Q69 of 231 Page 96

The chemical substance P is made and secreted by the meristematic tissue at the tip of stem (or shoot) of a plant. The chemical substance P is responsible for a phenomenon Q in plants in which the stem bends towards a source of light. The same chemical substance P has an opposite effect on the root of a plant. It causes the root of a plant to bend away from the source of light in a process called R.

(a) What is the chemical substance P?


(b) State whether P prefers to remains in the sunlight side of a stem or in shade.


(c) What is the effect of substance P on the rate of growth of (i) a root, and (ii) a stem?


(d) What is the name of process (i) Q, and (ii) R?


(e) What is the general name of chemical substances like P? Name another substance which belongs to this class of chemical substances.

(a) From the given question, the chemical substance P is Auxin which is produced at the shoot tip and can diffuse to other plant parts.

(b) Auxin moves to the darker side of the plant, causing the cells there to grow large.


(c) Auxin has adverse effect on growth rate of roots hence, it decrease the growth rate of roots. Unlike roots, the auxin has positive effect on growth rate of stem hence, it increases the growth rate of stem.


(d) Process Q is positive phototropism while process E is negative phototropism. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism.


(e) Chemical substance like P is Auxin, a Phytohormone or plant hormone. Like Auxin, Gibberellin is also an example of phytohormone.


More from this chapter

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67

Which of the following plant part exhibits negative phototropism?

 68

Which of the following are not tropisms?

(i) growing of pollen tube in response to a sugary substance


(ii) folding up of leaves of sensitive plant in response to touch


(iii) winding of tendril around a support in response to touch


(iv) opening up of the leaves of a daisy flower in response to light

 70

A potted plant is growing in a transparent glass jar. In this plant, X and Y are the two growing parts having a lot of meristematic tissue. It is observed that the part X of this plant exhibits positive geotropism but negative phototropism. On the other hand, part Y of this plant exhibits negative geotropism but positive phototropism.

(a) Name the part X of plant.


(b) Name the part Y of plant.


(c) Which part of the plant, X or Y, will exhibit positive hydrotropism?


(d) Which part of the plant, X or Y, can have tendrils on it?

 71

There are three plants A, B and C. The flowers of plant A open their petals in bright light during the day but close them when it gets dark at night. On the other hand, the flowers of plant B open their petals at night but close them during the day when there is bright light. The leaves of plant C fold up and droop when touched with fingers or any other solid object.

(a) Name the phenomenon shown by the flowers of (i) plant A, and (ii) plant B.


(b) Name one flower each which behaves like the flower of (i) plant A, and (ii) plant B.


(c) Name the phenomenon exhibited by the leaves of plant C.


(d) Name a plant whose leaves behave like those of plant C.


(e) Which plant/plants exhibit the phenomenon based on growth movements?