640 COMPARATIVE ELECTRO-PHYSIOLOGY 



however, carrying the substance acted upon to the phase of 

 D or E, will cause depression. Or it is conceivable that the 

 same depression might be more rapidly induced by more 

 intense stimulation that is to say, by a larger dose. 



Now that this is what actually takes place has already 

 been shown in several experiments which have been 

 described. We saw, for example (fig. 95) that the continued 

 action of the moderately stimulating agent, sodium car- 

 bonate, at first induced an exaltation of response, followed 

 later by depression. In the case of vegetal nerve, again 

 (cf. fig. 297), we found that the same agent, in smaller doses, 

 caused at first an enhancement of conductivity, followed 

 later by slow depression. A stronger dose of the same 

 reagent, however, was found to cause rapid depression 

 (fig. 298). Even in the case of poisons, so-called, the same 

 facts make their appearance. Here an agent which proves 

 toxic in large, appears as a stimulant when given in minute, 

 doses. Thus in studying the effect of various chemical 

 agents on growth-response, I found that while a one per cent, 

 solution of copper sulphate was toxic, the same reagent 

 proved stimulatory, if given in a solution of *2 per cent. 

 A more detailed account of these experiments will be found 

 in my work on ' Plant Response,' from which I quote the 

 following summing up : 



* A survey of the effects of drugs, both stimulating and 

 poisonous, reveals the striking fact that the difference 

 between them is [often] a question of quantity. Sugar, for 

 instance, which is stimulating when given in solutions of, 

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

 is very strbng. 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 connection 

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

 general metabolic processes, and thus removed from the 



