CHAP, xi Sensitivity and Stimulation 489 



ness have revealed a complicated mechanism associated 

 with them, proving that they play an important part in 

 modifying the ordinary phenomena of life, in causing pur- 

 poseful movements and in adjusting structure to varying 

 conditions of environment. 



In rapidly reviewing the progress of knowledge of these 

 various kinds of sensitiveness, we have passed somewhat 

 lightly over many features of general interest. 



Up to the end of the century comparatively little was 

 determined, except in a very general way, as to the sensory 

 zones, or areas of perception of stimuli. Their existence 

 was proved by many observers, but their localization and 

 particularly their character were not very closely examined. 

 The most complete instance afforded was the root-tip, to 

 which Darwin attributed, as we have seen, the possession 

 of such varied perceptive powers. The root-tip was 

 studied more recently by McDougal, who in 1896 deter- 

 mined it to be indebted for its sensitiveness to a mass of 

 periblem in the shape of an inverted cup which extends 

 axially for a distance of 1-2 mm., and reaches approxi- 

 mately to the forward edge of the motor zone. The bottom 

 of the cup is formed by the punctum vegetationis. Less- 

 studied parts are the tips of certain cotyledons, certain 

 epidermal outgrowths of leaves, special portions near the 

 tips of tendrils, certain pulvini, portions of the general 

 surface of leaves and stems, to which we have already 

 referred. The only fact that was conclusively shown is 

 that they exhibit no histological differentiation by which 

 at present they can be identified. Noll suggested that the 

 perceptive region of the protoplasm is the denser ectoplasm 

 which is relatively stable in its position in the cells. 



More extended observations, ranging over almost the 

 whole period under review, were made upon the mechanism 

 of movement or curvature, and the other results which 

 follow the reception of various stimuli. Some of these, of 



