When one looks at the Banyan tree it will be seen that the branches and sub-branches are growing towards the light, but the Prop roots originated from the branches growing towards the earth. If a gram seed is kept on the wet soil it will germinate within a few days. It will be seen that its radicle gradually develop and goes into the soil and plumule goes upwards against the soil. The movements of plant organ towards certain direction like light; earth or water is called tropism. The tropism maybe towards stimulant like light, earth or water or may against to those.
A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. Movement of plant organ towards the stimulant is called positive tropism. On the other hand movement against the stimulant is called negative tropism. Tropic movement towards light, earth, water and any chemical is described briefly: