Coordination and Response in Plants

• Marks in this topic are given for direction, order and restraint: the right direction of growth, the steps in the right order, and no claim larger than the evidence.
• Tropism = directional + growth + response. If a sentence you have written is missing any one of those three words, it is not yet a definition.
• Shoots chase light and flee gravity. Roots chase gravity and flee light. Four responses, two organs, two stimuli — and an advantage attached to each.
• Source → stimulus → side → action → length → bend → name. Seven words, in that order, and every mechanism question in this chapter is already half written.
• If two things in this chapter look alike, ask which stimulus is acting, which organ is responding, and which direction the growth took. Those three answers separate every pair above.
• Two organs. Two stimuli. One chain. One boundary. If you can say those four things in full, you can answer anything this topic asks.

• Define gravitropism as a response in which parts of a plant grow toward or away from gravity.
• Define phototropism as a response in which parts of a plant grow toward or away from light.
• Distinguish a positive tropism (growth toward the stimulus) from a negative tropism (growth away from it), using direction alone and never benefit or harm.
• State the typical responses of shoots and of roots to light and to gravity, and explain the biological advantage of each.
• Explain phototropism and gravitropism in a shoot as the result of auxin being made in the shoot tip, moving from the tip, becoming unequally distributed, and stimulating unequal cell elongation that bends the shoot.
• Apply that chain in the correct order to any new one-sided-light or horizontal-shoot scenario you are given.
• Recognise the limit of the syllabus mechanism: state the observable response of a root, and do not transfer the shoot auxin explanation to a root unless the question supplies the information.
• Describe an investigation into phototropism in shoots, naming the independent, dependent and controlled variables and a valid control.
• Describe an investigation into gravitropism in shoots and roots using germinating seeds, and explain why directional light must be excluded.
• Explain what a clinostat does and, just as importantly, what it does not do.
• Measure a response quantitatively: state a method, a unit, a time interval and a mean, rather than writing “measure how much it bends”.
• Separate an observation from an inference, and keep a conclusion inside the evidence the experiment actually provides.

One connection worth making now. Chapter 15 is the plant half of coordination and response. When you revise the human nervous and endocrine systems, come back to this page for ten minutes and write down three differences: speed, the nature of the signal, and whether the response is reversible. Contrasts learned in pairs stay learned.

Tropism

A directional growth response of part of a plant, in which the organ grows toward or away from a stimulus that itself has a direction. The bending is produced by cells on one side of the organ elongating more than cells on the other side, so a tropism is a change in growth rather than a movement of the whole plant.

Phototropism

A response in which parts of a plant grow toward or away from light. Shoots are usually positively phototropic, growing toward light, which positions the leaves where light for photosynthesis is available; roots may be negatively phototropic, growing away from light.

Gravitropism

A response in which parts of a plant grow toward or away from gravity. Roots are usually positively gravitropic, growing toward gravity into the soil for anchorage and for the absorption of water and mineral ions; shoots are usually negatively gravitropic, growing away from gravity so that the shoot rises clear of the soil.

Cell Elongation

The increase in length of an individual plant cell after it has been formed. Auxin stimulates cell elongation in shoots, so a side of a shoot receiving more auxin elongates more than the opposite side; the difference in length between the two sides bends the shoot.

Negative Gravitropism

The growth of a plant organ away from gravity. In a shoot laid horizontally, auxin becomes more concentrated on the lower side; auxin stimulates cell elongation, so the lower side elongates more, becomes longer than the upper side, and the shoot curves upward.

Auxin

A plant growth substance produced in the shoot tip. Auxin moves from the tip through the plant and stimulates cell elongation in shoots. When a directional stimulus such as one-sided light or gravity makes its distribution unequal, the two sides of the shoot elongate at different rates and the shoot bends.

Positive Phototropism

The growth of a plant organ toward light. In a shoot lit from one side, auxin becomes unequally distributed with more present on the shaded side; auxin stimulates cell elongation, so cells on the shaded side elongate more, that side becomes longer, and the shoot bends toward the light.

Controlled Variable

A factor deliberately kept the same throughout an investigation so that it cannot affect the result. Any factor left free to vary alongside the independent variable becomes an alternative explanation for the outcome, so the conclusion can no longer be attributed to the factor being tested.

Clinostat

An apparatus that rotates a seedling slowly about a horizontal axis so that the direction of gravity relative to the organ changes continually. It is used as a control in gravitropism investigations: it does not remove gravity, but it prevents gravity from acting persistently from one direction relative to the plant.

Bending Angle

The angle between the original orientation of a shoot and its orientation after a period of growth, measured in degrees from a marked starting position. It is the usual quantitative measure of a tropic response, and it is meaningful only when the starting orientation, the time interval and the measuring method are all stated.

Positive Gravitropism

The growth of a plant organ toward gravity. A root laid horizontally curves downward and grows in the direction in which gravity acts, which carries it into the soil where it anchors the plant and absorbs water and mineral ions.

Biological Variation

The natural differences between individual organisms of the same species, including differences in growth rate and in the size of a response. It is the reason several individuals are used in an investigation and a mean is calculated, since a single individual may not be representative.

