3. The CORKY LAYER or _Outer Bark_, the cells of which contain no chlorophyll, and are of the nature of _cork_. Common cork is the thick corky layer of the bark of the Cork-Oak of Spain. It is this which gives to the stems or twigs of shrubs and trees the aspect and the color peculiar to each,--light gray in the Ash, purple in the Red Maple, red in several Dogwoods, etc.

4. The EPIDERMIS, or skin of the plant, consisting of a layer of thick-sided empty cells, which may be considered to be the outermost layer, or in most herbaceous stems the only layer, of cork-cells.

[Ill.u.s.tration: Fig. 481. Magnified view of surface of a bit of young Maple wood from which the bark has been torn away, showing the wood-cells and the bark-ends of medullary rays.]

[Ill.u.s.tration: Fig. 482. Section in the opposite direction, from bark (on the left) to beginning of pith (on the right), and a medullary ray extending from one to the other.]

432. The green layer of bark seldom grows much after the first season.

Sometimes the corky layer grows and forms new layers, inside of the old, for years, as in the Cork-Oak, the Sweet Gum-tree, and the White and the Paper Birch. But it all dies after a while; and the continual enlargement of the wood within finally stretches it more than it can bear, and sooner or later cracks and rends it, while the weather acts powerfully upon its surface; so the older bark perishes and falls away piecemeal year by year.

433. So on old trunks only the inner bark remains. This is renewed every year from within and so kept alive, while the older and outer layers die, are fissured and rent by the distending trunk, weathered and worn, and thrown off in fragments,--in some trees slowly, so that the bark of old trunks may acquire great thickness; in others, more rapidly. In Honeysuckles and Grape-Vines, the layers of liber loosen and die when only a year or two old. The annual layers of liber are sometimes as distinct as those of the wood, but often not so.

434. =The Wood= of an exogenous trunk, having the old growths covered by the new, remains nearly unchanged in age, except from decay. Wherever there is an annual suspension and renewal of growth, as in temperate climates, the annual growths are more or less distinctly marked, in the form of concentric rings on the cross section, so that the age of the tree may be known by counting them. Over twelve hundred layers have been counted on the stumps of Sequoias in California, and it is probable that some trees now living antedate the Christian era.

435. The reason why the annual growths are distinguishable is, that the wood formed at the beginning of the season is more or less different in the size or character of the cells from that of the close. In Oak, Chestnut, etc., the first wood of the season abounds in dotted ducts, the calibre of which is many times greater than that of the proper wood-cells.

436. =Sap-wood, or Alburnum.= This is the newer wood, living or recently alive, and taking part in the conveyance of sap. Sooner or later, each layer, as it becomes more and more deeply covered by the newer ones and farther from the region of growth, is converted into

437. =Heart-wood, or Duramen.= This is drier, harder, more solid, and much more durable as timber, than sap-wood. It is generally of a different color, and it exhibits in different species the hue peculiar to each, such as reddish in Red-Cedar, brown in Black-Walnut, black in Ebony, etc. The change of sap-wood into heart-wood results from the thickening of the walls of the wood-cells by the deposition of hard matter, lining the tubes and diminishing their calibre; and by the deposition of a vegetable coloring-matter peculiar to each species. The heart-wood, being no longer a living part, may decay, and often does so, without the least injury to the tree, except by diminishing the strength of the trunk, and so rendering it more liable to be overthrown.

438. =The Living Parts of a Tree=, of the exogenous kind, are only these: first, the rootlets at one extremity; second, the buds and leaves of the season at the other; and third, a zone consisting of the newest wood and the newest bark, connecting the rootlets with the buds or leaves, however widely separated these may be,--in the tallest trees from two to four hundred feet apart. And these parts of the tree are all renewed every year. No wonder, therefore, that trees may live so long, since they annually reproduce everything that is essential to their life and growth, and since only a very small part of their bulk is alive at once. The tree survives, but nothing now living has been so long. In it, as elsewhere, life is a transitory thing, ever abandoning the _old_, and renewed in the _young_.

