REVIVAL OF GROWTH 83 



to feeble light, or to stronger light, converted the accelera- 

 tion into the normal retardation. 



Opposite effects of feeble and strong stimulation. —Thus 

 while strong stimulation induces retardation of the rate of 

 growth, feeble stimulation causes an enchancement of the 

 rate. In the wide range of stimulation between minimal 

 and maximal there is therefore a critical intensity above 

 which there is a retardation and below which there is 



Fig. 39. Acceleration of growth under subminimal stimulus of 



light (Kysoor). 



an acceleration. This critical intensity varies in different 

 species of plants. 



As chemical substances often cause stimulation, the 

 opposite effects of small or large doses of the same drug 

 may perhaps afford parallel phenom.ena. 



Revival of Growth Previously at Standstill 



Experiment 48.— An organ falls to a condition of extreme 

 subtonicity when it is maintained for a long time under 

 unfavourable conditions. A peduncle of Allium was kept 

 in the dark for a fortnight, after which its growth was found 

 to have been practically arrested. The plant attached to 

 the Crescograph then gave an almost horizontal record on 



