 — Chemotropism Of A Pollen Tube Longitudinal section of the pistil of a flower drawn as a line diagram, with a pollen grain resting on the stigma at the top and a pollen tube growing downwards from it through the style and into the ovary, ending at the ovule. Label pollen grain, stigma, style, pollen tube, ovary and ovule, with straight non-crossing lead lines.
Chemotropism is not confined to flowering plants. The hyphae of fungi grow towards regions of their food that are richer in nutrients, and the tentacles on the leaf of the sundew, Drosera, bend towards a source of nutrition caught on the leaf surface.
7.3.6 Heliotropism
Heliotropism is the movement of a plant part that follows the sun as it travels across the sky. The young flower heads of the sunflower show it well: through the day they turn slowly from east to west, keeping their faces to the sun.
The turning is brought about by auxins, which are sensitive to sunlight. Auxin moves out of the sunlit side of the stem below the head and gathers on the shaded side. The cells there elongate faster, that side lengthens, and the head is tipped over towards the sun. Once the flower is fully open the head stops travelling and settles facing east.
7.4 Summary
A plant stays in one place all its life. Its movements are the bending, twisting or elongation of certain parts — a stem, a root, a tendril or a flower stalk.
A plant part moves by growing. The cells along one side stretch their walls and become longer than those opposite, so the part curves. Plant movements are therefore slow and usually permanent, while animal movements, worked by muscles, are quick and reversible.
The difference goes back to nutrition. A plant makes its own food where it stands; an animal must search for food, escape and find a mate, so bodily movement is essential to it.
A stimulus is any change in the external or internal environment of an organism that the organism can detect. A response is the action or movement produced as a result.
A response may be positive, towards the stimulus, or negative, away from it. Different parts of one plant may respond to the same stimulus in opposite ways.
Plants respond to light, water, gravity, touch, chemicals, temperature and the change of seasons.
Plants have no nerves, no sense organs and no muscles. Their sensitivity and coordination are the result of chemical control — chemicals made in one part travel to another and alter the growth of the cells there.
A plant hormone, or phytohormone, is an organic substance produced in small amounts in one part of a plant and transported to another, where it regulates growth or some other activity. Plant hormones are also called plant growth regulators.
Growth begins in the meristems, the regions of actively dividing cells behind the tips of shoots and roots and along the sides of stems. Meristematic cells are sensitive to hormones.
The five plant hormones studied are auxins, gibberellins, cytokinins, ethylene and abscisic acid.
Auxins are powerful growth stimulants, effective at extremely low concentrations. They occur in the actively growing regions of a plant, and indole-3-acetic acid (IAA) is the main natural auxin.
Auxins promote the growth of stems, roots and fruits by cell elongation, delay senescence, cause apical dominance, make cuttings form roots, and can induce parthenocarpy.
Apical dominance is the suppression of the growth of the lateral buds by the apical bud. Removing the apical bud releases the lateral buds, and the plant becomes bushy.
Parthenocarpy is the development of a fruit without fertilisation. Such fruits are seedless.
Gibberellins occur in the meristematic regions of the plant; many forms are known, of which gibberellic acid, GA₃, is the most studied. They promote the elongation of internodes, break the dormancy of seeds, promote fruit growth, induce parthenocarpy and delay senescence.
Gibberellins are widely used in horticulture and the food industries — to lengthen the internodes of dwarf plants, to increase the length of grapes, to elongate apples, and to speed up malting.
Cytokinins are produced in the root tips and carried upwards through the xylem. They stimulate cell division, expand the cotyledons of seeds, break seed dormancy, promote chlorophyll formation, delay leaf senescence and inhibit apical dominance.
Auxins and gibberellins bring about growth by cell elongation; cytokinins bring it about by cell division.
Ethylene is the only plant hormone that is a gas at ordinary temperature. It is made by all living plant cells, and acts in the tissue that makes it.
Ethylene reduces stem elongation and accelerates senescence. It also ripens fruits, starts germination in peanut seeds, sprouts the potato tuber, promotes root growth and root-hair formation, and induces flowering in mango. It is the plant hormone most widely used in agriculture.