-- 4. ANATOMY OF LEAVES.

439. The wood in leaves is the framework of ribs, veins, and veinlets (125), serving not only to strengthen them, but also to bring in the sap, and to distribute it throughout every part. The cellular portion is the green pulp, and is nearly the same as the green layer of the bark.

So that the leaf may properly enough be regarded as a sort of expansion of the fibrous and green layers of the bark. It has no proper corky layer; but the whole is covered by a transparent skin or _epidermis_, resembling that of the stem.

440. The cells of the leaf are of various forms, rarely so compact as to form a close cellular tissue, usually loosely arranged, at least in the lower part, so as to give copious intervening s.p.a.ces or air pa.s.sages, communicating throughout the whole interior (Fig. 443, 483). The green color is given by the chlorophyll (417), seen through the very transparent walls of the cells and through the translucent epidermis of the leaf.

[Ill.u.s.tration: Fig. 483. Magnified section of a leaf of White Lily, to exhibit the cellular structure, both of upper and lower stratum, the air-pa.s.sages of the lower, and the epidermis or skin, in section, also a little of that of the lower face, with some of its stomates.]

441. In ordinary leaves, having an upper and under surface, the green cells form two distinct strata, of different arrangement. Those of the upper stratum are oblong or cylindrical, and stand endwise to the surface of the leaf, usually close together, leaving hardly any vacant s.p.a.ces; those of the lower are commonly irregular in shape, most of them with their longer diameter parallel to the face of the leaf, and are very loosely arranged, leaving many and wide air-chambers. The green color of the lower is therefore diluted, and paler than that of the upper face of the leaf. The upper part of the leaf is so constructed as to bear the direct action of the sunshine; the lower so as to afford freer circulation of air, and to facilitate transpiration. It communicates more directly than the upper with the external air by means of _Stomates_.

442. =The Epidermis= or skin of leaves and all young shoots is best seen in the foliage. It may readily be stripped off from the surface of a Lily-leaf, and still more so from more fleshy and soft leaves, such as those of Houseleek. The epidermis is usually composed of a single layer, occasionally of two or three layers, of empty cells, mostly of irregular outline. The sinuous lines which traverse it, and may be discerned under low powers of the microscope (Fig. 487), are the boundaries of the epidermal cells.

[Ill.u.s.tration: Fig. 484. Small portion of epidermis of the lower face of a White-Lily leaf, with stomata.]

[Ill.u.s.tration: Fig. 485. One of these, more magnified, in the closed state. 486. Another stoma, open.]

[Ill.u.s.tration: Fig. 487. Small portion of epidermis of the Garden Balsam, highly magnified, showing very sinuous-walled cells, and three stomata.]

443. =Breathing-pores, or Stomates, Stomata= (singular, a _Stoma_,--literally, a mouth) are openings through the epidermis into the air-chambers or intercellular pa.s.sages, always between and guarded by a pair of thin-walled guardian cells. Although most abundant in leaves, especially on their lower face (that which is screened from direct sunlight), they are found on most other green parts. They establish a direct communication between the external air and that in the loose interior of the leaf. Their guardian cells or lips, which are soft and delicate, like those of the green pulp within, by their greater or less turgidity open or close the orifice as the moisture or dryness varies.

444. In the White Lily the stomata are so remarkably large that they may be seen by a simple microscope of moderate power, and may be discerned even by a good hand lens. There are about 60,000 of them to the square inch of the epidermis of the lower face of this Lily-leaf, and only about 3000 to the same s.p.a.ce on the upper face. It is computed that an average leaf of an Apple-tree has on its lower face about 100,000 of these mouths.

-- 5. PLANT FOOD AND a.s.sIMILATION.

445. Only plants are capable of originating organizable matter, or the materials which compose the structure of vegetables and animals. The essential and peculiar work of plants is to take up portions of earth and air (water belonging to both) upon which animals cannot live at all, and to convert them into something organizable; that is, into something that, under life, may be built up into vegetable and animal structures.

