If it lies deep, the cracks in the bark are deep, and the ridges between them are high and rough, as they are on the oak.
The beech, on the contrary, wears its cork-under-robe just beneath its outer dress, and so the rents in the bark are shallow, while in the canoe-birch the cork layer lies on the surface of trunk and boughs, and can be peeled away in thin sheets.
In some trees curved plates of cork form deep beneath the surface, and as the woody tissue lying outside them dies and dries, masses of bark are, as it were, gouged out of the living trunk.
When these plates are long and narrow, and are formed horizontally, the bark cracks across the trunk, and peels away in broken rings. But if the long, curving cork-plates stand upright in the tissue of the tree, the bark which they cut off comes away in scales, as it does from the trunks of pines and larches. Sometimes the scales form, but remain clinging to one another upon the tree, so that, in the course of years, the trunk becomes covered with large plates of dead tissue, overlapping each other like shingles on a roof, and making what is called "scale-bark." Such dried-up flakes, clinging together, may be seen partly covering the trunks of old pine-trees.
The larger roots of the trees are wrapped in corky tissue, just as the trunk and branches are, but in summer the slenderest tips of growing rootlets are not. The chief use of the large roots is to anchor the tree to the spot where it grows. But the work of the little rootlets is to suck up moisture and nourishment from the surrounding soil, and if they were sheathed in cork they could not fulfil this office.
Each rootlet, just above its tiny tip end, is furred over with hairs (Fig. 101), slender and soft, yet tough enough to press in between the grains of close-packed soil, and draw food and drink out of it.
As winter approaches, these little "root-hairs" shrivel and drop off, and the root-tip from which they sprang becomes enwrapped, like the larger roots, with a layer of cork-cells. So the whole tree, from its highest to its lowest point, is enfolded with a slumber-robe of cork, which keeps the vegetable juices in and helps to keep the cold out.
When spring comes to wake the earth, the deeper layers of soil feel the sweet influence while the surface is still ice-bound. Then the least root-tips, far underground, cast off their slumber-robes and begin to absorb moisture from the soil, which seldom freezes for more than forty inches below the surface, even in the bitterest weather. And all winter, alive but sleeping, a group of active cells lies just behind the root-tip, ready to put out fresh root-hairs as soon as spring returns.
Fig. 101. - Lengthwise section of a root-tip, showing root-hairs.
(From the Vegetable World).
While the root-tips are being enclosed in cork-sheaths, preparations for a long winter sleep are going forward among the branches overhead.
All the trees which wear union suits of cork have on their youngest branches little ventilating holes, called "lenticels." These can be plainly seen on the twigs of birch, beech, cherry, and elder, as rough oval dots, slightly raised, and different in color from the bark around them (Fig. 102).
Those of the birch become greatly extended as time goes on and appear as sharply-drawn, blackish stripes, running horizontally around the trunk. But on the roughened older bark of most species of tree the lenticels are hard to find, though they are still there.
In the older bark of the cork-oak, however, we know them only too well, for the brown, powdery streaks which sometimes run through bottle-corks, and cause them to crumble vexatiously when one tries to draw them, were the lenticels of the growing tree.
A lenticel is a lens-shaped rift in the outer bark, filled in with a loose mass of cork-cells, which are not rectangular, and ranged in rows after the usual custom of cork-cells, but rounded and, as it were, flung together, like stones tipped out of a wheelbarrow. Between them lie many little chinks and spaces, and by way of these the air gets into the wood, and the moist breath exhaled by the living tissue of the tree reaches the outside air. But as summer wanes, the trees fit themselves for their approaching slumber by an action which might be compared to that of the Hindoo fakir of Eastern wonder-lore, who, before entering his death-like trance, stops his nostrils with plugs of wax.
Fig. 102. - Branches of alder (a) and poplar-leaved birch (b), showing numerous lenticels (e).
For at the end of the growing season a close layer of cork forms over the whole surface of each lenticel, and seals up the tree.
So the breathing away of the tree's moisture is checked, as it has need to be, at this season, for now no active little root-hairs are at work down below, sucking up water from the ground. And also the little seals of cork help to protect the tissues of the tree against sharp and sudden frost.
At the return of spring a number of new cork-cells will be formed under the seal which Nature has placed upon the lenticel. These will be a light, loose mass, like that which fills the lenticels in summer, and by their vigorous growth they will split the seal above them and open the lenticel once more. And as we have seen, a closing layer or seal of cork has grown across all the scars whence last summer's leaves have fallen.
