IRRITABILITY The term irritability is often applied to the visible reactions which the plant makes to its environment. The particular action of the environment is known as the stimulus, and irritable re sponses as responses to a stimulus. The stimulus may, for ex ample, be some change in the intensity of light or temperature or gravity or electricity. Some stimuli are necessary for the nor mal growth of the plant such as a suitable degree of temperature, light, etc. ; without such conditions acting more or less continually on the plant body as a whole the organism would not be in a state to react to stimuli. The term stimulus is to a large extent a term serving as a cloak for our ignorance. We do not speak of the stimulus of light causing assimilation because we have some physio-chemical picture of this process. When a shoot turns in response to unilateral light we speak of the light as a stimulus, largely because we are unable to picture the course of events which cause difference of illumination to bring about curvations of the shoot or root. Stimuli have, however, usually one point in
common that the stimulus does not itself supply the energy em ployed in the response to the stimulus, but the energy used in the response comes from a store pre-existing in the plant. The stimulus seems thus to be a releasing mechanism, in which the amount of energy employed has no relation to the energy re leased. The firing of a gun is a good example where the energy used in pulling the trigger has no relation at all to the amount of energy released in the explosion ; irritable mechanisms of this kind are therefore sometimes called trigger mechanisms.