All the food of animals is produced by plants. Animals live upon vegetables, directly or at second hand, the carnivorous upon the herbivorous; and vegetables live upon earth and air, immediately or at second hand.

446. =The Food of plants=, then, primarily, is earth and air. This is evident enough from the way in which they live. Many plants will flourish in pure sand or powdered chalk, or on the bare face of a rock or wall, watered merely with rain. And almost any plant may be made to grow from the seed in moist sand, and increase its weight many times, even if it will not come to perfection. Many naturally live suspended from the branches of trees high in the air, and nourished by it alone, never having any connection with the soil; and some which naturally grow on the ground, like the Live-forever of the gardens, when pulled up by the roots and hung in the air will often flourish the whole summer long.

447. It is true that fast-growing plants, or those which produce much vegetable matter in one season (especially in such concentrated form as to be useful as food for man or the higher animals) will come to maturity only in an enriched soil. But what is a rich soil? One which contains decomposing vegetable matter, or some decomposing animal matter; that is, in either case, some decomposing organic matter formerly produced by plants. Aided by this, grain-bearing and other important vegetables will grow more rapidly and vigorously, and make a greater amount of nourishing matter, than they could if left to do the whole work at once from the beginning. So that in these cases also all the organic or organizable matter was made by plants, and made out of earth and air. Far the larger and most essential part was air and water.

448. Two kinds of material are taken in and used by plants; of which the first, although more or less essential to perfect plant-growth, are in a certain sense subsidiary, if not accidental, viz.:--

_Earthy const.i.tuents_, those which are left in the form of ashes when a leaf or a stick of wood is burned in the open air. These consist of some _potash_ (or _soda_ in a marine plant), some _silex_ (the same as flint), and a little _lime_, _alumine_, or _magnesia_, _iron_ or _manganese_, _sulphur_, _phosphorus_, etc.,--some or all of these in variable and usually minute proportions. They are such materials as happen to be dissolved, in small quant.i.ty, in the water taken up by the roots; and when that is consumed by the plant, or flies off pure (as it largely does) by exhalation, the earthy matter is left behind in the cells,--just as it is left incrusting the sides of a teakettle in which much hard water has been boiled. Naturally, therefore, there is more earthy matter (i. e. more ashes) in the leaves than in any other part (sometimes as much as seven per cent, when the wood contains only two per cent); because it is through the leaves that most of the water escapes from the plant. Some of this earthy matter incrusts the cell-walls, some goes to form crystals or rhaphides, which abound in many plants (422), some enters into certain special vegetable products, and some appears to be necessary to the well-being of the higher orders of plants, although forming no necessary part of the proper vegetable structure.

_The essential const.i.tuents_ of the organic fabric are those which are dissipated into air and vapor in complete burning. They make up from 88 to 99 per cent of the leaf or stem, and essentially the whole both of the cellulose of the walls and the protoplasm of the contents. Burning gives these materials of the plant's structure back to the air, mainly in the same condition in which the plant took them, the same condition which is reached more slowly in natural decay. The chemical elements of the cell-walls (or cellulose, 402), as also of starch, sugar, and all that cla.s.s of organizable cell-material, are carbon, hydrogen, and oxygen (399). The same, with nitrogen, are the const.i.tuents of protoplasm, or the truly vital part of vegetation.

449. These chemical elements out of which organic matters are composed are supplied to the plant by water, carbonic acid, and some combinations of nitrogen.

_Water_, far more largely than anything else, is imbibed by the roots; also more or less by the foliage in the form of vapor. Water consists of oxygen and hydrogen; and cellulose or plant-wall, starch, sugar, etc., however different in their qualities, agree in containing these two elements in the same relative proportions as in water.

