Page:Researches on Irritability of Plants.djvu/87



of the effects brought about by varying external conditions on the excitability of the plant have now been noted. Certain other variations may, however, be induced in the excitability, in consequence of the after-effect of the stimulus itself, even when the external conditions are maintained constant. We may trace these induced internal changes in the modification of the response-records.

It is clear that we can only be assured of the occurrence of such internal changes from the observed variation of response-record if we have been able in the first place to keep the plant under unvarying external conditions. This, taking certain special precautions as regards light, temperature, and so forth, presents no difficulty. But, in the second place, we have to be specially careful that the testing-stimulus itself shall be absolutely constant in successive experiments. The problem then resolves itself into the successful devising of some arrangement by which records may be taken automatically at definite pre-determined intervals of time. The stimulus of unvarying intensity must also be made to act automatically upon the specimen. Under these conditions any variation that may be observed