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 ell'ect of stimulus is to zausc a retardation of growrh. It is probably on amount 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. Au induction shock, a condenser dis- charge, the make or break of a constant current, a sudden variation of tempe'ature, 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, lnuh't'a. We shill enquire whether the diverse forms of stimuli enumerated above, exert similar or dill'orent reactions on the growing organ.

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