Page:Researches on Irritability of Plants.djvu/88

Rh in the record will be due to changes of excitability induced as an after-effect of the stimulus itself.

Maintaining the stimulus-intensity absolutely constant is not so easy to secure with a single make- or break-shock, since the intensity of such a shock is liable to variation, according to the degree of suddenness with which it is effected; but the total additive value of a group of such shocks may be expected to be fairly constant. For this reason, therefore, tetanising shocks caused by a vibrating interrupter would be preferable, provided the duration of these shocks, depending on the duration of closure of current in the primary circuit, be maintained in successive experiments rigorously equal. Such constancy cannot be arrived at if the closure of the circuit be caried out by hand, or even by metronome. Some special mechanical device must therefore be adopted for this purpose.

It is further necessary, in order to maintain constancy of conditions, that identical periods of recovery should be allowed in successive records. For this the stimulus must be applied at accurate and pre-determined intervals of time. The ideal condition, then, for the final elimination of all uncertainties due to the personal factor, is that the plant attached to the recording apparatus should be automatically excited by a stimulus absolutely constant, make its own responsive records, go through its own period of recovery, and embark on the same cycle over again without assistance at any point on the part of the observer.

These demands have been fully met in the devices and adjustments now to be described, consisting as they do of two chief elements—namely, the Periodic Starter and the Automatic Exciter. By the former the time-interval between successive stimulations is regulated; by the latter the stimulation itself, of a definite duration, is applied.

In the case of Mimosa recovery from excitation is practically completed in a period of about 10 to 20 minutes,