526 LIFE MOVEMENTS IN PLANTS 



phenomenon of universal occurrence. I therefore extended 

 my investigation on various geotropically curved procumbent 

 stems of Ipcemia, Basella, and of Tropceolum majus. Here 

 also I found that diurnal variation of temperature induced 

 a periodic movement exactly similar to that in Palm trees. 



I next wished to find whether the Thermo-geotropic 

 reaction observed in stems was also exhibited by lateral 

 organs such as leaves, which being spread out in a 

 horizontal direction are subjected to the stimulus of gravity. 

 I found that in a large number of typical cases, a perio- 

 dic movement took place which was exactly similar to that 

 given by rigid trees and trailing stems. A standard 

 curve was thus obtained which was found to be charac- 

 teristic not only of trees and herbs, but also of leaves. 

 The stem and leaves fell continuously with the rise of 

 temperature, from the minimum at about 6 in the morning 

 to the maximum at about 2 p.m. They erected themselves 

 with falling temperature from 2 p.m. to 6 a.m. next 

 morning. 



In the diurnal record of Mimosa I met, however, with 

 an unaccountable deviation from the standard curve, for 

 which I could not for a long time find an adequate explana- 

 tion. Subsequent investigations showed that the deviation 

 was due to the introduction of additional factors of varia- 

 tion, namely of immediate and after-effects of light. 



COMPLEXITY OP THE PROBLEM. 



I have already referred to the great difficulty of ex- 

 planation of nyctitropism from the fact that the diurnal 

 movements may be brought about by different agencies 

 independent of each other. It is, moreover, not easy to 

 discriminate the effect of one agency from that of the 

 other. 



