230 CHAP. XXI. THE ' PRAYING * PALM 



leaves completely screen the deep-lying living tissue from 



light. 



2. The curve of the movement of the tree affords, how- 

 ever, data for correct inference as to the possible effect of 

 light. If the action of light is determined by its maximum 

 intensity, then there should be a climax at noon, and the 

 opposite at midnight. But the highest erection was reached 

 not at noon, but at 7 in the morning ; the lowest fall, on the 

 other hand, was attained not at midnight, but in the after- 

 noon. Again, if the movement was caused by the cumula- 

 tive action of light, then the maximum extent of movement, 

 either up or down, should be attained shortly before evening ; 

 but this was not the case, for it occurred several hours 

 earlier, at about 3 p.m. 



It is thus probable that light had very little influence 

 on the movement of the tree. I will describe additional 

 experiments in support of this conclusion. 



Effect of Variation of Temperature 



Turning next to the factor of thermal change, the 

 record shows that the curve of the movement of the 

 tree is practically a replica of the curve of variation of 

 temperature [see fig. 134). The rise of the tree followed the 

 fall of the temperature and vice versa. A lag will be 

 noticed at the turning-points of the movement ; thus, while 

 temperature began to rise at about 6 a.m., the tree did not 

 begin to fall till an hour afterwards. Again the turning- 

 point from rise to fall of temperature was after 2.45 p.m. ; the 

 downward movement of the tree was, however, not reversed 

 into one of erection till after 3.15 p.m., the lag being about 

 30 minutes. The delay is attributable to two causes : it 

 took some time for the thick trunk of the tree to attain 

 the temperature of its surroundings ; in addition to this, 

 physiological inertia delayed the reaction. 



In reference to the incidence of thermal noon, already 

 defined in the last chapter, it is modified (i) by the season ; 