Abscisic acid is a growth-retarding hormone found in angiosperms, gymnosperms, pteridophytes and some mosses. It slows down metabolism, induces seed dormancy, accelerates senescence and abscission, and closes the stomata. Because it raises the plant’s tolerance of stress, it is called the stress hormone.
Gibberellins and abscisic acid act against each other over dormancy, and auxins and cytokinins act against each other over apical dominance. What a growing region does depends on the balance between the hormones reaching it.
A tropic movement is a growth movement of a plant part in response to a stimulus coming from one direction, the direction of the movement being decided by the direction of the stimulus. The phenomenon is called tropism.
Phototropism is the response to light. Shoots are positively phototropic and roots negatively phototropic. Auxin from the shoot tip collects on the shaded side, the cells there elongate more, and the shoot curves towards the light.
Geotropism, also called gravitropism, is the response to gravity. Roots are positively geotropic and shoots negatively geotropic.
A clinostat is an instrument that rotates a potted plant slowly. A plant on a stationary clinostat shows the usual geotropic bending; one on a rotating clinostat does not bend, because every side of it is exposed equally to gravity and the stimulus has no single direction.
Hydrotropism is the response to water or moisture. Roots are positively hydrotropic and shoots negatively hydrotropic. When gravity and moisture pull a root in different directions, the root follows the moisture.
Thigmotropism is the response to touch. The tendrils of weak-stemmed plants such as the sweet pea perceive contact at the tip, pass the stimulus to the parts below, and coil around the support.
Chemotropism is the growth of a plant organ in response to a chemical. The pollen tube grows down the style towards chemicals secreted by the female tissues, and fungal hyphae grow towards richer food.
Heliotropism is the movement in which a part follows the sun across the sky, as the young flower heads of the sunflower do.
Flowchart
FC 7.6 — The Chapter In One Chain External stimulus arriving from one direction (light, gravity, water, touch or a chemical) → perceived by a growing region of the plant → plant hormone made and transported to the responding region → cells on one side elongate more than those on the other → that part bends: a tropic movement → named after the stimulus: light → phototropism; gravity → geotropism; water → hydrotropism; touch → thigmotropism; chemical → chemotropism
7.5 Key Terms
- Abscisic acid (ABA) — a growth-retarding plant hormone that induces dormancy, hastens senescence and abscission, and closes the stomata.
- Abscission — the falling away of a leaf, bud, flower or fruit from the plant.
- Apical dominance — the suppression of the growth of the lateral buds by the apical bud at the tip of the shoot.
- Auxin — the first plant hormone to be identified; it promotes growth chiefly by making cells elongate.
- Chemical control — control of the activities of a body by chemical messengers instead of by nerves.
- Chemotropism — the growth of a plant organ in response to a chemical substance.
- Clinostat — an instrument that rotates a potted plant slowly, so that no one side of it stays turned towards gravity.
- Coordination — the working together of the parts of an organism, in the right amount and at the right time.
- Cytokinin — a plant hormone, made in the root tips, that stimulates growth by cell division.
- Dormancy — the resting state in which a seed or bud stays alive but does not grow.
- Ethylene — the only plant hormone that is a gas at ordinary temperature; it ripens fruit and hastens senescence.
- Geotropism — the growth movement of a plant part in response to gravity.
- Gibberellin — a plant hormone that lengthens internodes and breaks the dormancy of seeds and buds.
- Gravitropism — another name for geotropism.
- Heliotropism — the movement of a plant part that follows the sun as it travels across the sky.
- Hydrotropism — the growth movement of a plant part in response to water or moisture.
- Indole-3-acetic acid (IAA) — the chief auxin produced naturally by plants.
- Meristem — a region of small, actively dividing cells from which the growth of a plant begins.
- Parthenocarpy — the development of a fruit without fertilisation, which gives a seedless fruit.
- Phototropism — the growth movement of a plant part in response to light.
- Phytohormone — a plant hormone; the term keeps it distinct from the hormones of animals.
- Plant hormone — an organic substance made in small amounts in one part of a plant and carried to another, where it regulates growth or some other activity.
