STIMULATION AND ITS KESULTS 385 



affect more or less the whole plant, we find instances of 

 special sensitiveness in various parts to influences which 

 are not appreciated by the whole of the living substance, 

 but are especially received by the young growing regions. 

 Of these the most prominent are lateral light, gravity, contact 

 ivith foreign bodies, moisture, and certain chemical stimuli. 

 The effects produced, however, must be regarded in all 

 these cases as the action of the whole organism, which 

 modifies and co-ordinates the behaviour of its several parts 

 for the good of the plant. The usual form of response 

 consists in various modifications of local growth. 



One or two other cases of special sensitiveness affecting 

 only particular organisms may also be discussed. 



LATERAL LIGHT. The effect of the lateral incidence of 

 light may be studied very easily in the case of young seed- 

 lings. When one of these is so placed that one side of its 

 stem is more brightly illuminated than the opposite, a 

 curvature soon appears in the part which is actively growing. 

 This is of such a nature, and takes place to such an extent, 

 as to cause the axis of the plant to take up a position in 

 which it is parallel to the direction of the incident rays. 

 It manifests itself in some cases very rapidly, in others 

 more slowly. This response to the stimulus of a lateral 

 illumination is not confined to the stems of seedlings, but 

 may be seen to a greater or less degree in many adult plants. 

 It is a matter of common observation that geraniums grown 

 in a window all bend their stems and petioles towards 

 the illuminated side. 



In other cases the same stimulus may produce an opposite 

 effect. When certain young roots are exposed to it, they 

 curve so as to place themselves in the same position with 

 regard to the incident rays, but with their growing apices 

 in the opposite direction. Steins are said accordingly to 

 grow towards, and roots away from, the light-source. This 

 behaviour is not, however, confined to roots ; it is exhibited 

 by the tendrils of Bignonia capreolata, the peduncles of 

 Cyclamen persicum, and by many other organs. 



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