In general, tropism is an involuntary orienting response of an organism to a stimulus. It often involves the growth rather than the movement of an organism. In tropism, the response of the organism is often by its growth rather than by its movement. It may grow either towards or away from the stimulus. There are many forms of tropisms and one of them is the so-called chemotropism. In chemotropism, the growth of the organism is based upon its response to a chemical stimulus. The growth response may involve the entire organism or parts of the organism. The growth response may also be either positive or negative. A positive chemotropism is one in which the growth response is towards the stimulus whereas a negative chemotropism is when the growth response is away from the stimulus.
Chemotropism can be observed during the growth of the pollen tube towards the ovules. This is due to the ovary releasing chemicals that influence the development of pollen tubes. The roots growing towards useful minerals are also exhibiting chemotropism, i.e. positive. There are instances though when the roots grow away from harmful acids and this is an example of negative chemotropism. It should not be confused with chemotaxis and chemokinesis. Chemotaxis pertains to the directional movement of an organism or a living motile cell in response to certain diffusible chemicals in the environment. Chemokinesis is a behavioral response of a cell or an organism to a soluble chemical that leads to random movement.
Word origin: chemo- (relating to chemicals) + tropism