Geotropism

Paratonic growth movements

These are the induced movements of growth caused by external stimuli such as gravity, light, water or contact. The movements show a directional relation to the stimulus. The stimuli are effective in causing growth movements only when they are unidirectional. Such movements are generally known as tropic movements and the phenomenon is referred to as tropism. Based on the nature of stimulus, tropisms may be of various types.

(A)           Geotropism

(B)           Phototropism

(C)           Hydrotropism

(D)           Chemotropism

(E)     Thigmotropism

Geotropism

This is also called gravitropism and it is the ability of a plant organ to assume a definite position in relation to the force of gravity. Plants show five different types of geotropic responses.

(i)       Positive Geotropism 

This is observed in primary roots where the taproot grows downwards, along the force of gravity.

(ii)       Negative Geotropism 

This is exhibited by the sporangiophores of fungi, shoots of mosses, pneumatophore of mangrove plants and the stems of flowering plants all of which grow upward against the force of gravity.

(iii) Diageotropism 

Shown by rhizomes and runners which grow at right angles to the force of gravity.

(iv) Plagiogeotropism

This is the property of secondary lateral roots which grow obliquely downwards.

(v)      Apogeotropism 

The lateral roots and branches of higher plants are not sensitive to the stimulus of gravity.

Experiment to demonstrate negative geotropism in aerial stems

The effect of gravity on the plant can be demonstrated using an apparatus called klinostat.

Klinostat: The klinostat has a rotating pot like container mounted on an axial rod. A potted plant is fitted horizontally on the klinostat and is slowly rotated at about four rotations per hour. By this, the effect of gravity is completely eliminated, as all the sides of the plant are equally stimulated by gravity. If the rotation of the klinostat is stopped for a considerable period of time, then the tip of the stem is observed to curve and grow upwards. This proves that the stem tip is negatively geotropic.