LATERAL ILLUMINATION : HELlOTROPlSM 253 



cupboard. Note that the heads close after being kept an hour or two in 

 darkness. Remove them to a bright situation and observe if they open 

 again. 



(iii) Movements induced by lateral illumination; heliotropism. 



— When a plant is allowed to grow undisturbed in the window 

 of an ordinary room, one side of its stem is illuminated much 

 more than the other ; in consequence of such lateral illumination 

 the growing part slowly bends over towards the light so that the 

 tip and a certain amount of the stem behind it ultimately points 

 in the direction from which the light comes. Similar curvature 

 is seen in stems of plants growing near walls, and in other situa- 

 tions where they receive light on one side more than the other. 



This bending like other cases of the curvature of growing 

 members is due to a difference in the rate and amount of 

 growth on the two sides of the stems, and like the movements of 

 leaves and roots mentioned below, is effected in response to the 

 stimulus of light falling upon the stem from one side. A small 

 portion near the tip of the stem is specially sensitive to lateral 

 illumination, and the stimulus it receives appears to be conducted 

 back to the part which bends in the peculiar manner described. 



If the tip of the stem of a seedling which exhibits such move- 

 ments is cut off or carefully covered by a cap of some material 

 through which light cannot pass, the characteristic curvature 

 does not occur. 



The same stimulus of lateral light when applied to roots- 

 induces an opposite movement from that observable in the 

 growing part of a stem. The growing part of a root curves 

 away from the stimulating light, and the tip and a small portion 

 near it, although they lie in the line of the incident light, point 

 away from it. 



The movements in response to the stimulus of lateral light 

 in which the plant members turn towards the light, like stems, 

 are spoken of as heliotropism or positive heliotropism, while the 

 terra apheliotropism or negative heliotropism is applied to move- 



