378 VEGETABLE PHYSIOLOGY 



contact with foreign bodies, moisture, and certain chemical 

 stimuli. 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 

 seedlings. 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. Stems 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. 



Leaves in many cases show a similar sensitiveness, but 

 the position they assume is different again. They place 

 themselves so as to present their upper surfaces at right 

 angles to the incident rays. 



These phenomena, thus associated with the incidence 

 of a lateral light, are spoken of as heliotropism, aphelio- 

 tropism, and diaheliotropism respectively. The advantages 

 thus secured are in some cases very obvious ; thus the 

 heliotropism of a stem places its leaves in the most favour- 

 able position for the action of the chlorophyll in the 



