Page:Researches on Irritability of Plants.djvu/108



order to investigate the effects of various gases in modifying the excitability of Mimosa, a series of responses, more or less uniform, is first obtained under uniform stimuli, at intervals of 15 minutes. The given gas is now introduced into the plant-chamber, and another series of responses are once more obtained by the action of the same stimuli as before. The variation of amplitude of responses then gives an indication of the excitatory or depressing action of the agent.

In carrying out the experimental investigation in this manner, we proceed on the assumption that the stimuli applied are invariable, and that the external conditions are maintained constant, with the sole exception of the change induced by the introduction of the given gas. In order to complete a single investigation a period of nearly two hours is often necessary, which is the time required to take eight responses at intervals of 15 minutes. Of these, the first two give the normal responses, the next four the modified responses under the influence of the gas, and the last two