Life Of Trees
( Originally Published 1927 )
The swift unfolding of the leaves in spring is always a miracle. One day the budded twigs are still wrapped in the deep sleep of winter. A trace of green appears about the edges of the bud scales—they loosen and fall, and the tender green shoot looks timidly out and begins to unfold its crumpled leaves. Soon the delicate blade broadens and takes on the texture and familiar appearance of the grown-up leaf. Behold! while we watched the single shoot the bare tree has clothed itself in the green canopy of summer.
How can this miracle take place? How does the tree come into full leaf, sometimes within a fraction of a week? It could never happen except for the store of concentrated food that the sap dissolves in spring and carries to the buds, and for the remarkable activity of the cambium cells within the buds.
What is a bud? It is a shoot in miniature its leaves or flowers, or both, formed with wondrous completeness in the previous summer. About its base are crowded leaves so hardened and overlapped as to cover and protect the tender shoot. All the tree can ever express of beauty or of energy comes out of these precious little "growing points," wrapped up all winter, but impatient, as spring approaches, to accept the invitation of the south wind and sun.
The protective scale leaves fall when they are no longer needed. This vernal leaf fall makes little show on the forest floor, but it greatly exceeds in number of leaves the autumnal defoliation.
Sometimes these bud scales lengthen before the shoot spares them. The silky, brown scales of the beech buds sometimes add twice their length, thus protecting the lengthening shoot which seems more delicate than most kinds, less ready to encounter unguarded the wind and the sun. The hickories, shagbark, and mockernut, show scales more than three inches long.
Many leaves are rosy, or lilac tinted, when they open—' the waxy granules of their precious "leaf green "screened by these colored pigments from the full glare of the sun. Some leaves have wool or silk growing like the pile of velvet on their surfaces. These hairs are protective also. They shrivel or blow away when the leaf comes to its full development. Occasionally a species retains the down on the lower surface of its leaves, or, oftener, merely in the angles of its veins.
The folding and plaiting of the leaves bring the ribs and veins into prominent: The delicate green web sinks into folds between and is therefore protected from the weather. Young leaves hang limp, never presenting their perpendicular surfaces to the sun.
Another protection to the infant leaf is the pair of stipules at its base. Such stipules enclose the leaves of tulip and magnolia trees. The beech leaf has two long strap-like stipules. Linden stipules are green and red—two con-cave, oblong leaves, like the two valves of a pea pod. Elm stipules are conspicuous. The black willow has large, leaf-like, heart-shaped stipules, green as the leaf and saw-toothed.
Most stipules shield the tender leaf during the hours of its helplessness, and fall away as the leaf matures. Others persist, as is often seen in the black willows.
With this second vernal leaf fall (for stipules are leaves) the leaves assume independence, and take up their serious work. They are ready to make the living for the whole tree. Nothing contributed by soil or atmosphere—no matter how rich it is—can become available for the tree's use until the leaves receive and prepare it.
Every leaf that spreads its green blade to the sun is a laboratory, devoted to the manufacture of starch. It is, in fact, an outward extension of the living cambium, thrust out beyond the thick, hampering bark, and specialized to do its specific work rapidly and effectively.
The structure of the leaves must be studied with a microscope. This laboratory has a delicate, transparent, enclosing wall, with doors, called stomates, scattered over the lower surface. The "leaf pulp" is inside, so is the framework of ribs and veins, that not only supports the soft tissues but furnishes the vascular system by which an incoming and outgoing current of sap is kept in constant circulation. In the upper half of the leaf, facing the sun, the pulp is in "palisade cells," regular, oblong, crowded together, and perpendicular to the flat surface. There are sometimes more than one layer of these cells.
In the lower half of the leaf's thickness, between the palisade cells and the under surface, the tissue is spongy. There is no crowding of cells here. They are irregularly spherical, and cohere loosely, being separated by ample air spaces, which communicate with the outside world by the doorways mentioned above. An ordinary apple leaf has about one hundred thousand of these stomates to each square inch of its under surface. So the ventilation of the leaf is provided for.
The food of trees comes from two sources—the air and the soil. Dry a stick of wood, and the water leaves it. Burn it now, and ashes remain. The water and the ashes came from the soil. That which came from the air passed off in gaseous form with the burning. Some elements from the soil also were converted by the heat into gases, and escaped by the chimneys.
