Question

Give examples of situations in plants responding to a stimulus.

Answer 1

Hint: A stimulus is something that causes a reaction. It causes a change in an organism’s physical pattern due to external surroundings. A tropism is a reaction to a stimulus that an organism produces.

Complete answer: There are 5 ways of tropism observed in plants. They are as follows:
a) Phototropism (where sunlight is a stimulus): Here, the plant stem moves towards the sunlight, whereas roots turn away from the stimulus. The hormone auxin plays an important role in phototropism.
b) Hydrotropism (where the trigger is water): Here, the root of a plant in search of water spread deep into the earth.
c) Thigmotropism (where the stimulus is contact): In thigmotropism, when we touch the plant, the plant folds. For example, the mimosa plant folds up its leaves almost instantly.
d) Geotropism: Here, the gravity stimulation makes the roots move in the direction of gravity, while gravity shifts away from the stem of the same plant.
e) Chemotropism (where stimuli are chemicals): In chemotropism, during the growth of the pollen tube, motion can be seen towards the ovule, i.e. it grows in response to certain chemicals.
f) Nastic gestures are non-directional stimulus responses (e.g. temperature, humidity, light irradiance).

Note: Although growth is a response of plants towards stimuli while shedding of leaves, dropping of fruits, and peeling of bark is a part of the process known as 'abscission'. Abscission occurs when in a plant region, auxin is exhausted, which makes the region more susceptible to ethylene, causing leaves to fall, flowers to wilt and fruit to ripen. Abscission is a plant process that cuts off the undesired portion of the plant to conserve its energy.