Page:Researches on Irritability of Plants.djvu/48

Rh experiments, or varied in a definite and known manner. Another great obstacle to be overcome in practice is the avoidance of injury which is caused by the stimulus itself. The application of stimulus above a critical intensity induces a depression or abolition of excitability of the tissue.

As the result of long investigation for the purpose of securing various forms of quantitative stimulus, I find that one mode of thermal and three modes of electrical stimulation may be rendered practicable for our purpose. These four different methods will be described in some detail below.

It is evident that touching the specimen with a hot wire, though effective, is not a form of stimulus that is capable of quantitative application or of repetition. It is apt, moreover, unless very great precautions are taken, to injure the tissue.

The thermal mode of stimulation can, however, be rendered practicable by the electrical mode of the generation of heat. A loop of fine platinum-wire is made to clasp round the petiole which is to be excited, and is connected with an electrical circuit by means of fine flexible silver-wire (fig. 6). The circuit can be completed by a metronome interrupter, the current from the battery flowing for a definite length of time during, say, a single or definite number of beats of the metronome. This produces a sudden thermal shock, enough to cause excitation. Successive uniform stimuli can