In the middle of a forest, trees grow straight, tall, and slender, as in Fig. 12, because it is necessary for them to push up the tops in order that they may receive sufficient sunlight, to enable the leaves to digest the plant-food and increase the diameter and height of the stem. Lower branches last only a few years, then die, and are broken off (b, Fig. 12). On the margins of the forest and in open places, trees send out numerous branches, and stems become large in diameter, but remain short (Fig. 11). The bordering trees, while they serve as a protection from the wind for those inside, furnish knotty and cross-grained lumber; those inside produce the straight-grained and valuable wood (Fig. 13). Members of the palm group rarely have branching stems. In growth, the stems remain long and slender, but frequently larger at the top than at the base.

Fig. 11.

Fig. 12.

Fig. 13.

Fig. 11. - Shape of a tree on the border of a forest, a, broken branch exposing surfaces for boring insects or fungus spores. Fig. 12. - Young forest tree. b. b, branches die for want of sunlight. Fig. 13. - Shape of forest tree with straight stem and crown of small branches and leaves.