Anna B. Comstock

A Pacific Coast live oak showing the effects of constant, strong winds from one direction.  Photo by G. K. Gilbert.   Courtesy of U. S. Geological Survey.

Tree Study

Teacher's Story

ATURAL is our love for trees! A tree is a living being, with a life comparable to our own. In one way it differs from us greatly: it is stationary, and it has roots and trunk instead of legs and body; it is obliged to wait to have what it needs come to it, instead of being able to search the wide world over to satisfy its wants.

A tree with parts named.

The roots, which extend out in every direction beneath the surface of the ground, have two quite different offices to perform: First, they absorb the water which contains the tree food dissolved from the soil; second, they hold the tree in place against the onslaught of the winds. If we could see a tree standing on its head with its roots spread in the air in the same manner as they are in the ground, we could then better understand that there is as much of the tree hidden below ground as there is in sight above ground, although of quite different shape, being flatter and in a more dense mass. The roots seem to know in which direction to grow to reach water; thus, the larger number of the roots of a tree are often found to extend out toward a stream flowing perhaps some distance from the tree; when they find plenty of food and water the rootlets interlace forming a solid mat. On the Cornell Campus are certain elms which, every six or seven years, completely fill and clog the nearby sewers; these trees send most of their roots in the direction of the sewer pipe. The fine rootlets upon the tree-roots are covered with root-hairs, which really form the mouths by which the liquid food is taken into the tree.

To understand how firm a base the roots form to hold up the tall trunk, we need to see an uprooted tree. The great roots seem to be molded to take firm grasp upon the soil. It is interesting to study some of the "stump fences" which were made by our forefathers, who uprooted the white pines when the land was cleared of the primeval forest, and made fences of their widespreading but rather shallow extending roots. Many of these fences stand to-day with branching, out-reaching roots, white and weather-worn, but still staunch and massive as if in memory of their strong grasp upon the soil of the wilderness.

The upturned roots of a white pine; a part of a stump fence a century old.

The trunk, or bole, or stem of the tree has also two chief offices: It holds the branches aloft, rising to a sufficient height in the forest so that its head shall push through the leaf canopy and expose the leaves to the sunlight. It also is a channel by which the water containing the food surges from root to leaf and back again through each growing part. The branches are divisions of the trunk, and have the same work to do.

In cross-section, the tree trunk shows on the outside the layer of protective bark; next to this comes the cambium layer, which is the vital part of the trunk; it builds on its outside a layer of bark, and on its inside a layer of wood around the trunk. Just within the cambium layer is a lighter colored portion of the trunk, which is called the sap-wood because it is filled with sap which moves up and down its cells in a mysterious manner; the sap-wood consists of the more recent annual rings of growth. Within the sap-wood are concentric rings to the very center or pith; this portion is usually darker in color and is called the heartwood; it no longer has anything to do with the life of the tree, but simply gives to it strength and staunchness. The larger branches, if cut across, show the same structure as the trunk,—the bark on the outside, the cambium layer next, and within this the rings of annual growth. Even the smaller branches and twigs show similar structure, but they are young and have not attained many annual rings.

The leaves are borne on the outermost parts of the tree. A leaf cannot grow, and if it could would be of no use, unless it can be reached by the sunlight. Therefore the trunk lifts the branches aloft, and the branches hold the twigs far out, and the twigs divide into the fine spray, so as to spread the leaves and hold them out into the sunshine. In structure, the leaf is made up of the stem, or petiole, and the blade, or widened portion of the leaf, which is sustained usually with a framework of many ribs or veins. The petioles and the veins are sap channels like the branches and twigs.

The Way a Tree Grows

HE places of growth on a tree may be found at the tips of the twigs and the tips of the rootlets; each year through this growth the tree pushes up higher, down deeper and out farther at the sides. But in addition to all of these growing tips, there is a layer of growth over the entire tree—over every root, over the trunk, over the limbs and over each least twig, just as if a thick coat of paint had been put over the complete tree. It is a coat of growth instead, and these coats of growth make the concentric rings which we see when the trunks or branches are cut across. Such growth as this cannot be made without food; but the tree can take only liquid food from the soil; the root-hairs take up the water in which the "fertilizer" is dissolved, and it is carried up through the larger roots, up through the sap-wood of the trunk, out through the branches to the leaves, where in the leaf-factories the water and free oxygen is given off to the air, and the nourishing elements retained and mixed with certain chemical elements of the air, thus becoming tree food. The leaf is a factory; the green pulp in the leaf cells is part of the machinery; the machinery is set in motion by sunshine power; the raw materials are taken from the air and from the sap containing food from the soil; the finished product is largely starch. Thus, it is well, when we begin the study of the tree, to notice that the leaves are so arranged as to gain all the sunlight possible, for without sunlight the starch factories would be obliged to "shut down." It has been estimated that on a mature maple of vigorous growth there is exposed to the sun nearly a half acre of leaf surface. Our tree appears to us in a new phase when we think of it as a starch factory covering half an acre.

Starch is plant food in a convenient form for storage, and it is stored in sap-wood of the limbs, the branches and trunk, to be used for the growth of the next year's leaves. But starch cannot be assimilated by plants in this form, it must be changed to sugar before it may be used to build up the plant tissues. So the leaves are obliged to perform the office of stomach and digest the food they have made for the tree's use. In the mysterious laboratory of the leaf-cells, the starch is changed to sugar; and nitrogen, sulphur, phosphorus and other substances are taken from the sap and starch added to them, and thus are made the proteids which form another part of the tree's diet. It is interesting to note that while the starch factories can operate only in the sunlight, the leaves can digest the food and it can be transported and used in the growing tissues in the dark.  The leaves are also an aid to the tree in breathing, but they are not especially the lungs of the tree. The tree breathes in certain respects as we do; it takes in oxygen and gives off carbon dioxide; but the air containing the oxygen is taken in through the numerous pores in the leaves called stomata, and also through lenticels in the bark; so the tree really breathes all over its active surface.

The tree is a rapid worker and achieves most of its growth and does most of its work by midsummer. The autumn leaf which is so beautiful has completed its work. The green starch-machinery or chlorophyl, the living protoplasm in the leaf cells, has been withdrawn and is safely secluded in the woody part of the tree. The autumn leaf which glows gold or red, has in it only the material which the tree can no longer use. It is a mistake to believe that the frost causes the brilliant colors of autumn foliage; they are caused by the natural old age and death of the leaves—and where is there to be found old age and death more beautiful? When the leaf assumes its bright colors, it is making ready to depart from the tree; a thin, corky layer is being developed between its petiole and the twig, and when this is perfected, the leaf drops from its own weight or the touch of the slightest breeze.

A stump showing rings of growth.

A tree, growing in open ground, records in its shape, the direction of the prevailing winds. It grows more luxuriantly on the leeward side. It touches the heart of the one who loves trees to note their sturdy endurance of the onslaughts of this, their most ancient enemy.

Reference Books for Tree Study—The Tree Book, Julia Rogers; Our Native Trees, Harriet Keeler; Our Northern Shrubs, Harriet Keeler; The Trees of the Northern States, Romayne Hough. The Trees, N. L. Britton; Getting Acquainted with the Trees, J. Horace McFarland; Familiar Trees and their Leaves, Schuyler Mathews; Our Trees and How to Know Them, Clarence Moores Weed; A Guide to the Trees, Alice Lounsberry; The First Book of Forestry, Filibert Roth; Practical Forestry, John Gifford; Trees in Prose and Poetry, Stone & Fickett; The Primers of Forestry, Pinchot.