/O COMPARATIVE ELECTRO-PHYSIOLOGY 



stimulator was brought nearer and nearer, the two effects 

 became superposed, and one was masked by the other. 

 Even in this case, however, on careful examination, it is 

 possible to infer the results due to the action of the internal 

 factor. 



Thus, we may suppose stimulus to be applied directly on 

 the pulvinus of Mimosa, bringing about a responsive fall of 

 the leaf. The expelled water from the excited pulvinus will 

 now be forced into the neighbouring tissues, making them 

 over- turgid, and raising their energy above par. On the 

 cessation of stimulus, the water that has been forced away 

 will flow back from the region of heightened, to that of 

 lowered tension, and re-establish the normal turgidity of the 

 pulvinus, which had undergone a negative variation. The 

 erectile recovery of the leaf is thus seen to be, not a merely 

 passive process, but an effect dependent on the internal energy 

 of the plant. This is also shown by the fact that in autumn 

 and winter, when the internal energy is low, the period of 

 recovery is very long, being sometimes as much as eighteen 

 minutes ; whereas in summer, on the other hand, with the 

 increased internal energy of the plant, it takes place nearly 

 three times as quickly. I have elsewhere shown that it is 

 this internal energy which is vaguely referred to as the tonic 

 condition of the tissue, and that it consists of the sum total 

 of energy derived from external stimuli previously absorbed 

 and held latent by the plant. The different forms of stimulus 

 may be very various. We have for instance tonicity, as 

 imparted by light, or phototonus ; by favourable temperature, 

 thermotonus ; by electrical current, electrotonus ; by internal 

 hydrostatic pressure, hydrotonus ; or by the presence of favour- 

 able chemical substances, chemotonus. 



We have seen that, as a general rule, external stimulus 

 and internal energy find responsive expressions of opposite 

 sign, the former inducing a negative, and the latter, a positive 

 turgidity-variation. It is, nevertheless, important to demon- 

 strate the extensive applicability of this law, by many different 

 results and different modes of manifestation. And such a 



