440 PLANT RESPONSE 



mitted to the growing region, and finds expression in the 

 work of growth. 



A rise of temperature, up to the optimum, enhances the 

 rate of growth. At a maximum temperature of about 44 C. 

 growth is apparently arrested. This is not, however, due to 

 any rigor or arrest of internal activity, but to the fact that in 

 each pulsation of growth the constituent response and recovery 

 are now equal. There is thus no resultant growth-elongation. 

 At such a temperature the amplitude of pulsation is re- 

 duced, and the frequency increased, as in Desmodium. 



Longitudinal tension has the effect, up to an optimum, of 

 increasing the rate of growth. 



The effect of external stimulus on a growing, is precisely 

 the same as on a stationary, organ, that is to say, a respon- 

 sive contraction. On account of this contraction, and con- 

 comitant negative turgidity-variation, growth is retarded, as 

 the direct effect of the action of external stimulus. 



External stimulus, however, when absorbed and held 

 latent by the tissue, has the effect of increasing the internal 

 energy of the plant. This indirect effect of stimulus causes 

 acceleration of errowth. 



