Under the Maples. John Burroughs
are mainly for strength and to hold it to the ground. How they grip the rocks, fitting themselves to them, as Lowell says, like molten metal! The tree's life is in the fine hairlike rootlets that spring from the roots. Darwin says those rootlets behave as if they had minute brains in their extremities. They feel their way into the soil; they know the elements the plant wants; some select more lime, others more potash, others more magnesia. The wheat rootlets select more silica to make the stalk; the pea rootlets select more lime: the pea does not need the silica. The individuality of plants and trees in this respect is most remarkable. The cells of each seem to know what particular elements they want from the soil, as of course they do.
The vital activity of the tree goes on at three points—in the leaves, in the rootlets, and in the cambium layer. The activity of the leaf and rootlet furnishes the starchy deposit which forms this generative layer—the milky, mucilaginous girdle of matter between the outer bark and the wood through which the tree grows and increases in size. Generation and regeneration take place through this layer. I have called it the girdle of perpetual youth. It never grows old. It is annually renewed. The heart of the old apple-tree may decay and disappear, indeed the tree may be reduced to a mere shell and many of its branches may die and fall, but the few apples which it still bears attest the fact that its cambium layer, at least over a part of its surface, is still youthful and doing its work. It is this layer that the yellow-bellied woodpecker, known as the sapsucker, drills into and devours, thus drawing directly upon the vitality of the tree. But his ravages are rarely serious. Only in two instances have I seen dead branches on an apple-tree that appeared to be the result of his drilling.
What we call the heart of a tree is in no sense the heart; it has no vital function, but only the mechanical one of strength and support. It adds to the tree's inertia and power to resist storms. The trunk of a tree is like a community where only one generation at a time is engaged in active business, the great mass of the population being retired and adding solidity and permanence to the social organism. The rootlets of a plant or a tree are like the laborers in the field that produce for us the raw material of our food, while the leaves are like our many devices for rendering it edible and nourishing. The rootlets continue their activity in the fall, after the leaves have fallen, and thus gorge the tree with fluid against the needs of the spring. In the growing tree or vine the sap, charged with nourishment, flows down from the top to the roots. In the spring it evidently flows upward, seeking the air through the leaves. Or rather, we may say that the crude sap always flows upward, while the nutritive sap flows downward, thus giving the tree a kind of double circulation.
A tree may be no more beautiful and wonderful when we have come to a knowledge of all its hidden processes, but it certainly is no less so. We do not think of the function of the leaves, nor of the bark, nor of the roots and rootlets, when we gaze upon a noble oak or an elm; we admire it for its form, its sturdiness, or its grace; it is akin to ourselves; it is the work of a vast community of cells like those that build up our own bodies; it is a fountain of living matter rising up out of the earth and splitting up and spreading out at its top in a spray of leaves and flowers; and if we could see its hidden processes we should realize how truly like a fountain it is. While in full leaf a current of water is constantly flowing through it, and flowing upward against gravity. This stream of water is truly its life current; it enters at the rootlets under the ground and escapes at the top through the leaves by a process called transpiration. All the mineral salts with which the tree builds up its woody tissues,—its osseous system, so to speak,—the instruments with which it imprisons and consolidates the carbon which it obtains from the air, are borne in solution in this stream of water. Its function is analogous to that of the rivers which bring the produce and other material to the great cities situated upon their banks. A cloud of invisible vapor rises from the top of every tree and a thousand invisible rills enter it through its myriad hairlike rootlets. The trees are thus conduits in the circuit of the waters from the earth to the clouds. Our own bodies and the bodies of all living things perform a similar function. Life cannot go on without water, but water is not a food; it makes the processes of metabolism possible; assimilation and elimination go on through its agency. Water and air are the two ties between the organic and the inorganic. The function of the one is mainly mechanical, that of the other is mainly chemical.
As the water is drawn in at the roots, it flows out at the top, to which point it rises by capillary attraction and a process called osmosis. Neither of them is a strictly vital process, since both are found in the inorganic world; but they are in the service of what we call a vital principle. Some physicists and biochemists laugh at the idea of a vital principle. Huxley thought we might as well talk about the principle of aqueosity in water. We are the victims of words. The sun does not shoot out beams or rays, though the eye reports such; but it certainly sends forth energy; and it is as certain that there is a new activity in matter—some matter—that we call vital.
Matter behaves in a new manner; builds up new compounds and begets myriads of new forms not found in the inorganic world, till it finally builds up the body and mind of man. Death puts an end to this activity alike in man and tree, and a new kind of activity sets in—a disorganizing activity, still with the aid of water and air and living organisms. It is like the compositor distributing his type after the book is printed. The micro-organisms answer to the compositor, but they are of a different kind from those which build up the body in the first instance. But the living body as a whole, with its complex of coördinating organs and functions—what attended to that? The cells build the parts, but what builds the whole?
How many things we have in common with the trees! The same mysterious gift of life, to begin with; the same primary elements—carbon, nitrogen, oxygen, and so on—in our bodies; and many of the same vital functions—respiration, circulation, absorption, assimilation, reproduction. Protoplasm is the basis of life in both, and the cell is the architect that builds up the bodies of both. Trees are rooted men and men are walking trees. The tree absorbs its earth materials through the minute hairs on its rootlets, called fibrillæ, and the animal body absorbs its nutriment through analogous organs in the intestines, called lacteals.
Whitman's expression "the slumbering and liquid trees" often comes to my mind. They are the words of a poet who sees hidden relations and meanings everywhere. He knows how fluid and adaptive all animate nature is. The trees are wrapped in a kind of slumber in winter, and they are reservoirs of living currents in summer. If all living bodies came originally out of the sea, they brought a big dower of the sea with them. The human body is mainly a few pinches of earth salts held in solution by several gallons of water. The ashes of the living tree bulk small in comparison with the amount of water it holds. Yes, "the slumbering and liquid trees." They awaken from their slumber in the spring, the scales fall from their buds, the fountains within them are unsealed, and they again become streams of living energy, breaking into leaf and bloom and fruit under the magic of the sun's rays.
II
THE PLEASURES OF A NATURALIST
I
How closely every crack and corner of nature is packed with life, especially in our northern temperate zone! I was impressed with this fact when during several June days I was occupied with road-mending on the farm where I was born. To open up the loosely piled and decaying laminated rocks was to open up a little biological and zoölogical museum, so many of our smaller forms of life harbored there. From chipmunks to ants and spiders, animal life flourished. We disturbed the chipmunks in their den a foot and a half or more beneath the loosely piled rocks. There were two of them in a soft, warm nest of dry, shredded maple-leaves. They did not wait to be turned out of doors, but when they heard the racket overhead bolted precipitately. Two living together surprised me, as heretofore I had never known but one in a den. Near them a milk snake had stowed himself away in a crevice, and in the little earthquake which we set up got badly crushed. Two little red-bellied snakes about one foot long had also found harbor there.
The ants rushed about in great consternation when their eggs were suddenly exposed. In fact, there was live natural history under every stone about us. Some children brought me pieces of stone, which they picked up close by, which sheltered a variety of cocoon-building spiders. One small, dark-striped spider was carrying about its ball of eggs, the size of a large pea, attached to the hind part