Preparations for repose have taken place, not only on the surface of the tree, but in its inner tissues. The fluid which, in summer, mounts slowly from the tiniest rootlets toward the leaves, is the "crude sap." It is water, holding in solution chemical substances derived from the soil. In the leaves, as we remember, it is worked over into the "elaborated-sap" which builds up and feeds plant-tissues. And this, creeping blindly from cell to cell, finds its way to the tips of roots and branches where growth is being actively carried on.
So in latter spring and summer there is a constant slow movement of fluids in the trees, first from the roots upward and then from the leaves downward.
Though this movement is connected functionally with the tree's feeding and digestion, it resembles the circulation of the blood in one respect. Crude sap, like arterial-blood, flows always through one set of channels, while elaborated sap, like veinous-blood, flows always through another. Crude sap, as we have seen, travels via the young wood; elaborated sap moves through the inner bark or "bast," where, in most trees, a way is prepared for it through what are technically known as "sieve-cells." These are long and narrow, and run lengthwise of trunk and boughs.
As the sap moves through them, it comes to places where the partition-wall between cell and cell is "punched full" of holes, like the top of a pepper-pot. Fine fibrils of plant-jelly reach through these, joining the contents of neighboring cells, and in summer, plant-fluids pass easily all along the route. But as autumn approaches, Nature seals these holes and isolates the "sieve-cells".
About midsummer, a glutinous plate, called the callus-plate, begins to form upon the little sieve, stopping up its pores. This gains thickness and solidity all through the waning of the year, and by time the leaves fall the route through the sieve-cells is closed as completely as is the route to Klondike in midwinter. This sealing of the little sieves has a beneficent purpose. At almost any time throughout the winter, in our latitudes, we may have a false promise or mocking similitude of early spring. We have seen that several gullible or foolhardy herbs may be cheated or dared into blooming any month of the year. Their foliage is practically evergreen, so that their untimely energy results in nothing worse than the production of a few futile flowers, which ripen no seed. But if the trees were to put forth when summer was not nigh at hand, their indiscretion might cost us the bloom of spring orchards and the luxuriance of midsummer woods.
When vegetable life resumes its functions the starches and other food-substances stored in the wood follow the route of the elaborated sap. The starch-grains are dissolved and changed into fluid • glucose, which, with other nutrient fluids, feels its way into the inner bark, and then creeps along through it into the buds where life is stirring.
But were the little sieves all open through the winter the plant-food stored in the wood could make its way to the buds at any time, and the buds, thus generously fed, could unfold in a few days. Lured by the false promise of a January thaw, baby-blossoms and delicate leaves would issue, all too quickly, into what would speedily prove a cold and inhospitable world. And all the energy used in putting them forth would be so much dead loss to the tree. So wise Nature keeps the stores of food within the plant-tissues safely locked up throughout the winter.
And thus the minute pieces of callus in the inner bark help to preserve the beauty of the forests.
I have not been able to find any recorded case of the reopening of a little sieve which has once been closed and sealed.
It seems probable that the very first growth of spring buds is fed, as is the unseasonable growth of too forth-putting autumn ones, by the nourishment drawn from closely neighboring cells. By time the unfolding blossoms and leaves of March or April have exhausted this slender store the cambium, which is formed each spring, has come into being and has taken up its work. New sieve-cells have been formed just inside the old ones which were sealed up last autumn, and there is a newly organized bark-route from end to end of every trunk and bough. So nourishment travels on unchecked to the expanding buds, and when the trees are fully aroused by April sunshine, they all at once begin to leaf out and to blossom, as the awakened servants, in the palace of the Sleeping Beauty, took up each his task again.
When next spring's new bark is formed, last spring's sieve-cells will be pushed a very little way outward, and each successive season's growth will force them still further from the centre of trunk or bough. So after awhile the sealed and disused sieve-cells of long-vanished summers find their way into the outer bark, and are sloughed off.
The forest where "frost hath wrought a silence," and where every tree is wrapped in its slumber-robe, sleeps as one who expects to be aroused and loves the expectation.
The danger guarded against is not that the trees will sleep too late, but that they may awaken too soon.
For the Earth's heart wakes for the Sun-prince, who is coming from the South, and the woods, hushed by winter, dream of spring.
And, as sometimes in summer nights day-birds rouse, call to their fellows, and sleep again, we can fancy that the trees now and then half awake, and whisper to one another, "Is spring drawing near?"
Then the great pine, which looks southward from the hill top, sends down through many branches the murmurous message, "Not yet," "Not yet".