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. 63, 630). 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, 
known as geotropism, which guides them into the trans- 
verse position. The climbing stem, like the erect one, 
