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
Fig. 2.—Upward Curvature of a Culm of Grass which has been placed in a Horizontal 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 stimulus has, moreover, induced in the plant a delicacy of irritability toward light far beyond that exhibited toward gravity. The manner in which leaves react to light may be seen in Fig. 3.
Fig. 3.—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-
