94 Conclusions 



stimulating concentrations varying both with the poisons used and 

 the plants on which they act. 



It is quite possible for a stimulation m. one respect to be correlated 

 with a retardation in another. In the Kothamsted experiments on the 

 action of manganese sulphate on barley the. weaker concentrations of 

 the salt improved the vegetative gro.wth, as was shown by the increase 

 in the dry weights, but with the same strengths of the poison the 

 ripening of the grain was retarded, so that, while certain of the 

 physiological functions were expedited, others were hindered by the 

 action of the poison. 



Thus it is evident that it is exceedingly difficult sharply to charac- 

 terise either toxic or stimulant action. In neither case is the reaction 

 simple — many factors may come into play and many processes are 

 concerned, while the effect of a so-called poison may vary in respect 

 of each of the functions and processes concerned. If the poison is 

 presented in great strength the toxic action is dominant, and probably 

 affects many functions in the same sense, so that the action is, so to 

 speak, cumulative. Lower down in the scale of concentration differ- 

 entiation of action may set in, and while some processes may still 

 be hindered, others may be stimulated. If the two actions balance 

 one another an apparent indifference may be manifested, so that it 

 seems that such strengths of the poison have no effect on growth, either 

 harmful or beneiicial. At still lower concentrations, with certain plants 

 and certain poisons, the stimulative action overpowers the toxic effect, 

 so that in some respect or other improvement occurs in growth. 



It is quite conceivable, however, that some poisons are truly indif- 

 ferent in weak concentrations, as no stimulation makes itself evident 

 under any circumstances. In these cases one is inclined to suspect 

 that the action is somewhat more simple, in that the toxic effects 

 gradually diminish until no poisonous action is manifest at very 

 weak concentrations, and as no stimulation is present to bring the 

 growth above the normal with these very weak concentrations the 

 plant is similar to those grown without any addition of the poison. 



The modus operandi of these stimulative agents is not yet fully 

 understood. Perhaps at the present time two main theories hold the 

 field: (1) that they act as catalytic agents, being valueless on their 

 own account, but valuable in that they aid in the procuring of es- 

 sential food substances; (2) that the stimulants themselves are of 

 integral value for nutrition. The French school, with Bertrand at 

 the head, hold strongly to the catalytic theory, maintaining that 



