230 THE POPULAR SCIENCE MONTHLY. 



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 sup- 

 ports 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 Eod 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 ques- 

 tion as to the mechanism by which they are accomplished becomes 

 one of very great interest. The first point which naturally pre- 

 sents 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, fur- 

 ther, 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- 



