Phototropism and geotropism

The plants respond to the stimulus from the external environment. The response can be the movement either in the same direction or in the reverse direction from the stimulus. If the movement of the plant is against the movement of the stimulus, then it is called as a nastic movement.

The growth of plants towards the direction of light is investigated by Charles Darwin and his son. The movement of plants towards light stimulus is called as phototropism. Darwin observed that the plant tip is curved and the curve in the stem has extended down into the stem. If the plant tips are covered with foil then the curving will not take place. It was observed that some element present in the tip region traveled towards the lower region of the stem making the plant to bend or curve.

The experiments conducted by Frits Went in 1926 indicate that the Auxin is released by the stem tip. Auxin moves to the stem parts where light does not fall and make the cells to divide there than the cells present in the plant side that is covered by light. This will make the plant to bend towards the side on which light falls. This curving and movement of the plant towards light is called phototropism.

The response of the plant to gravity is termed as geotropism. The roots of the plants are found as positively geotropic while the plant shoots are negatively geotropic. Geotropism is considered as the phenomenon where the gravity influences the concentration of Auxin.