EFFECT OF LIGHT 229 



movement of the tree was considerable, means had to be 

 employed for the reduction of the movement recorded by 

 the lower writing lever. On the oscillating plate successive 

 dots were made at intervals of 15 minutes, the thicker 

 dots being inscribed at intervals of an hour. The erectile 

 movement of the tree is represented as an up-curve, while 

 rise of temperature is recorded as a down-curve. 



The Diurnal Record of the Tree 



Experiment 126. — Hasty observation led people to 

 beheve that the tree hfted itself at sunrise and prostrated 

 itself at sunset ; but the continuous record obtained with 

 my apparatus proved that the tree was never at rest, but 

 in a state of continuous movement which underwent 

 periodic reversal (fig. 134). The tree attained its maximum 

 erection at 7 in the morning, after which there was a rapid 

 fall. The downward movement reached its maximum at 

 3.15 P.M., after which it began to hft itself very slowly, then 

 more rapidly, until it hfted itself to the highest position at 

 7 next morning. This diurnal periodicity was maintained 

 day after day. TJie movement was by no means passive, hi.t 

 was effected with an active force sufficient to lift a man off the 

 ground. 



The next point is the determination of the relative 

 effects of variations of light and of temperature in inducing 

 the diurnal movement. 



The Relative Effect of Light 



The following considerations will help in deciding 

 whether or not light had any perceptible influence in the 

 production of the periodic movement of the tree. 



I. Since the movement has been shown to be a physio- 

 logical phenomenon, the stimulus of light, in order to be 

 effective, must act directly on the living tissue. In the 

 case of the Palm this is impossible, for the bark of the tree 

 is of considerable thickness, and the thick bases of dead 



