THE GROWTH OF A TREE.
The great chestnut tree on the hillside has cast its bur den of ripe nuts, flung down the empty burs, and given its yellow leaves to the autumn winds. Now the owner has cut down its twin, which was too near a neighbor for the well-being of either, and is converting it into lumber. The lopped limbs have gone to the woodpile, and the boards will be dressed and polished and used for the woodwork of the new house. Here is our opportunity to see what the bark of the living tree conceals—to study the anatomy of the tree to learn something of grain and wood rings and knots.
The most amazing fact is that this "too, too solid flesh" of the tree body was all made of dirty water and carbonic acid gas. Well may we feel a kind of awe and reverence for the leaves and the cambium—the builders of this wooden structure we call a tree. The bark, or outer gar ment, covers the tree completely, from tip of farthest root to tip of highest twig. Under the bark is the slimy, colorless living layer, the cambium, which we may define as.
the separation between wood and bark. It seems to have no perceptible diameter, though it impregnates with its substance the wood and bark next to it. This cambium is a continuous undergarment, lining the bark everywhere, covering the wood of every root and every twig as well as of the trunk and all its larger divisions.
Under the cambium is the wood, which forms the real body of the tree. It is a hard and fibrous substance, which in cross section of root or trunk or limb or twig is seen to be in fine, but distinctly marked, concentric rings about a central pith. This pith is most conspicuous in the twigs.
Now, what does the chestnut tree accomplish in a single growing season? We have seen its buds open in early spring and watched the leafy shoots unfold Many of these bore clusters of blossoms in midsummer, long yellow spikes, shaking out a mist of pollen, and falling away at length, while the inconspicuous green flowers developed into spiny, velvet-lined burs that gave up in their own good time the nuts which are the seeds of the tree.
The new shoots, having formed buds in the angles of their leaves, rest from their labors. The tree had added to the height and breadth of its crown the exact measure of its new shoots. There has been no lengthening of limb or
trunk. But underground the roots have made a season's growth by extending their tips. These fresh rootlets clothed with the velvety root hairs are new, just as the shoots are new that bear the leaves on the ends of the branches.
There is a general popular impression that trees grow in height by the gradual lengthening of trunk and limbs. If this were true, nails driven into the trunk in a vertical line would gradually become farther apart. They do not, as observation proves. Fence wires stapled to growing trees are not spread apart nor carried upward, though the may serve as posts for years, and the growth in diameter may swallow up staple and wire in a short time. Normal wood fibres are inert and do not lengthen. Only the season's rootlets and leafy shoots are soft and alive and capable of lengthening by cell division.
The work of the leaves has already been described. The return current, bearing starch in soluble form, flows freely among the cells of the cambium. Oxygen is there also. The cambium cell in the growing season fulfils its life mis sion by absorbing food and dividing. This is growth— and the power to grow comes only to the cell attacked by oxygen. The rebuilding of its tissues multiplies the sub stance of the cambium at a rapid rate. A cell divides, producing two "daughter cells." Each is soon as large as its parent, and ready to divide in the same way. A cam bium cell is a microscopic object, but in a tree there are millions upon millions of them. Consider how large an area of cambium a large tree has. It is exactly equivalent to the total area of its bark. Two cells by dividing make four. The next division produces eight, then sixteen, thirty-two, sixty-four, in geometric proportion. The cell's power and disposition to divide seems limited only by the food and oxygen supply. The cambium layer itself remains a very narrow zone of the newest, most active cells. The margins of the cambium are crowded with cells whose walls are thickened and whose protoplasm is no longer active. The accumulation of these worn-out cells forms the total of the season's growth, the annual ring of wood on one side of the cambium and the annual layer of dark on the other.