bility to contact or impact has been developed in a number of carnivorous plants, which entrap and hold insects which serve as food; in the tendrils of climbing plants, which coil around supports and lift the foliage and flowers into sunlight; in a large class of "sensitive plants," which quickly fold their leaves on the reception of such a stimulus, thus avoiding injury from hail or grazing animals. The need of delicacy is much greater here than
Fig. 4.—Tendril of Passiflora Forty Seconds after Contact of Wooden Rod against Lower Surface. The dotted outline designates the original position of the organ.
in the previous forms described, and the response is much more marked and rapid. As an example of this form of irritability may be cited the contact reaction of tendrils. In Fig. 4 is shown the curvature of a tendril forty seconds after it has been lightly touched with a wooden rod.
With the general features of these reactions at hand, the question as to the mechanism by which they are accomplished becomes one of very great interest. The first point which naturally presents itself is the reception of the stimulus by the plant. Is every cell in the plant or organ affected by the stimulus, and do all bear an equal and similar part in the resulting movement? It is found by experiment that if the terminal region (see Fig. 1) of a root is cut away with a sharp razor, the root will no longer respond to gravity, but will remain in whatever position it may be placed. After a time, when the tip has been rehabilitated, the root regains its power of response to this force as before. The results of this and other experiments tend to show that the only part of the root which can receive the stimulus of gravity is a small mass of cells in the center of the tip. It may be seen, further, on reference to Fig. 1, that the curvature occurs in the fourth and fifth divisions from the tip. Here, then, is an instance in which a distinct mass of cells—"the perceptive zone"—receives the stimulus, and curvature results from the action of another mass of cells—"the motor zone"—several millimetres distant. This would, of course, imply that the impulse from the stimulus received by the perceptive zone was transmitted to the motor zone, and that the reception of the stimulus, the transmission of the im-