- Response — the action or movement an organism produces as a result of a stimulus.
- Senescence — the ageing of a plant part, ending in its yellowing and fall.
- Stimulus — any change in the external or internal environment of an organism that the organism can detect.
- Stress hormone — the name given to abscisic acid, because it raises a plant’s tolerance of unfavourable conditions.
- Tendril — a slender, thread-like structure of a weak-stemmed plant that coils around a support.
- Thigmotropism — the growth movement of a plant part in response to touch or contact.
- Tropic movement — a growth movement whose direction is decided by the direction from which the stimulus comes.
- Tropism — the phenomenon of a plant part responding by growth to a stimulus that arrives from one direction.
7.6 Points To Ponder
A plant answers its surroundings by growing, and growth cannot be undone. A stem that has bent towards a window stays bent even after the window is shuttered. What does a plant gain by responding in a way that is permanent, and what does it give up?
Auxin at a certain concentration makes the cells of a stem elongate, while a root needs far less of it. If one and the same substance can mean ‘grow longer’ in one organ and something quite different in another, where does the meaning lie — in the hormone, or in the cell that receives it?
Almost every effect in this chapter is opposed by a second hormone: cytokinin works against auxin over the lateral buds, and gibberellin against abscisic acid over dormancy. Why might a plant be better served by a pair of opposing signals than by a single signal that is simply turned up or down?
The plant on the rotating clinostat was pulled on by gravity exactly as much as the plant on the stationary one, yet it did not bend. What does this suggest about the difference between receiving a stimulus and receiving information from it?
In the wire-netting experiment the radicle abandoned gravity and followed moisture instead. When two stimuli pull a plant part in different directions, what might decide which of them the part treats as the stronger?
A hormone is defined as a messenger that travels within one organism. Ethylene, being a gas, drifts out of the fruit that makes it and ripens the fruit lying beside it. Is it still acting as a hormone when it does that, and what would you call it if it is not?
7.7 Objective Questions
Multiple Choice Type
- Which plant hormone is produced in the root tips and carried upwards through the xylem? (1 mark)
- Auxin
- Cytokinin
- Ethylene
- Abscisic acid
- A stem tendril winding itself around a bamboo stick is an example of: (1 mark)
- Chemotropism
- Hydrotropism
- Thigmotropism
- Geotropism
- The hormone that brings about the closing of the stomata when the soil dries is: (1 mark)
- Gibberellic acid
- Indole-3-acetic acid
- Abscisic acid
- Cytokinin
- A potted plant fixed horizontally on a rotating clinostat does not bend, because: (1 mark)
- Gravity does not act on a rotating plant
- Every side of the plant is exposed equally to gravity
- Rotation destroys the auxin in the shoot tip
- The roots are no longer in contact with the soil
- Which pair of hormones works against each other in deciding whether a seed remains dormant? (1 mark)
- Auxin and ethylene
- Gibberellin and abscisic acid
- Cytokinin and gibberellin
- Auxin and cytokinin
Assertion And Reason Type
In each of the questions below, an Assertion (A) is followed by a Reason (R). Choose the correct option from the codes given here: (1) Both A and R are true, and R is the correct explanation of A (2) Both A and R are true, but R is not the correct explanation of A (3) A is true but R is false (4) A is false but R is true
- Assertion (A): The root of a seedling grows away from a source of light. Reason (R): Roots are negatively phototropic. (1 mark)
- Assertion (A): A parthenocarpic fruit carries no seeds. Reason (R): Parthenocarpy is the development of a fruit without fertilisation. (1 mark)
- Assertion (A): Ethylene must be transported from the place where it is made to the place where it acts. Reason (R): Ethylene is the only plant hormone that is a gas at ordinary temperature. (1 mark)
Match The Following
- Match each item in Column A with the one that belongs to it in Column B. (4 marks)
| Column A | Column B |
|---|---|
| a. Cytokinin | i. Ripening of fruits |
| b. Gibberellic acid | ii. Speeding up of malting |
| c. Abscisic acid | iii. Expansion of the cotyledons |
| d. Ethylene | iv. Closing of the stomata |
Fill In The Blanks
- Complete each sentence with the correct word. (4 marks)
- The chief auxin produced naturally by plants is ____________ .
