3 Chemical coordination in plants

Question 1

Why do most plants not respond very quickly?

Answer 1

This is because their response normally involves a change in their rate of growth.

Question 2

How may plants respond to a stimulus?

Answer 2

By increasing growth near the tip of its shoots or roots.

Question 3

What is an example of how plants grow?

Answer 3

Imagine a plant growing normally in a pot. Usually, most light will be falling on the plant from above. If you turn the plant on its side and leave it for a day or so, you will see that its shoot starts to grow upwards.

Question 4

What kind of stimuli are light and gravity?

Answer 4

Directional stimuli.

Question 5

What is the growth response of a plant to a directional stimulus called?

Answer 5

A tropism.

Question 6

What is a tropism?

Answer 6

The growth of a plant in response to a directional stimulus.

Question 7

What kind of tropism is it if the growth response is towards the direction of the stimulus?

Answer 7

Positive tropism.

Question 8

What kind of tropism is it if the growth response is away from the direction of the stimulus?

Answer 8

Negative tropism.

Question 9

What is phototropism?

Answer 9

It is the growth movement of a plant in response to a directional light stimulus.

Question 10

What is geotropism?

Answer 10

It is the growth movement of a plant in response to the directional stimulus of gravity.

Question 11

Why does a shoot grow towards the light?

Answer 11

It ensures that the leaves, held out at an angle to the stem, will receive the maximum amount of sunlight.

Question 12

Why are the roots positively geotropic?

Answer 12

To ensure that the roots grow down into the soil, where they can reach water and mineral ions. Roots also fix a plant firmly in place - they provide ‘anchorage’.

Question 13

What is the name of the response to light?

Answer 13

Phototropism.

Question 14

What is the response of the shoots to light?

Answer 14

Positive phototropism.
They grow towards the light source.

Question 15

What is the response of the roots to light?

Answer 15

Negative phototropism.
Most species show no response; some grow away from light.

Question 16

What is the name of the response to gravity?

Answer 16

Geotropism.

Question 17

What is the response of shoots to gravity?

Answer 17

Negative geotropism.
They grow away from the direction of gravity.

Question 18

What is the response of the roots to gravity?

Answer 18

Positive geotropism.
They grow towards the direction of gravity.

Question 19

Why is it obvious that plants had to have some way of sensing?

Answer 19

Since they respond to stimuli such as light and gravity, they must have some way of detecting them and coordinating the response.

Question 20

What is a coleoptile?

Answer 20

It is a protective sheath that covers the first leaves of a cereal seedling. It protects the delicate leaves as the shoot emerges through the soil.

Question 21

What are coleoptiles used for?

Answer 21

They have a simple structure and are easy to grow, so they are often used to investigate tropisms.

Question 22

What did Darwin find out about where the detection of unidirectional light was?

Answer 22

It was detected by the tip of the coleoptile and transmitted to a growth zone, just behind the tip.

Question 23

What is an image of Darwins experiments with phototropism?

Question 24

What did scientists start to do since plants don’t have nervous systems?

Answer 24

Biologists began to look for a chemical messenger (or plant hormone) that might be the cause of phototropism in coleoptiles.

Question 25

What is an example of experiment 1 with coleoptiles?

Question 26

Why were these results found from experiment 1?

Answer 26

The stimulus for growth was found to pass through materials such as gelatine, which absorbs water-soluble chemicals, but not through materials such as mica (a mineral), which is impermeable to water. This made the biologists think that the stimulus was a chemical that was solubel in water.

Question 27

What is an example of experiment 2 with coleoptiles?

Question 28

Why were these results found from experiment 2?

Answer 28

It was shown that the phototropic response could be brought about, even without unidirectional light, by removing a coleoptile tip and placing the tip on one side of the decapitated stalk.

Question 29

What is an example of experiment 3 with coleoptiles?

Question 30

Why were these results found from experiment 3?

Answer 30

It was found that the hormone could be collected in another water-absorbing material (a block of agar jelly). Placing the agar block on one side of the decapitated coleoptile stalk caused it to bend.

Question 31

What did experiments like 2 and 3 cause scientists to believe?

Answer 31

That the hormone caused bending by stimulation growth on the side of the coleoptile furthest from the light.

Question 32

What is the theory concerning the growth hormone in coleoptiles?

Answer 32

The hormone is produced in the tip of the shoot, and diffuses back down the shoot. If the shoot is in the dark, or if light is all around the shoot, the hormone diffuses at equal rates on each side of the shoot, so it stimulates the shoot equally on all sides. However, if the shoot is receiving light from one direction, the hormone moves away from the light as it diffuses downwards. The higher concentration of hormone on the ‘dark’ side of the shoot stimulates cells there to grow, making the shoot bend towards the light.

Question 33

What is the hormone responsible?

Answer 33

The auxin.

Question 34

What is the auxin?

Answer 34

The plant hormone involved in tropisms and other growth responses.

Question 35

Like auxins, several other types of plant hormone have been found, so what have they called these substances.

Answer 35

Plant growth substances.

Question 36

What are plant growth substances?

Answer 36

They are plant ‘hormones’ that affect various aspects of plant growth.

Question 37

What is a diagram that shows how movement of a plant hormone causes phototropism?

Question 38

How could you show tropisms?

Answer 38

Using a clinostat.

Question 39

What is a clinostat?

Answer 39

It is a piece of apparatus consisting of an electric motor which turns a cork disc.

Question 40

How do you set up the clinostat to show tropisms?

Answer 40

Germinating seeds are attached to the disc. The motor turns the disc and seeds around very slowly, so that the movement eliminates the effect of any directional stimulus that may be acting on the seeds. The clinostat can be turned through 90*, so that the disc rotates either horizontally or vertically.

Question 41

What results do we expect to get from the clinostat with beans?

Answer 41

The radicles of the beans on the control clinostat will have grown downwards, under the influence of gravity. Those on the moving clinostat will have grown straight out horizontally. The continuously changing direction of the gravitational stimulus acting on the seeds of the moving clinostat cancels out the geotropic response.