When the movement in plants is growth dependent then it is called tropic movement. It is a directional movement in response to a stimulus. For example:

(i) Phototropism- Movement towards the direction of light. Example: Movement of the shoot of plant upwards towards the light.

It can be explained by the help of this diagram. This kind of movement is induced by light. The stem mostly grows and turns towards the source of light, therefore it is said to be positively phototropic. Auxin distribution controls phototropism.

In 1, Sunlight strikes the plant from directly above. Auxin (pink dots) encourages growth straight up. In 2, 3, 4, Sunlight strikes the plant at an angle. Auxin is concentrated on one side, encouraging growth at an angle from the preceding stem. More growth of the stem on the shaded side is due to more accumulation of auxin on the shaded side than on the illuminated side.

This unequal distribution of auxins on the two sides may be due to following two reasons:

(i) Translocation of auxins from illuminated side to the shaded side.

(ii) Inhibition of auxin synthesis in the illuminated side(light side).

(ii) Geotropism- Movement in the direction of gravity or away from gravity. Example: Root growing downward.

The effects of gravity on plants are not like those of light and temperature because it is both continuous in action and constant in strength. Primary roots and certain other portions of the root system tend to grow directly toward the centre of gravity and hence called positively geotropic.

Mechanism of Geotropism:

Geotropism too can be explained on the basis of differential (unequal) distribution of auxins. In a horizontally placed plant the auxin accumulation on the lower side of the stem and root apices due to gravity. In the stem higher concentration of auxin on the lower side of the apex stimulates its growth on that side. This results in the growth of the stem in the upward direction showing negative geotropism.

On the other hand, in the case of root, the higher concentration of auxin on the lower side inhibits the growth on the upper side than on the lower side. As a result the root apex grows downward (i.e., Positive geotropism)

When a potted seedling was placed horizontally within a dark wooden chamber, the stem was seen to bent upwards, i.e., in a direction opposite to the pull of gravity. This is a negative geotropism. When roots were taken out of the pot, they showed a curvature or bend opposite to the bend of the stem. This curvature is in the direction of the pull of gravity. It is positive geotropism.