- The growth of a pollen tube towards chemicals from the ovule is an example of ____________ .
- ____________ is the resting state in which a seed stays alive but does not grow.
- Cytokinins are produced in the ____________ and are carried upwards through the xylem.
True Or False
- State whether each statement is true or false. (3 marks)
- Shoots are positively geotropic.
- Auxins are effective only when present in large amounts.
- The tip of a tendril perceives a touch stimulus before the parts below it do.
Name The Following
- Name the following. (4 marks)
- The plant hormone that induces flowering in mango.
- The instrument used in the laboratory to demonstrate the response of a plant to gravity.
- The phenomenon in which the apical bud holds back the growth of the lateral buds.
- The tissue through which cytokinins travel from the root to the rest of the plant.
Odd One Out
- Pick the odd one out in each group and give a reason for your choice. (4 marks)
- Auxin, gibberellin, cytokinin, abscisic acid.
- Phototropism, thigmotropism, senescence, hydrotropism.
7.8 Check Your Concepts
Define a tropic movement. (2 marks)
Name the plant hormone responsible for each of the following: (a) the sprouting of a potato tuber, and (b) the lengthening of the internodes of a dwarf plant. (2 marks)
Give a reason why a root grows towards a source of moisture. (2 marks)
State what is meant by apical dominance. (2 marks)
Differentiate between a positive and a negative tropic response, stating the basis of the difference, and give one example of each. (3 marks)
A gardener wants a hedge to grow thick rather than tall. Explain what should be cut, and why cutting it works. (3 marks)
What would happen to a plant during a long dry spell if it produced no abscisic acid at all? Give reasons for your answer. (3 marks)
Differentiate between growth by cell elongation and growth by cell division, stating the basis of the difference, naming the hormones that bring each about, and explaining why a plant needs both. (4 marks)
Explain, step by step, how auxin makes a shoot bend towards light that falls on one side of it. (4 marks)
Why does a plant on a stationary clinostat bend while an identical plant on a rotating clinostat does not? Explain what the rotating plant proves about tropic movements. (4 marks)
Explain the physiological effects of gibberellins on the growth of plants, and state two uses made of them in horticulture or in industry. (5 marks)
Ethylene is called the plant growth hormone most widely used in agriculture. Explain, with reference to its properties and its effects, why it is so useful to growers and traders of fruit. (5 marks)
7.9 Figure-Based Questions
- Study Fig. 7.2-1, which shows an experiment on the tip of an oat seedling, and answer the following. (4 marks)
- What collects in the agar block while the cut tips stand on it?
- Why was the seedling kept in darkness during the experiment?
- Why did the seedling curve away from the side that carried the block?
- What was shown by the seedling that was given a block of plain agar?
- Study Fig. 7.3-2, which shows the tip of a shoot lit evenly and lit from one side. (4 marks)
- Name the substance shown as dots in the diagram.
- On which side of the second shoot has this substance gathered?
- Why are the cells on that side drawn longer than the cells opposite them?
- Name the movement shown by the second shoot and define it.
- Study Fig. 7.3-4, in which two potted plants are fixed horizontally on clinostats, A and B. (3 marks)
- Which of the two shows no bending at all?
- Name the response shown by the root and by the shoot of the other plant.
- Give a reason for the difference between A and B.
- Study Fig. 7.3-7, which shows a pollen tube growing through the pistil of a flower. (4 marks)
- Name the phenomenon shown and define it.
- Name, in order, the parts of the pistil through which the pollen tube passes on its way from the stigma to the ovule.
- Name two chemicals that guide the growth of the pollen tube.
- Name one other group of plants in which the pollen tube behaves in this way.
- Refer to Fig. 7.3-6. Draw a neat, labelled diagram of a weak-stemmed plant climbing a support by means of a leaf tendril. Label the leaf tendril, the leaf, the weak stem and the support, keeping the label lines straight and free of crossings. Name the tropic movement your diagram illustrates. (4 marks)