THE LIFE OF THE TREES 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 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 de velopment. 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 prominence. 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 special ized 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 arc sometimes more than one layer of these cells.