Page:Life Movements in Plants.djvu/225

In plant physiology, the word 'stimulus' is often used in a very indefinite manner. This is probably due to the different meanings which have been attached to the word. An agent is said to stimulate growth, when it induces an acceleration. But the normal effect of stimulus is to cause a retardation of growth. It is probably on account of lack of precision in the use of the term that we often find it stated, that a stimulus sometimes accelerates, and at other times, retards growth. In order to avoid any ambiguity, it is very desirable that the term stimulus should always be used in the sense as definite as in animal physiology. An induction shock, a condenser discharge, the make or break of a constant current, a sudden variation of temperature, and a mechanical shock bring about an excitatory contraction in a muscle. These various forms of stimuli cause, as we have. seen, a similar excitatory contraction of the motile pulvinus of Mimosa pudica. We shall enquire whether the diverse forms of stimuli enumerated above, exert similar or different reactions on the growing organ.

The form of stimulus which is extensively used in physiological investigations, is the electric stimulus of