Read Ebook: Trees Worth Knowing by Rogers Julia Ellen

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Ebook has 796 lines and 78188 words, and 16 pages

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, H^O. The carbon comes in as carbon dioxide, CO^. 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 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 complex molecules. These are molecules of starch, C^H^O^.

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 CO^2 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.

The great chestnut tree on the hillside has cast its burden 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.

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 trees 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 fulfills its life mission 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 substance 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 cambium 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 bark on the other.

What was once a delicate cell now becomes a hollow wood fibre, thin walled, but becoming thickened as it gets older. For a few years the superannuated cell is a part of the sap wood and is used as a tube in the system through which the crude sap mounts to the leaves. Later it may be stored full of starch, and the sap will flow up through newer tubes. At last the walls of the old cell harden and darken with mineral deposits. Many annual rings lie between it and the cambium. It has become a part of the heart wood of the tree.

The cells of its own generation that were crowded in the other direction made part of an annual layer of bark. As new layers formed beneath them, and the bark stretched and cracked, they lost their moisture by contact with the outer air. Finally they became thin, loose fibres, and scaled off.

The years of a tree's life are recorded with fair accuracy in the rings of its wood. The bark tells the same story, but the record is lost by its habit of sloughing off the outer layers. Occasionally a tree makes two layers of wood in a single season, but this is exceptional. Sometimes, as in a year of drought, the wood ring is so small as to be hardly distinguishable.

Each annual ring in the chestnut stump is distinct from its neighboring ring. The wood gradually merges from a dark band full of large pores to one paler in color and of denser texture. It is very distinct in oak and ash. The coarser belt was formed first. The spring wood, being so open, discolors by the accumulation of dust when exposed to the air. The closer summer wood is paler in color and harder, the pores almost invisible to the unaided eye. The best timber has the highest percentage of summer wood.

If a tree had no limbs, and merely laid on each year a layer of wood made of parallel fibres fitted on each other like pencils in a box, wood splitting would be child's play and carpenters would have less care to look after their tools. But woods differ in structure, and all fall short of the woodworker's ideal. The fibres of oak vary in shape and size. They taper and overlap their ends, making the wood less easily split than soft pine, for instance, whose fibres are regular cylinders, which lie parallel, and meet end to end without "breaking joints."

Fibres of oak are also bound together by flattened bundles of horizontal fibres that extend from pith to cambium, insinuated between the vertical fibres. These are seen on a cross-section of a log as narrow, radiating lines starting from the pith and cutting straight through heart wood and sap wood to the bark. A tangential section of a log shows these "pith rays," or "medullary rays" as long, tapering streaks. A longitudinal section made from bark to centre, as when a log is "quarter-sawed," shows a full side view of the "medullary rays." They are often an inch wide or more in oak; these wavy, irregular, gleaming fibre bands are known in the furniture trade as the "mirrors" of oak. They take a beautiful polish, and are highly esteemed in cabinet work. The best white oak has 20 per cent. to 25 per cent. of its substance made up of these pith rays. The horny texture of its wood, together with its strength and durability, give white oak an enviable place among timber trees, while the beauty of its pith rays ranks it high among ornamental woods.

The grain of wood is its texture. Wide annual rings with large pores mark coarse-grained woods. They need "filling" with varnish or other substance before they can be satisfactorily polished. Fine-grained woods, if hard, polish best. Trees of slow growth usually have fine-grained wood, though the rule is not universal.

When a twig breaks off, the bark heals the wound and the grain becomes straight over the place. Trees crowded in a forest early divest themselves of their lower branches. These die for lack of sun and air, and the trunk covers their stubs with layers of straight-grained wood. Such timbers are the masts of ships, telegraph poles, and the best bridge timbers. Yet buried in their heart wood are the roots of every twig, great or small, that started out to grow when the tree was young. These knots are mostly small and sound, so they do not detract from the value of the lumber. It is a pleasure to work upon such a "stick of timber."

A tree that grows in the open is clothed to the ground with branches, and its grain is found to be warped by hundreds of knots when it reaches the sawmill. Such a tree is an ornament to the landscape, but it makes inferior, unreliable lumber. The carpenter and the wood chopper despise it, for it ruins tools and tempers.

Besides the natural diversion of straight grain by knots, there are some abnormal forms to notice. Wood sometimes shows wavy grain under its bark. Certain trees twist in growing, so as to throw the grain into spiral lines. Cypresses and gum trees often exhibit in old stumps a veering of the grain to the left for a few years, then suddenly to the right, producing a "cross grain" that defies attempts to split it.

