488 PLANT RESPONSE 



Opposite effects of large and small doses. — I have 

 shown the toxic effect of a I per cent, solution of copper 

 sulphate at the normal temperature of 30 C. If, however, 

 the dose be reduced to "2 per cent, we shall find that its 

 action becomes stimulatory (fig. 202). This is an interesting 

 illustration of the general fact that a poisonous reagent, if 

 given in sufficiently minute doses, will act as an excitant. 



In conclusion, a survey of the effects of drugs, both 

 stimulatory and poisonous, reveals the striking fact that the 

 difference between them is a question of quantity. Sugar, 



for instance, which is stimulat- 

 ing when given in solutions of, 

 say, 1 to 5 per cent., becomes 

 depressing when the solution 

 is very strong. Copper sul- 

 Fig. 202. Opposite Effects of phate again, which is regarded 



Large and Small Doses of i - . 



Poi g on as a poison, is only so at 1 per 



A solution of 1 per cent, copper cent - and Upwards, a solution of 



sulphate produces depression - 2 pe r cent, being actually a 



and death, but '2 per cent. 



exercises stimulating action. stimulant. The difference be- 



tween 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 sphere of action. Thus continuous absorp- 

 tion of sugar could not for a long time bring about sufficient 

 accumulation to cause depression. With copper sulphate, 

 however, the case is different. Here, the constant absorption 

 of the sub-toxic stimulatory dose would cause accumulation 

 in the system, and thus ultimately bring about the death of 

 the plant. 



Summary 



Chemical reagents are found to have the same effect on 

 the response of growth as on ordinary response, or as on the 

 autonomous response of Desmodium gyrans. 