Take that same stick of wood, and, instead of burning it in an open fireplace or stove, smother it in a pit and burn it slowly, and it comes out a stick of charcoal, having its shape and size and grain preserved. It is carbon, its only impurity being a trace of ashes. What would have escaped up a chimney as carbonic-acid gas is confined here as a solid, and fire can yet liberate it.
The vast amount of carbon which the body of a tree contains came into its leaves as a gas, carbon dioxide. The soil furnished various minerals, which were brought up in the "crude sap." Most of these remain as ashes when the wood is burned. Water comes from the soil. So the list of raw materials of tree food is complete, and the next question is: How are they prepared for the tree's use?
The ascent of the sap from roots to leaves brings water with mineral salts dissolved in it. Thus potassium, calcium, magnesium, iron, sulphur, nitrogen, and phosphorus are brought to the leaf laboratories—some are useful, some useless. The stream of water contributes of itself to the laboratory whatever the leaf cells demand to keep their own substance sufficiently moist, and those molecules that are necessary to furnish hydrogen and oxygen for the making of starch. Water is needed also to keep full the channels of the returning streams, but the great bulk of water that the roots send up escapes by evaporation through the curtained doorways of the leaves.
Starch contains carbon, hydrogen, and oxygen, the last two in the exact proportion that they bear to each other in water, H2O. The carbon comes in as carbon dioxide, CO2. There is no lack of this familiar gas in the air. It is exhaled constantly from the lungs of every animal, from chimneys, and from all decaying substances. It is diffused through the air, and, entering the leaves by the stomates, comes in contact with other food elements in the palisade cells.
The power that runs this starch factory is the sun. The chlorophyll, or leaf green, which colors the clear protoplasm of the cells, is able to absorb in daylight (and especially on warm, sunny days) some of the energy of sunlight, and to enable the protoplasm to use the energy thus captured to the chemical breaking down of water and carbon dioxide, and the reuniting of their free atoms into new and more complexmolecules. These are molecules of starch, C6H10O5.
The new product in soluble form makes its way into the current of nutritious sap that sets back into the tree. This is the one product of the factory—the source of all the tree's growth—for it is the elaborated sap, the food which nourishes every living cell from leaf to root tip. It builds new wood layers, extends both twigs and roots, and perfects the buds for the coming year.
Sunset puts a stop to starch making. The power is turned off till another day. The distribution of starch goes on. The surplus is unloaded, and the way is cleared for work next day. On a sunless day less starch is made than on a bright one.
Excess of water and of free oxygen is noticeable in this making of starch. Both escape in invisible gaseous form through the stomates. No carbon escapes, for it is all used up, and a continual supply of CO2 sets in from outside. We find it at last in the form of solid wood fibres. So it is the leaf's high calling to take the crude elements brought to it, and convert them into food ready for assimilation.
There are little elastic curtains on the doors of leaves, and in dry weather they are closely drawn. This is to prevent the free escape of water, which might debilitate the starch-making cells. In a moist atmosphere the doors stand wide open. Evaporation does not draw water so hard in such weather, and there is no danger of excessive loss. "The average oak tree in its five active months evaporates about 28,000 gallons of water"—an average of about 187 gallons a day.
In the making of starch there is oxygen left over—just the amount there is left of the carbon dioxide when the carbon is seized for starch making. This accumulating gas passes into the air as free oxygen, "purifying" it for the use of all animal life, even as the absorption of carbon dioxide does.
When daylight is gone, the exchange of these two gases ceases. There is no excess of oxygen nor demand for carbon dioxide until business begins in the morning. But now a process is detected that the day's activities had obscured.
The living tree breathes—inhales oxygen and exhales carbonic-acid gas. Because the leaves exercise the function of respiration, they may properly be called the lungs of trees, for the respiration of animals differs in no essential from that of plants.
The bulk of the work of the leaves is accomplished before midsummer. They are damaged by whipping in the wind, by the ravages of fungi and insects of many kinds. Soot and dust clog the stomates. Mineral deposits cumber the working cells. Finally they become sere and russet or "die like the dolphin," passing in all the splendor of sunset skies to oblivion on the leaf mould under the trees.