6.4 Plant Hormones

Name the plant hormone responsible for both phototropism and gravitropism.

What effect do auxins have on plants?

A group of students set up an experiment to investigate plant growth.

They attached a plant pot to a machine called a clinostat so that it could be rotated slowly on its side as shown in Figure 1. The apparatus was then placed in a darkened room.

Figure 1

Which of the following images would show the expected result of this experiment after two days?

Figure 2 represents a plant shoot growing, it is illuminated from the side.

Figure 2

Which of the following images best represents how auxins will start to accumulate in the shoot tip in response to light?

A group of students wanted to investigate geotropism in bean seedling roots.

The set-up of their experiment is shown in the diagram below.

The students used the following method:

• • Measure the starting length of the roots of 5 bean seedlings
  • Pin the 5 bean seedlings to a cork mat as shown in Apparatus A
  • Place Apparatus A in a dark cupboard for 3 days
  • After 3 days, measure the length of the root in each seedling and make a drawing of each seedling

The students then set up Apparatus B in exactly the same way as Apparatus A, but used a clinostat to rotate the cork board constantly.

Explain the purpose of Apparatus B.

The seedlings in the experiment in part (a) were placed in a dark room to remove the effect of light. This is a control variable.

State why this stage is necessary.

The diagram below shows the appearance of the seedlings at the end of the 3 days.

Explain the appearance of the seedlings in the diagram.

The students carried out their experiment using 10 bean seedlings split across two sets of apparatus; Apparatus A and Apparatus B (the control).

The results of the student’s investigation are shown in the table below.

Calculate the mean change in length for the bean seedlings in Apparatus B.