20 



PLANTS 



the neighboring tree. If the tree is closely surrounded by others 

 of approximately the same age, as in a forest, the trunk will be 

 taller in proportion to its diameter and there will be no large 

 limbs near the ground. These facts show clearly that the larger 

 branches develop only where they can reach the light, whereas 



Fig. 3. — Three oak- trees in a group, showing the effect of one tree on 

 another with regard to the development of the branches. In the middle tree 

 the branches extend toward, and away from, the observer. 



those twigs which appear from time to time in shaded situ- 

 ations fail to develop into large branches because of the lack of 

 light, and, after a few seasons' struggle against adverse circum- 

 stances, ultimately die and fall away. 



36. The stem of erect plants is usually a cylinder of woody 



