SHOOTS 



69 



This has the general result of placing the leaf blades at 

 right angles to the rays of light, and in this respect the 

 heliotropism of the stem aids in securing a favorable leaf 

 position (see Figs. G3, G3a). Prostrate stems are differently 

 affected by the light, however, being directed transversely 

 to the rays of light. The same is true of many foliage 



Fig. 60. An oak in its winter condition, showing the wide branching. The various 

 directions of the branches have been determined by the light-relations. 



branches, as may be seen by observing almost any tree in 

 ■which the lower branches are in the general transverse posi- 

 tion. These branches generally tend to turn upwards when 

 they are beyond the region of shading. Subterranean stems 

 are also mostly horizontal, but they are out of the influence 

 of light, and under the influence of gravity, the response to 

 which is known as geotropism, which guides them into the 

 transverse position. The climbing stem, like the erect one, 