• 1. “Plants move toward the light.” Why it is wrong: a plant is anchored and cannot travel or lean. The change of direction is produced entirely by growth. SayThe shoot grows toward the light, because cells on one side elongate more than cells on the other.
• 2. “Any movement of a plant is a tropism.” Why it is wrong: a tropism needs a directional stimulus and a growth response. Wilting and a flower closing at dusk are responses, but nothing has grown and no direction was involved. SayA tropism is a directional growth response. Other responses are not tropisms.
• 3. “Positive means the response is good for the plant.” Why it is wrong: the words describe geometry, not value. Positive means growth toward the stimulus, and nothing more. SayPositive means growth toward the stimulus. Whether it benefits the plant is a separate question, answered by stating the advantage.
• 4. “Negative means the response harms the plant.” Why it is wrong: a shoot is negatively gravitropic, and that response is what gets it out of the soil. Negative responses are frequently the useful ones. SayNegative means growth away from the stimulus — for example, a shoot growing upward, away from gravity, which lifts it into the light.
• 5. “Shoots are negatively phototropic.” Why it is wrong: shoots grow toward light, which is growth toward the stimulus. SayShoots are usually positively phototropic, which positions the leaves where light for photosynthesis is available.
• 6. “Roots are negatively gravitropic.” Why it is wrong: a root grows downward, which is the direction in which gravity acts — toward the stimulus. SayRoots are usually positively gravitropic, growing toward gravity into the soil for anchorage and for water and mineral ions.
• 7. “Light destroys all the auxin on the illuminated side.” Why it is wrong: it is an absolute claim that the syllabus does not make and that no experiment described in this chapter tests. SayAuxin becomes unequally distributed, with more auxin present on the shaded side.
• 8. “Auxin pulls the shoot toward the light.” Why it is wrong: auxin exerts no force. Its stated action is to stimulate cells to elongate. SayAuxin stimulates cell elongation, so the side with more auxin grows longer and the shoot curves.
• 9. “The illuminated side contracts.” Why it is wrong: plant cells that have elongated do not shorten again. Nothing in the shoot contracts. SayThe illuminated side elongates less than the shaded side. Both sides grow; they grow by different amounts.
• 10. “The shaded side becomes shorter, so the shoot bends toward the light.” Why it is wrong: this reasoning predicts the opposite curvature. A shorter shaded side would bend the shoot away from the light. SayThe shaded side becomes longer, so it forms the outside of the curve and the tip swings toward the light.
• 11. “Auxin is an enzyme.” Why it is wrong: it is neither a catalyst nor food nor energy nor a nerve impulse. It is a plant growth substance. SayAuxin is a plant growth substance produced in the shoot tip that stimulates cell elongation in shoots.
• 12. “The root must work the same way as the shoot.” Why it is wrong: applying “more auxin on the lower side, so it elongates more” to a root predicts the root curving upward, which is the opposite of what is observed. SayThe root grows toward gravity — positive gravitropism — which anchors the plant and reaches water and mineral ions. State the observable response and its advantage, and stop there.
• 13. “A clinostat removes gravity.” Why it is wrong: no laboratory apparatus removes gravity. Rotation changes the direction from which it acts relative to the organ, not whether it acts. SayA clinostat rotates the seedling slowly so that gravity does not act continuously from one direction relative to the organ, so no persistent one-sided stimulus is maintained.
• 14. “Light does not need controlling in a gravitropism experiment.” Why it is wrong: if directional light is present, any curvature could be a phototropic response instead, and the experiment cannot answer its own question. SayThe seeds are kept in darkness, or in even all-round light, so that gravity is the only directional stimulus acting.
• 15. “One seedling is enough to show the response.” Why it is wrong: individuals of the same species differ in growth rate and in the size of their response, so a single seedling may be unrepresentative. SaySeveral seedlings are used and a mean calculated, so that biological variation does not determine the result.
• 16. “Cutting off the tip proves that auxin causes the bending.” Why it is wrong: no substance was detected, identified or measured. Removing a tip is also major damage, which may reduce growth for unrelated reasons. SayThe result supports the conclusion that the tip is involved in detecting or controlling the response. It does not identify the substance responsible.
• 17. “Measure how much it bends.” Why it is wrong: no quantity, no instrument, no unit and no reference point. Two students following it would produce numbers that cannot be compared. SayMeasure the angle in degrees between the marked starting orientation and the shoot's direction, with a protractor, at equal intervals, for each of several seedlings, and calculate a mean.

• Coordination in plants is a small topic that behaves like a large one, because it is nearly always tested through unseen material: a photograph of a seedling, a diagram of an apparatus, a table of bending angles. You are rarely asked to recite; you are asked to apply.
• Classification traps. A stem, a root and an arrow — choose the correct pair of terms. The wrong options are always “negative gravitropism” for a root and “negative phototropism” for a shoot.
• Define a tropism for one mark. State a shoot or root response and give one advantage. Predict the growth of a seedling laid on its side.
• Four to six marks for the auxin chain. Marks are awarded for ordered steps, so an answer containing the right words in the wrong order loses them.
• Design or improve an investigation. Marks sit on the control, the controlled variables, the replicates and a stated measurement, not on the apparatus drawing.
• Read a bending-angle table or curve, calculate a mean or a change, identify an anomaly, and compare treatment with control.