SENSITIVENESS OF ROOTS. 415 



1073. It has been shown (1024) that young shoots react, 

 although somewhat sluggishly, against mechanical shock, their 

 change of form or direction depending on the character or direc- 

 tion of the blows received. In certain delicate tissues, especially 

 those which possess much simplicity of structure, change of form 

 and of direction may be produced in response to comparatively 

 slight mechanical or chemical irritation. It is to these that the 

 term sensitive tissues is properly applied. 



1074. Sensitiveness of roots. The tip of the caulicle is gen- 

 erally sensitive to contact and to caustics. There are, however, 

 great differences in the degree of sensitiveness ; in some cases 

 slight contact being sufficient to cause reaction, while in others 

 the contact must be prolonged and accompanied by direct pres- 

 sure. If the caulicle with its unformed root is placed under 

 conditions where growth can take place with great rapidity, the 

 sensitiveness is much impaired and sometimes is wholly lost ; 

 it is partially lost also when the caulicle grows slowly, or is 

 forced to grow out of season. Under natural conditions and at 

 a normal rate of growth the tip is sensitive for about one twen- 

 tieth of an inch. If a piece of caustic is applied to the tip (not 

 more than 1.5 mm. from the very end), the caulicle will curve 

 awa3* from the irritated side. The reaction is as plainly seen in 

 those cases where the caulicle does not elongate, but where the 

 root itself descends. 



1075. The length of the portion of these organs which reacts is 

 about ten millimetres. The time of reaction varies for different 

 plants, being sometimes in five hours, and, according to Darwin, 

 almost always within twentj'-four hours. 



1076. " The curvature often amounts to a rectangle ; that is, 

 the terminal part bends upwards until the tip, which is but little 

 curved, projects almost horizontally. Occasionally the tip, from 

 the continued irritation of the attached object, continues to bend 

 up until it forms a hook with the point directed towards the 

 zenith, or a loop, or even a spire. After a time the radicle 

 apparently becomes accustomed to the irritation, as occurs in 

 the case of tendrils ; for it again grows downwards, although 

 the bit of card or other object may remain attached to the 

 tip." 1 



1077. The tip of the radicle has been shown (1046) to be 

 constantly circumnutating. By this movement the sensitive tip 

 is brought into contact with different sides of minute crevices in 



1 Power of Movement in Plants, 1880, p. 193. 