_Carbonic acid_ gas (Carbon dioxide) is one of the components of the atmosphere,--a small one, ordinarily only about 1/2500 of its bulk,--sufficient for the supply of vegetation, but not enough to be injurious to animals, as it would be if acc.u.mulated. Every current or breeze of air brings to the leaves expanded in it a succession of fresh atoms of carbonic acid, which it absorbs through its mult.i.tudinous breathing-pores. This gas is also taken up by water. So it is brought to the ground by rain, and is absorbed by the roots of plants, either as dissolved in the water they imbibe, or in the form of gas in the interstices of the soil. Manured ground, that is, soil containing decomposing vegetable or animal matters, is constantly giving out this gas into the interstices of the soil, whence the roots of the growing crop absorb it. Carbonic acid thus supplied, primarily from the air, is the source of the carbon which forms much the largest part of the substance of every plant. The proportion of carbon may be roughly estimated by charring some wood or foliage; that is, by heating it out of contact with the air, so as to decompose and drive off all the other const.i.tuents of the fabric, leaving the large bulk of charcoal or carbon behind.

_Nitrogen_, the remaining plant-element, is a gas which makes up more than two thirds of the atmosphere, is brought into the foliage and also to the roots (being moderately soluble in water) in the same ways as is carbonic acid. The nitrogen which, mixed with oxygen, a little carbonic acid, and vapor of water, const.i.tutes the air we breathe, is the source of this fourth plant-element. But it is very doubtful if ordinary plants can use any nitrogen gas directly as food; that is, if they can directly cause it to combine with the other elements so as to form protoplasm.

But when combined with hydrogen (forming ammonia), or when combined with oxygen (nitric acid and nitrates) plants appropriate it with avidity.

And several natural processes are going on in which nitrogen of the air is so combined and supplied to the soil in forms directly available to the plant. The most efficient is _nitrification_, the formation of nitre (nitrate of potash) in the soil, especially in all fertile soils, through the action of a bacterial ferment.

450. =a.s.similation= in plants is the conversion of these inorganic substances--essentially, water, carbonic acid, and some form of combined or combinable nitrogen--into vegetable matter. This most dilute food the living plant concentrates and a.s.similates to itself. Only plants are capable of converting these mineral into organizable matters; and this all-important work is done by them (so far as all ordinary vegetation is concerned) only

451. _Under the light of the sun, acting upon green parts or foliage_, that is, upon the chlorophyll, or upon what answers to chlorophyll, which these parts contain. The sun in some way supplies a power which enables the living plant to originate these peculiar chemical combinations,--to organize matter into forms which are alone capable of being endowed with life. The proof of this proposition is simple; and it shows at the same time, in the simplest way, what a plant does with the water and carbonic acid it consumes. Namely, 1st, it is only in sunshine or bright daylight that the green parts of plants give out oxygen gas,--then they regularly do so; and 2d, the giving out of this oxygen gas is required to render the chemical composition of water and carbonic acid the same as that of _cellulose_, that is, of the plant's permanent fabric. This shows why plants spread out so large a surface of foliage.

Leaves are so many workshops, full of machinery worked by sun-power. The emission of oxygen gas from any sun-lit foliage is seen by placing some of this under water, or by using an aquatic plant, by collecting the air bubbles which rise, and by noting that a taper burns brighter in this air. Or a leafy plant in a gla.s.s globe may be supplied with a certain small percentage of carbonic acid gas, and after proper exposure to sunshine, the air on being tested will be found to contain less carbonic acid and just so much the more oxygen gas.

452. Now if the plant is making cellulose or any equivalent substance,--that is, is making the very materials of its fabric and growth, as must generally be the case,--all this oxygen gas given off by the leaves comes from the decomposition of carbonic acid taken in by the plant. For cellulose, and also starch, dextrine, sugar, and the like are composed of carbon along with oxygen and hydrogen in just the proportions to form water. And the carbonic acid and water taken in, less the oxygen which the carbon brought with it as carbonic acid, and which is given off from the foliage in sunshine, just represents the manufactured article, cellulose.

