Chapter 7 · ICSE Class 10 Biology (Selina Concise)

Chemical Coordination in Plants

Plants don't have nerves or muscles, yet they respond to light, gravity, and touch. Plant hormones make this possible.

🌱 Chapter 7

A plant shoot bending toward light due to auxin A shoot grown with light from one side bends toward the light source, because auxin accumulates on the shaded side and causes those cells to elongate more Light source Shoot bends toward light more auxin, more elongation (shaded side)
7.1

Plant Hormones

Plant hormones are chemical messengers that coordinate growth and responses. Auxin promotes cell elongation, while gibberellins promote stem elongation.

1

Which plant hormone is primarily responsible for cell elongation?

Auxin

7.2

Phototropism

Phototropism is the growth response of a plant shoot toward light, caused by auxin accumulating more on the shaded side of the shoot, making those cells elongate more than the lit side.

1

Why does a shoot bend toward light rather than growing straight?

Auxin accumulates more on the shaded side, causing those cells to elongate more than the cells on the lit side, bending the shoot toward the light

Practice

Exercises

1. What is a plant hormone?

2. Which hormone is mainly responsible for cell elongation in plants?

3. What is phototropism?

4. Why does a shoot bend toward a light source?

5. Which plant hormone promotes stem elongation?

6. Is phototropism in a shoot usually toward or away from light?

7. Why don't plants need a nervous system to respond to their environment?

8. Name one environmental factor, besides light, that plants can respond to using hormones.

9. If a seedling is placed on its side, which part of the stem usually bends upward?

10. Why might a plant kept with light coming from only one direction end up leaning noticeably?

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