"Bird's-eye" and "curly maple" are prizes for the furniture maker. Occasionally a tree of swamp or sugar maple keeps alive the crowded twigs of its sapling for years, and forms adventitious buds as well. These dwarfed shoots persist, never getting ahead further than a few inches outside the bark. Each is the centre of a wood swelling on the tree body. The annual layers preserve all the inequalities. Dots surrounded by wavy rings are scattered over the boards when the tree is sawed. This is bird's-eye grain, beautiful in pattern and in sheen and coloring when polished. It is cut thin for veneer work. Extreme irregularity of grain adds to the value of woods, if they are capable of a high polish. The fine texture and coloring, combined with the beautiful patterns they display, give woods a place in the decorative arts that can be taken by no other material.

It is November, and the glory of the woods is departed. Dull browns and purples show where oaks still hold their leaves. Beech trees in sheltered places are still dressed in pale yellow. The elfin flowers of the witch hazel shine like threads of gold against the dull leaves that still cling. The trees lapse into their winter sleep.

Last week a strange thing happened. The wind tore the red robes from our swamp maples and sassafras and scattered them in tatters over the lawn. But the horse-chestnut, decked out in yellow and green, lost scarcely a leaf. Three days later, in the hush of early morning, when there was not a whiff of a breeze perceptible, the signal, "Let go!" came, and with one accord the leaves of the horse-chestnut fell. In an hour the tree stood knee deep in a stack of yellow leaves; the few that still clung had considerable traces of green in them. Gradually these are dropping, and the shining buds remain as a pledge that the summer story just ended will be told again next year.

Perhaps such a sight is more impressive if one realizes the vast importance of the work the leaves of a summer accomplish for the tree before their surrender.

The shedding of leaves is a habit broad-leaved trees have learned by experience in contact with cold winters. The swamp magnolia is a beautiful evergreen tree in Florida. In Virginia the leaves shrivel, but they cling throughout the season. In New Jersey and north as far as Gloucester, where the tree occurs sparingly, it is frankly deciduous. Certain oaks in the Northern states have a stubborn way of clinging to their dead leaves all winter. Farther south some of these species grow and their leaves do not die in fall, but are practically evergreen, lasting till next year's shoots push them off. The same gradual change in habit is seen as a species is followed up a mountain side.

The horse-chestnut will serve as a type of deciduous trees. Its leaves are large, and they write out, as if in capital letters, the story of the fall of the leaf. It is a serial, whose chapters run from July until November. The tree anticipates the coming of winter. Its buds are well formed by midsummer. Even then signs of preparation for the leaf fall appear. A line around the base of the leaf stem indicates where the break will be. Corky cells form on each side of this joint, replacing tissues which in the growing season can be parted only by breaking or tearing them forcibly. A clean-cut zone of separation weakens the hold of the leaf upon its twig, and when the moment arrives the lightest breath of wind--even the weight of the withered leaf itself--causes the natural separation. And the leaflets simultaneously fall away from their common petiole.

There are more important things happening in leaves in late summer than the formation of corky cells. The plump green blades are full of valuable substance that the tree can ill afford to spare. In fact, a leaf is a layer of the precious cambium spread out on a framework of veins and covered with a delicate, transparent skin--a sort of etherealized bark. What a vast quantity of leaf pulp is in the foliage of a large tree!

As summer wanes, and the upward tide of sap begins to fail, starch making in the leaf laboratories declines proportionately. Usually before midsummer the fresh green is dimmed. Dust and heat and insect injuries impair the leaf's capacity for work. The thrifty tree undertakes to withdraw the leaf pulp before winter comes.

But how?

It is not a simple process nor is it fully understood. The tubes that carried the products of the laboratory away are bound up with the fibres of the leaf's skeleton. Through the transparent leaf wall the migration of the pulp may be watched. It leaves the margins and the net veins, and settles around the ribs and mid vein, exactly as we should expect. Dried and shriveled horse-chestnut leaves are still able to show various stages in this marvellous retreat of the cambium. If moisture fails, the leaf bears some of its green substance with it to the earth. The "breaking down of the chlorophyll" is a chemical change that attends the ripening of a leaf. The waxy granules disintegrate, and a yellow liquid shows its colors through the delicate leaf walls. Now other pigments, some curtained from view by the chlorophyll, others the products of decomposition, show themselves. Iron and other minerals the sap brought from the soil contribute reds and yellows and purples to the color scheme. As drainage proceeds, with the chemical changes that accompany it, the pageant of autumn colors passes over the woodlands. No weed or grass stem but joins in the carnival of the year.

Crisp and dry the leaves fall. Among the crystals and granules that remain in their empty chambers there is little but waste that the tree can well afford to be rid of--substances that have clogged the leaf and impeded its work.

We have been mistaken in attributing the gay colors of autumnal foliage to the action of frost. The ripening of the leaves occurs in the season of warm days and frosty nights, but it does not follow that the two phenomena belong together as cause and effect. Frost no doubt hastens the process. But the chemical changes that attend the migration of the carbohydrates and albuminous materials from the leaf back into twig and trunk and root for safe keeping go on no matter what the weather.