453. It comes to the same if the first product of a.s.similation is sugar, or dextrine which is a sort of soluble starch, or starch itself. And in the plant all these forms are readily changed into one another. In the tiny seedling, as fast as this a.s.similated matter is formed it is used in growth, that is, in the formation of cell-walls. After a time some or much of the product may be acc.u.mulated in store for future growth, as in the root of the turnip, or the tuber of the potato, or the seed of corn or pulse. This store is mainly in the form of starch. When growth begins anew, this starch is turned into dextrine or into sugar, in liquid form, and used to nourish and build up the germinating embryo or the new shoot, where it is at length converted into cellulose and used to build up plant-structure.

454. But that which builds plant-fabric is not the cellular structure itself; the work is done by the living protoplasm which dwells within the walls. This also has to take and to a.s.similate its proper food, for its own maintenance and growth. Protoplasm a.s.similates, along with the other three elements, the nitrogen of the plant's food. This comes primarily from the vast stock in the atmosphere, but mainly through the earth, where it is acc.u.mulated through various processes in a fertile soil,--mainly, so far as concerns crops, from the decomposition of former vegetables and animals. This protoplasm, which is formed at the same time as the simpler cellulose, is essentially the same as the flesh of animals, and the source of it. It is the common basis of vegetable and of animal life.

455. _So plant-a.s.similation produces all the food and fabric of animals._ Starch, sugar, the oils (which are, as it were, these farinaceous matters more deoxidated), chlorophyll, and the like, and even cellulose itself, form the food of herbivorous animals and much of the food of man. When digested they enter into the blood, undergo various transformations, and are at length decomposed into carbonic acid and water, and exhaled from the lungs in respiration,--in other words, are given back to the air by the animal as the very same materials which the plant took from the air as its food,--are given back to the air in the same form that they would have taken if the vegetable matter had been left to decay where it grew, or if it had been set on fire and burned; and with the same result, too, as to the heat,--the heat in this case producing and maintaining the proper temperature of the animal.

456. The protoplasm and other products containing nitrogen (gluten, legumine, etc.), and which are most acc.u.mulated in grains and seeds (for the nourishment of their embryos when they germinate), compose the most nutritious vegetable food consumed by animals; they form their proper flesh and sinews, while the earthy const.i.tuents of the plant form the earthy matter of the bones, etc. At length decomposed, in the secretions and excretions, these nitrogenous const.i.tuents are through successive changes finally resolved into mineral matter, into carbonic acid, water, and ammonia or some nitrates,--into exactly or essentially the same materials which the plants took up and a.s.similated. Animals depend upon vegetables absolutely and directly for their subsistence; also indirectly, because

457. _Plants purify the air for animals._ In the very process by which they create food they take from the air carbonic acid gas, injurious to animal respiration, which is continually poured into it by the breathing of all animals, by all decay, by the burning of fuel and all other ordinary combustion; and they restore an equal bulk of life-sustaining oxygen needful for the respiration of animals,--needful, also, in a certain measure, for plants in any work they do. For in plants, as well as in animals, work is done at a certain cost.

-- 6. PLANT WORK AND MOVEMENT.

458. As the organic basis and truly living material of plants is identical with that of animals, so is the life at bottom essentially the same; but in animals something is added at every rise from the lowest to highest organisms. Action and work in living beings require movement.

459. Living things move; those not living are only moved. Plants move as truly as do animals. The latter, nourished as they are upon organized food, which has been prepared for them by plants, and is found only here and there, must needs have the power of going after it, of collecting it, or at least of taking it in; which requires them to make spontaneous movements. But ordinary plants, with their wide-spread surface, always in contact with the earth and air on which they feed,--the latter everywhere the same, and the former very much so,--might be thought to have no need of movement. Ordinary plants, indeed, have no locomotion; some float, but most are rooted to the spot where they grew. Yet probably all of them execute various movements which must be as truly self-caused as are those of the lower grades of animals,--movements which are overlooked only because too slow to be directly observed.

Nevertheless, the motion of the hour-hand and of the minute-hand of a watch is not less real than that of the second-hand.

[Ill.u.s.tration: Fig. 488. Two individuals of an Oscillaria, magnified.]