The growth movements of plant parts in which the direction of the stimulus determines the direction of the response is known as tropic movements or tropism. Plants also have non-directional movements which may not be growth dependent. (I) Name the movement which causes 'X' and 'Y' to grow downwards and upwards respectively. (II) Write the name of a hormone that plays a major role in (i) falling of leaves (ii) rapid cell division. (III) (a) Leaves of the sensitive plant move very quickly in response to 'touch'. How is this stimulus of touch communicated and explain how the movement takes place. OR (III) (b) Name the plant hormone which is synthesized at the shoot tip. How does this hormone helps the plant to bend towards light?

The movement that causes 'X' and 'Y' to grow downwards and upwards respectively is called geotropism. The hormone that plays a major role in (i) falling of leaves is ethylene, and (ii) rapid cell division is auxin. (III) (a) The stimulus of touch is communicated through the plant's nervous system, which consists of specialized cells called neurons. When a leaf is touched, it sends a signal to the brain, which then sends a response back to the leaf, causing it to move. The movement takes place through the contraction and relaxation of muscles in the leaf. OR (III) (b) The plant hormone synthesized at the shoot tip is auxin. Auxin helps the plant to bend towards light by promoting cell elongation on the side of the stem opposite the light source. This causes the stem to bend towards the light, a process known as phototropism.