POSITIVE AND NEGATIVE EFFECTS 51 



The positive A effect is externally manifested — 



(a) By expansion, as manifested by the erectile 



movement of the leaf of Mimosa ; 



(b) By increase of turgor, and by increase in the 



normal rate of growth ; 



(c) By electromotive variation of galvanometric 



positivity; and 



(d) By increase of electric resistance of the tissue. 



The negative D reaction, on the other hand, is shown — 



(a) By contraction and the fall of the leaf of Mimosa ; 



(b) By diminution of turgor, and by retardation of 



the rate of growth ; 



(c) By electromotive change of galvanometric 



negativity ; 



(d) By diminution of the electric resistance of the 



tissue. 



The widened outlook resulting from the establishment 

 of the correspondence of the characteristic expression of 

 the positive, as also of the negative, in various modes of 

 response has led to the discovery of several important 

 phenomena, notably in photosynthesis, which at first sight 

 would appear inexplicable. Feeble and strong electric 

 stimulus will be shown in the next chapter to induce opposite 

 effects in photosynthesis. Again, in a moderately vigorous 

 specimen of Hydrilla, the after-effect of strong intensity of 

 light on photosynthesis is a depression of activity. But 

 in sub-tonic specimens, in which physiological condition is 

 below par, the after-effect of strong intensity of light is an 

 enhancement of activity instead of a depression (cf. Chapter 

 XVIII.). Still more curious is the effect of rapidly intermit- 

 tent light, which photosynthetically is even more effective 

 than continuous light. Apparent anomalies like these are 

 frequently met with, and their explanation is to be sought 

 in the general characteristics of protoplasmic irritability. 



Some of the most important of these characteristics will 

 be described in the following order : (1) the relation between 



