CHAPTER VIII 



CONCLUSIONS 



IN the foregoing chapters a very limited number of plant poisons 

 have been considered, yet there is sufficient evidence to show that 

 even these few differ considerably in their action upon plant-life. 

 This action is most variable, and it is impossible to foretell the effect 

 of any substance upon vegetative growth without experiments. The 

 degree of toxicity of the different poisons is not the same, and also 

 one and the same poison varies in the intensity and nature of its 

 action on different species of plants. While certain compounds of 

 copper, zinc and arsenic are exceedingly poisonous, compounds of 

 manganese and boron are far less deleterious, so that a plant can 

 withstand the presence of far more of the latter substances than of 

 the former. Again, the tested compounds of copper, zinc and arsenic 

 do not seem to stimulate growth, even when they are applied in the 

 smallest quantities, whereas very dilute solutions of manganese and boron 

 compounds decidedly increase growth. But, differentiation occurs even 

 in this stimulative action, for while manganese is the more effective in 

 stimulating barley, boric acid is far more potent for peas, the shoots 

 being particularly improved. 



A consideration of the experimental work that has been done on 

 this subject of poisoning and stimulation leads one to the inevitable 

 conclusion that it is not true to maintain the hypothesis that all 

 inorganic plant poisons act as stimulants when they are present in 

 very small quantities, for while some poisons do increase plant growth 

 under such conditions, others fail to do so in any circumstances. It 

 is probable that what has been found true with the few substances 

 tested would prove to be similarly true over a much wider range of 

 poisons, and at any rate the hypothesis must be dismissed in its 

 universal application. A more accurate statement would be that some 

 inorganic poisons act as stimulants when present in small amounts, the 



