The movement
or growth of any part of a plant in response to an external stimulus is called
tropism or tropic movement. The shoot system of any plant responds towards the
stimulus of light. i.e it grows in the direction of source of light. When a
potted plant is kept near window in a room you would observe that the stem
bends slightly towards the window. This movement exhibited by the plant is
called phototropic movement.
When light falls on a part of the plant, a hormone called
auxin, which is synthesized at the tip of the shoot helps the cell to grow
longer. As the light is falling on the one side of the plant, this hormone
auxin diffuses towards the side of the shoot which is not in the sunlight and
stimulates the cells to grow longer. Hence the plant appears to bend towards
the light. On the other hand, the root system of the plants responds to the
stimulus of gravity and water. These responses are called gravitropic and
hydrotropic movements respectively.
Chemotropism is
the movement of plant part in response to certain chemicals. e.g. the growth of
pollen tubes towards the ovules.
Other examples of plant hormones are gibberellins,
cytokinins, abscissic acid, etc. Gibberellins help in the growth of the stem,
cytokinins promote cell division and hence is found in greater concentration in
areas where rapid cell division takes place such as fruits and seeds. Auxins,
Gibberellins and cytokinins help in promoting growth. Abscissic acid is a plant
hormone which inhibits growth leading to wilting of leaves.
