RESPONSIVE GROWTH-CURVATURES IN PLANTS 727 



Effect of temperature on growth.— That growth is an 

 excitatory phenomenon is seen, again, in the fact that it is 

 increased by any circumstance that tends to increase excit- 

 ability. Thus, for example, in the case of most tropical 

 phanerogamous plants, it is found that responsive excitatory 

 contraction is greatest at a temperature of about 35° C. ; and 

 this is also found to be the optimum temperature, at which 

 the natural rate of growth is at its maximum. 



I have described a method of obtaining a Thermo- 

 CRESCENT CURVE for the determination of the various rates 

 of growth which correspond to different temperatures. The 

 continuous record thus obtained in the course of about half 

 an hour affords us not only the rate of growth at any tem- 

 perature, but also a means of determining its optimum and 

 maximum points. The optimum temperature may also be 

 determined, with an accuracy within one-tenth of a degree, 

 by means of the Balanced Crescographic record. The 

 results obtained by all the different methods employed are 

 found to concur. The optimum point is thus shown, under 

 normal conditions, to be very constant (p. 451). It may be 

 said here that in the case also of plants which exhibit tor- 

 sional growth-response, the rate of torsional movement is 

 greatest at this optimum point. 



The arrest of growth which occurs at the maximum tem- 

 perature does not appear to be due to any cessation of 

 activity as brought on by rigor ; for we found in a record 

 taken from a seedling of Balsam at 44 C. that at this 

 temperature the constituent growth-pulsations had actually 

 become more frequent than before, the resultant abolition of 

 growth being due to the fact that response and recovery 

 were now equal. It was likewise shown that the apparent 

 arrest of the pulsatory movements of Desmodium at certain 

 high temperatures was not due to the cessation of activity, 

 but that at such temperatures the pulsations had become 

 more frequent and very minute (p. 431). The fact that at 

 the maximum temperature growth is not arrested by rigor 

 receives curious illustration, again, when the application of 



