404 COMPARATIVE ELECTRO-PHYSIOLOGY 



nerve gives the normal negative response, the same tissue, 

 if its tonic condition be below par, gives a more or less per- 

 sistent positive response. It is only when the tonic condition 

 of the nerve has again been raised, by long-continued 

 stimulation, that it will once more give normal response. 

 Fatigue is another condition which is liable to give rise to 

 the abnormal positive response. 



The use of the stimulus of light carries with it, also, a 

 further limitation. A mechanical stimulus, say vibrational, 

 throws into activity the whole mass of tissue, not only in its 

 superficial, but also in its deeper lying strata. Now we have 

 seen that the epidermal layer of living tissues is less excitable 

 than those which are deeper seated. It may even, in fact, on 

 local excitation, give positive response (p. 298). It is to be 

 noticed, moreover, that light acts from outside, its excitatory 

 influence affecting the outmost tissue first, and only by 

 gradual percolation passing to the subjacent. Owing to 

 these two facts, then, of the moderateness of this stimulus 

 and the superficial character of its action, the tissue, if not 

 highly excitable, is apt, under its application, to give positive 

 response. We have seen, further, that various circumstances, 

 such as age and season, have an important effect in varying 

 the excitatory reaction of a tissue. We saw the effect of 

 age exemplified in the responses given by two different 

 specimens of roots (p. 353), in which a young root gave nega- 

 tive and an older positive responses. Again, we shall see 

 presently that there is a diurnal period, on account of which 

 the state of turgor, the excitability, and the sign of response, 

 are all alike liable to undergo periodic variations. Under 

 the stimulus of light these varying excitabilities may be ex- 

 pected to find varying expressions. 



That sub-tonicity tends to make the response, under 

 moderate stimulation, positive, is seen in the fact that an 

 etiolated petiole of celery gives positive response under light 

 Again, I have noticed that leaves of Bryophylhijn, which 

 usually give normal negative responses, sometimes exhibit 

 positive, if the plant, during the previous night, have been 



