The trees in a forest help each other by enriching the soil in which they stand with their fallen leaves and twigs, which are not quickly blown or washed away as are those under a tree in the open. This collection of "duff" or "the forest floor" retains the moisture about their roots, and this moist mass tends to keep the temperature of the forest warmer in winter and cooler in summer. The forest cover, Fig. 55, p. 212, consisting largely of foliage, has the same effect, and in addition protects the bark, the roots, and the seedlings of the trees from the direct and continuous hot rays of the sun. Without the shade of the leaves, many trees, as white pine, would quickly die, as may readily be seen by transplanting them to the open. The mass of standing trees tempers the force of the wind, which might overthrow some of them, and hinders the drying up of the duff. [Illustration: Fig. 70. Shallow Roots of Hemlock. Bronx Park, New York, N. Y.] But trees hinder as well as help each other. There is a constant struggle between them for nourishment and light. To get food and water, some trees, as spruces and hemlocks, Fig. 70, spread their roots out flat; others, as oak and pine, send down a deep tap root. Those succeed in any environment that find the nourishment they need. Still more evident is the struggle for light and air. However well a tree is nourished thru its roots, unless its leaves have an abundance of light and air it will not thrive and make wood. [Illustration: Fig. 71. Long-bodied White Oak of the Forest. _U. S. Forest Service._] Even the trees most tolerant of shade in youth, like spruce, must have light later or perish, and hence in a forest there is the constant upward reach. This produces the characteristic "long-bodied" trunk of the forest tree, Fig. 71, in contrast to the "short-bodied" tree of the open, where the branches reach out in all directions, Fig. 72. In this constant struggle for existence is involved the persistent attempt of scattered seeds to sprout whenever there is an opening. The result is that a typical forest is one in which all sizes and ages of trees grow together. Scattered among these are bushes and scrubby trees, called "forest weeds," such as mountain maple and dogwood, Fig. 80, p. 234, which do not produce timber. [Illustration: Fig. 72. Short-bodied White Oak of the Open. Fort Lee, N. J.] By foresters the trees themselves are classified according to their size into: Seedlings, less than 3' high, Saplings, Small, 3'-10' high. Large, 4" in diameter, at breast height (4' 6"). Poles, Small, 4"-8" in diameter, at breast height. Large, 8"-12" in diameter, at breast height. Standards, 1'-2' in diameter, at breast height. Veterans, over 2' in diameter, at breast height.