EXCITABILITY UNDER CHEMICAL REAGENTS 137 



While pursuing another line of inquiry on the effect of 

 various strengths of solution of Na 2 Co 3 on the natural 

 current, I obtained results which were parallel (p. 122). 

 It was there shown that dilute solution of NaXo, induced 



- o 



a positive variation of the natural current ; a strong solution, 

 a negative variation, and that a solution of intermediate 

 strength induced a preliminary positive followed by a negative 

 variation. Thus the positive variation in the last-named 

 experiments, already shown to be indicative of increased 

 excitability, was here seen to correspond with heightened 

 amplitude of response, while the negative variation on the 

 other hand is seen to coincide with depression of excitability. 

 The application of a strong solution inducing excitation, 

 carries the molecular condition of the tissue to the stage 

 E, where, as we know, the excitability is depressed. 



Another fact elucidated by this and similar inquiries, 

 which I have pursued elsewhere, 1 lies in the fact that the 

 difference between stimulants and poisons, so called, is often 

 one merely of degree. Thus a stimulatory reagent, if given 

 in large quantities, will be found to induce a profound 

 depression, whereas a poisonous reagent in minute quantities 

 may be found to act as a stimulant. In carrying out a 

 similar investigation with regard to growth response, I found 

 that sugar, for instance, which is stimulating in solutions of, 

 say, i to 5 per cent, becomes depressing when the solution 

 is very strong. Copper sulphate again, which is regarded as 

 a poison, is only so at I per cent, and upwards, a solution of 

 2 per cent, being actually a stimulant The difference 

 between sugar and copper sulphate is here seen to lie in the 

 fact that in the latter case the range of safety is very narrow. 

 Another fact, which must be borne in mind in this connec- 

 tion, is that a substance like sugar is used by the plant for 

 general metabolic processes, and thus removed from the 

 sphere of action. Thus continuous absorption of sugar could 

 not for a long time bring about sufficient accumulation to 

 cause depression. With copper sulphate, however, the case 

 1 Bose, Plant Response, p. 488. 



