454 PLANT RESPONSE 



tion of growth at the different required temperatures. Three 

 responses were taken at each temperature, and were found to 

 be practically the same. Some of these records will be 

 given in the next chapter (fig. 185). I have given the 

 results of four such experiments, carried out on different 

 specimens. 



The translocation of the optimum point. — We have 

 thus seen how constant is the optimum point in the same 

 species, under normal conditions ; but, since we found that 

 the otherwise constant death-point was liable to be shifted 

 under the disturbance caused by the sudden variation of 

 external conditions (p. 172), so it would appear probable that 

 the optimum point also would be liable to transposition under 

 the influence of similarly disturbing causes. The optimum 

 point of the Crinum Lily has been seen to lie, normally 

 speaking, between 35*4° C. and 35'5° C. After a night of 

 heavy rain and gale, however, I found that the optimum 

 point of a specimen of this Lily had fallen to 34*6° C. Under 

 the action of a poison like copper sulphate, again, admi- 

 nistered in such dilution as not to kill, but only to retard 

 growth, I have observed the optimum point to be lowered 

 to 34'5° C. In the case of dilute solution of sugar, however, 

 which induces — as we shall see in the next chapter — an 

 increase of growth-activity, I have found the optimum point 

 to be raised to 36'6° C. 



Thus under normal conditions the optimum temperature 

 for each species is extremely definite. But circumstances 

 which increase or decrease the rate of growth abnormally, 

 operate also to transpose the optimum point, in the same 

 manner as the death -point was found to be translocated by 

 external influences. 



Summary 



The difficulties usually encountered in the accurate deter- 

 mination of the effect of temperature on growth have been 

 successfully overcome in the case of four distinct methods. 



In taking records of growth at different temperatures, it 



