chapter 16 pt 4

Question 1

geotropism

Answer 1

The growth of plants in response to gravity

Question 2

phototropism

Answer 2

The growth of plants in response to light which comes from one direction only

Question 3

chemotropism

Answer 3

The growth of plants in response to chemicals

Question 4

thigmotropism.

Answer 4

The growth of plants in response to touch

Question 5

Tropisms as a response to environmental cues
p 1

Answer 5

• To be able to make the maximum use of the environmental conditions, plants must grow and respond to variations in those conditions.
• For example, once a seed begins to germinate in the soil, the shoot and root must keep growing in the right direction if the developing plant is to survive.
• The shoot must grow up towards the light source for photosynthesis to take place.
• The roots must grow downwards into the soil which will provide support, minerals, and water for the plant.
• The movements of the root and shoot take place in direct response to environmental stimuli.
• The direction of the response is related to the direction from which the stimulus comes.
• These responses are examples of tropisms.
• Much of the research on tropisms uses germinating seeds and very young seedlings.

Question 6

Tropisms as a response to environmental cues
p 2

Answer 6

• They are easy to work with and manipulate and as they are growing and responding rapidly, any changes show up quickly.
• Changes also tend to affect the whole organism rather than a small part (as with a mature plant) and this makes any tropisms much easier to observe and measure.
• The seedlings of monocotyledonous plants - usually cereals such as oats and wheat - are most commonly used as the shoot that emerges is a single spike with no apparent leaves known as a coleoptile.
• It is easier to manipulate and observe than a dicotyledonous shoot.
• However, coleoptiles are relatively simple plant systems, so it is important to remember that the control of the responses to light in an intact adult plant may be more complex.

Question 7

Phototropism:
1

Answer 7

• The basic model of the way plants respond to light as they grow was based on experiments where shoots were kept entirely in the dark or in full illumination.
• However, this is rarely the case in real life.
• Phototropisms are the result of the movement of auxins across the shoot or root if it is exposed to light that is stronger on one side than the other.
• If plants are grown in bright, all-round light in normal conditions of gravity they grow more or less straight upwards.
• In even but low light they will also grow straight upwards - in fact in these conditions they will grow faster and taller than in bright light.

Question 8

Phototropism:
2

Answer 8

• If plants, however, are exposed to light which is brighter on one side than another, or to unilateral light that only shines from one side, then the shoots of the plant will grow towards that light and the roots, if exposed, will grow away.
• Shoots are said to be positively phototropic and roots are negatively phototropic.
• This response has an obvious survival value for a plant.
• It helps to ensure that the shoots receive as much all-round light as possible, allowing the maximum amount of photosynthesis to take place.
• Also, if the roots should emerge from the soil - as they might do after particularly heavy rain, for example - they will rapidly turn back to the soil.

Question 9

diagram of experimental observations

Answer 9

Question 10

The effect of unilateral light:
pt 1

Answer 10

Examples of the response of plants to unilateral light can be seen in any garden or woodland.
Where plants are partially shaded the shoots grow towards the light and then grow on straight towards it.
This response appears to be the result of the way auxin moves within the plant under the influence of light.
Figure 2 shows that the side of a shoot exposed to light contains less auxin than the side which is not illuminated.

Question 11

The effect of unilateral light:
pt 2

Answer 11

It appears that light causes the auxin to move laterally across the shoot, so there is a greater concentration on the unilluminated side.
This in turn stimulates cell elongation and growth on the dark side, resulting in observed growth towards the light.
Once the shoot is growing directly towards the light, the unilateral stimulus is removed.
The transport of auxin stops and the shoot then grows straight towards the light.
The original theory was that light destroyed the auxin, but this has been disproved by experiments showing that the levels of auxin in shoots are much the same regardless of whether they have been kept in the dark or under unilateral illumination.

Question 12

the effect of unilateral light diagram

Answer 12

Question 13

diagram showing impact on auxin levels to light

Answer 13

Question 14

Practical investigations into phototropisms

Answer 14

• Germinate and grow seedlings in different conditions of dark, all-round light, and unilateral light. Observe, measure, and record the patterns of growth.
  Timelapse photography can give a good record of the changes as they take place.
• Germinate and grow seedlings in unilateral light with different colour filters to see which wavelengths of light trigger the phototropic response.
• Repeat some of the classic experiments (Figure 1) - cover the tips of coleoptiles with foil, remove the tips of some coleoptiles, place auxin-impregnated agar jelly blocks or lanolin on decapitated coleoptiles, place auxin-impregnated agar blocks on one side only of decapitated coleoptiles.