PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 747 



of growth, but only after an abnormal relaxation or expan- 

 sion. Alkali, on the other hand, induced arrest, but after 

 contraction. And, finally, the arrest induced by one was 

 counteracted by the effect of the application of the other 

 (p. 484). These facts, and others which have already been 

 fully described, afford a conclusive demonstration of the 

 essential unity of the physiological effects of drugs on plant 

 and animal tissues. 



The existence of such a unity having been established, it 

 is evident that much may be gathered from investigations 

 carried out on plants, as to the obscure question known to 

 medical practice as the modification of the effect of drugs 

 by individual constitutions ; for while a given individual will 

 succumb quickly to the action of a certain poison, another, 

 as is well known, will throw off its influence and survive. 

 Again, a particular dose of a given drug may have the effect 

 of producing excitation in one case, and in another profound 

 depression. The effect on a tissue of any given reagent, then, 

 does not merely represent the action of that reagent as such, 

 but is further determined also by the reacting power of the 

 responding tissue itself. And this reacting power is modified 

 by what is known as the individual constitution of the 

 organism. Thus no result can be definitely predicted of a 

 reagent, unless we have a precise knowledge of the action 

 of the same drug on various definite constitutions. This 

 problem, of the variations induced in the effects of drugs by 

 the different reacting powers of different constitutions, may 

 now be attacked, therefore, through the study of the plant, in 

 which, as I have shown, it is possible to induce known 

 differences of constitution by artificial means. 



It was shown, for example, in one case that, while a 5 

 per cent, solution of the poisonous reagent, copper sulphate, 

 produced an immediate depression, quickly followed by death, 

 another similar plant, whose tonic condition had been raised 

 to the optimum, was found to withstand the action of this 

 poison for a considerable time, the immediate effect being an 

 actual exaltation of its response. The opposite effects of the 



