STEMS AND BRANCHES AND THEIR USES 195 



most light. The result of this tendency is that the branches 

 are more or less spread out. If the upper part of the trunk 

 of a tree is cut off, one (or more) of the uppermost branches 

 turns and grows upward, because that is now the direction 

 from which it receives most light. 



The effect of the stimulus of gravity upon a branch is usu- 

 ally quite different from its effect on the main stem. Instead 

 of tending to grow in a direction opposite to that of the force 

 of gravity, as the stem does, the branch tends to grow at a 

 right angle, or at some other definite angle, to the direction 

 of the force. Thus the stimulus of gravity is an important 

 factor in inducing branches to grow horizontally or diagonally. 

 The final shape of a branch results from the combined effects 

 of the stimuli of light and gravity (and in a measure doubtless 

 of some other stimuli as well). The influences of light and 

 gravity may have worked in the same direction, but oftener 

 they have been more or less opposed, and the actual direction 

 of growth of the branch is a compromise between the con- 

 flicting tendencies. 



215. Hollow Upright Stems. Many members of the grass 

 family, such as wheat, have upright stems which are cylindri- 

 cal and hollow except for a cross partition at each node. 

 The outer solid part of the stem is thin, but it contains a large 

 proportion of thick- walled cells. The stem is also supported 

 in part by the bases of the leaves, each of which forms a 

 sheath about the stem, extending for some distance upward 

 from the node to which the leaf is attached. The stem is 

 made still more stiff by silica, which is deposited in the walls 

 of its epidermal cells, and which makes the surface of the 

 stem feel hard and harsh. Some grass stems contain so much 

 silica that when one of them is burned a skeleton of silica re- 

 mains that has the exact form of the stem. Grasses gain a 

 great advantage from their hollow cylindrical stems, because 

 this form gives the greatest resistance to sidewise strains with 

 the use of the least material. The same mechanical principle 



