474 PLANT RESPONSE 



the growth-rate rose more or less continuously till 8 A.M. The 

 growth-rate then began to fall during the course of the day. 



The second record was taken with a seedling of Tama- 

 rindus indica fourteen days old, the diurnal periodicity being 

 thus deeply impressed. It was placed in the dark room, 

 mounted on the float, and the balanced record begun at 

 3 P.M. It will be seen that, as the positive after-effect of 

 the day's illumination, there was a depression of the rate 



FlG. 191. Continuous Photographic Record of Variation of Rate ot 

 Growth in Seedling of Tamarindus indica, a Fortnight Old, from 

 3 p.m. to 3 A.M. 



Owing to positive after-effect of daylight, there is a depression of rate of 

 growth, although the seedling was now placed in a dark room. In the 

 evening, however, the rale began to rise. 



of growth, though the plant was kept in the dark. This 

 persisted for two hours, till 5 P.M., after which the rate showed 

 increase, there being three pulsations before the end of the 

 record, at 3 A.M. (fig. 191). 



Annual rings of wood and seasonal periodicity. — The 

 different growths of wood in spring and autumn, leading to 

 the formations known as ' annual rings,' constitute a pheno- 

 menon of growth not yet fully explained. I may here point 

 out an important factor in connection with this subject. It 



