As regards stimulation of vegetable tissues, there are various agencies besides electric shock, which induce excitatory contraction; these agencies I shall designate as stimuli. Excitation is detected in Mimosa by the downward movement of the leaf. It will be found that such excitatory movement is caused by a mechanical blow, by a prick or a cut, by the application of certain chemical agents, by the action of electric current and by the action of strong light. The study of the action of these stimuli will be given in greater detail in subsequent Papers.
I shall give below a general classification of different stimuli which cause excitation in vegetable tissues.
Electric Stimulus.—Induction shock, condenser discharge, the make of kathode and the break of anode.
Mechanical Stimulus.—Mechanical blow, friction, prick or cut.
Chemical Stimulus.—Effect of certain acids and of other chemical substances.
Thermal Stimulus.—Sudden variation of temperature; application of heated wire.
Radiation Stimulus.—Luminous radiation of the more refrangible portion of the spectrum; ultra-violet rays; thermal radiation in the infra-red region.
All these different forms of stimulus induce an excitatory contraction, a diminution of turgor, and a negative mechanical response or fall of a motile leaf.