In countries having a moist atmosphere autumn colors are less vivid. England and our own Pacific Coast have nothing to compare with the glory of the foliage in the forests of Canada and the Northeastern states, and with those on the wooded slopes of the Swiss Alps, and along the Rhine and the Danube. Long, dry autumns produce the finest succession of colors. The most brilliant reds and yellows often appear long before the first frost. Cold rains of long duration wash the colors out of the landscape, sometimes spoiling everything before October. A sharp freeze before the leaves expect it often cuts them off before they are ripe. They stiffen and fall, and are wet and limp next day, as if they had been scalded; all their rich cell substance lost to the tree, except as they form a mulch about its roots. But no tree can afford so expensive a fertilizer, and happily they are not often caught unawares.

The leaves of certain trees in regions of mild winters persist until they are pushed off by the swelling buds in spring. Others cling a year longer, in sorry contrast with the new foliage. We may believe that this is an indolent habit induced by climatic conditions.

Leaves of evergreens cling from three to five years. Families and individuals differ; altitude and latitude produce variations. An evergreen in winter is a dull-looking object, if we could compare it with its summer foliage. Its chlorophyll granules withdraw from the surface of the leaf.

They seek the lower ends of the palisade cells, as far as they can get from the leaf surface, assume a dull reddish brown or brownish yellow color, huddle in clumps, their water content greatly reduced, and thus hibernate, much as the cells of the cambium are doing under the bark. In this condition, alternate freezing and thawing seem to do no harm, and the leaves are ready in spring to resume the starch-making function if they are still young. Naturally, the oldest leaves are least capable of this work, and least is expected of them. Gradually they die and drop as new ones come on. As among broad-leaved trees, the zone of foliage in evergreens is an outer dome of newest shoots; the framework of large limbs is practically destitute of leaves.

Nine out of every ten intelligent people will see nothing of interest in a row of bare trees. They casually state that buds are made in the early spring. They miss seeing the strength and beauty of tree architecture which the foliage conceals in summertime. The close-knit, alive-looking bark of a living tree they do not distinguish from the dull, loose-hung garment worn by the dead tree in the row. All trees look alike to them in winter.

Yet there is so much to see if only one will take time to look. Even the most heedless are struck at times with the mystery of the winter trance of the trees. They know that each spring re?<

A tree has no centre of life, no vital organs corresponding to those of animals. It is made up, from twig to root, of annual, concentric layers of wood around a central pith.

The bark protects the cambium, and the cambium is the tissue which by cell multiplication in the growing season produces the yearly additions of wood and bark. Buds are growing points set along the twigs. They produce leafy shoots, as a rule. Some are specialized to produce flowers and subsequently fruits. Leaves are extensions of cambium spread in the sun and air in the season when there is no danger from frosts. The leaves have been called the stomachs of a tree. They receive crude materials from the soil and the air and transmute them into starch under the action of sunlight. This elaborated sap supplies the hungry cambium cells during the growing season, and the excess of starch made in the leaf laboratories is stored away in empty wood cells and in every available space from bud to root tip, from bark to pith.

The tree's period of greatest activity is the early summer. It is the time of growth and of preparation for the coming winter and for the spring that follows it. Winter is the time of rest--of sleep, or hibernation. A bear digs a hollow under the tree's roots and sleeps in it all winter, waking in the spring. In many ways the tree imitates the bear. Dangerous as are analogies between plants and animals, it is literally true that the sleeping bear and the dormant tree have each ceased to feed. The sole activity of each seems to be the quiet breathing.

Do trees really breathe? As truly and as incessantly as you do, but not as actively. Other processes are intermittent, but breathing must go on, day and night, winter and summer, as long as life lasts. Breathing is low in winter. The tree is not growing. There is only the necessity of keeping it alive.

The skin is the efficient "third lung" of animals. The closing of its pores causes immediate suffocation. The bark of trees carries on the work of respiration in the absence of the leaves. Bark is porous, even where it is thickest.

There is a popular fallacy that trees have no buds until spring. Some trees have very small buds. But there is no tree in our winter woods that will not freely show its buds to any one who wishes to see them. A very important part of the summer work of a tree is the forming of buds for next spring. Even when the leaves are just unfolding on the tender shoots a bud will be found in each angle between leaf and stem. All summer long its bud is the especial charge of each particular leaf. If accident destroy the leaf, the bud dies of neglect. When midsummer comes the bud is full grown, or nearly so, and the fall of the leaf is anticipated. The thrifty tree withdraws as much as possible of the rich green leaf pulp, and stores it in the twig to feed the opening buds in spring.

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