Tropism means movement in response to stimuli. Plants show movement in response to stimuli.  Chemotropism is the movement of plants in response to chemicals. It can be away or towards chemicals, e.g. growth of pollen tubes towards ovules in response to some chemical substances that are secreted by ovules to attract pollen tubes.

Ethylene is a fruit ripening hormone.

Auxin is synthesised in shoot tips. Indole-3-acetic acid (IAA) is the most abundant and the basic auxin natively occurring and functioning in plants. It generates the majority of auxin effects in intact plants, and is the most potent native auxin.

Ethylene is an inhibitor which causes abscission by destructive effect on auxins. Some fruits will produce ethylene as ripening begins. Apples and pears are examples of fruits that produce ethylene with ripening.

Growth of a plant is inhibited by abscisic acid. It functions in many plant-developmental processes, including bud dormancy.

Phototropism is movement of plants in response to light. Movement can be towards or away from light. For example, the growth of shoots towards light shows positive phototropism whereas the growth of roots away from light shows negative phototropism.

Cytokinins are plant growth hormones that encourage cell division.

‘Touch-me-not’ plants are sensitive plants. They do not show growth as they fold up and droop in response to touch. This is because of the reason that when such plants are touched or disturbed, certain specialised cells in the stem become sensitive and release certain chemicals. These chemicals help in removing the water from vacuoles of the cells. As a result of this, water diffuses out of the plant cells, making them shrink.

When we touch the leaves of chhui–mui (the ‘sensitive’ or ‘touch-me–not’ plant of the Mimosa family), they begin to fold up and droop. The reason is that when such plants are touched or disturbed, certain specialised cells in the stem become sensitive and release certain chemicals. These chemicals remove water from vacuoles of the cells. As a result of this, water diffuses out of the plant cells, making them shrink.

Geo - earth or gravity Tropism – movement towards stimulus Plants show movements in response to stimuli. The roots of a plant always grow downwards, while the shoots usually grow upwards and away from the earth. This upward and downward growth of shoots and roots, respectively in response to the pull of the earth or gravity is known as geotropism.