IRRITABILITY AND MOVEMENT IN PLANTS. 229 



plants in response to this stimulus are comparatively simple. 

 Light has its source in the sun, which varies its position through 

 one hundred and eighty degrees during the daytime and is wholly 





11 



Fig. 2. Upward Curvature of a Cclm of Grass which has been placed in a Hori- 

 zontal Position. The dotted outline denotes the original position of the plant. 



absent at night. In consequence the movements of the plant to 

 adjust its surface to this stimulus of ever-varying direction and 

 intensity are of great complexity. This variability of the stimu- 

 lus has, moreover, induced in the plant a delicacy of irritability 

 toward light far beyond that exhibited toward gravity. The man- 

 ner in which leaves react to light may be seen in Fig. 3. 



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Fig. 8. Diagram of Light Position of Leaves. The arrows indicate the direction of the 

 rays which have fallen on each plant separately. 



In a large and varied category of plants it has become of great 

 importance that they execute certain movements when solid bodies 

 come in contact with them or strike their surfaces. This irrita- 



