Part 13

408. =Wood.= This is found in all common herbs, as well as in shrubs and trees, but the former have much less of it in proportion to the softer cellular tissue. It is formed very early in the growth of the root, stem, and leaves,--traces of it appearing in large embryos even while yet in the seed. Those cells that lengthen, and at the same time thicken their walls form the proper WOODY FIBRE or WOOD-CELLS; those of larger size and thinner walls, which are thickened only in certain parts so as to have peculiar markings, and which often are seen to be made up of a row of cylindrical cells, with the partitions between absorbed or broken away, are called DUCTS, or sometimes VESSELS. There are all gradations between wood-cells and ducts, and between both these and common cells. But in most plants the three kinds are fairly distinct.

409. The proper cellular tissue, or _parenchyma_, is the ground-work of root, stem, and leaves; this is traversed, chiefly lengthwise, by the strengthening and conducting tissue, wood-cells and duct-cells, in the form of bundles or threads, which, in the stems and stalks of herbs are fewer and comparatively scattered, but in shrubs and trees so numerous and crowded that in the stems and all permanent parts they make a solid mass of wood. They extend into and ramify in the leaves, spreading out in a horizontal plane, as the framework of ribs and veins, which supports the softer cellular portion or parenchyma.

410. =Wood-Cells, or Woody Fibres=, consist of tubes, commonly between one and two thousandths, but in Pine-wood sometimes two or three hundredths, of an inch in diameter. Those from the tough bark of the Basswood, shown in Fig. 444, are only the fifteen-hundredth of an inch wide. Those of Buttonwood (Fig. 447) are larger, and are here highly magnified besides. The figures show the way wood-cells are commonly put together, namely, with their tapering ends overlapping each other,--spliced together, as it were,--thus giving more strength and toughness. In hard woods, such as Hickory and Oak, the walls of these tubes are very thick, as well as dense; while in soft woods, such as White Pine and Basswood, they are thinner.

411. Wood-cells in the bark are generally longer, finer, and tougher than those of the proper wood, and appear more like fibres. For example, Fig. 446 represents a cell of the wood of Basswood of average length, and Fig. 444 one (and part of another) of the fibrous bark, both drawn to the same scale. As these long cells form the principal part of fibrous bark, or _bast_, they are named _Bast-cells_ or _Bast-fibres_. These give the great toughness and flexibility to the inner bark of Basswood (i. e. Bast-wood) and of Leatherwood; and they furnish the invaluable fibres of flax and hemp; the proper wood of their stems being tender, brittle, and destroyed by the processes which separate for use the tough and slender bast-cells. In Leatherwood (Dirca) the bast-cells are remarkably slender. A view of one, if magnified on the scale of Fig. 444, would be a foot and a half long.

412. The wood-cells of Pines, and more or less of all other Coniferous trees, have on two of their sides very peculiar disk-shaped markings (Fig. 448-450) by which that kind of wood is recognizable.

413. =Ducts=, also called VESSELS, are mostly larger than wood-cells: indeed, some of them, as in Red Oak, have calibre large enough to be discerned on a cross section by the naked eye. They make the visible porosity of such kinds of wood. This is particularly the case with

_Dotted_ ducts (Fig. 451, 452), the surface of which appears as if riddled with round or oval pores. Such ducts are commonly made up of a row of large cells more or less confluent into a tube.

_Scalariform_ ducts (Fig. 458, 459), common in Ferns, and generally angled by mutual pressure in the bundles, have transversely elongated thin places, parallel with each other, giving a ladder-like appearance, whence the name.

_Annular_ ducts (Fig. 457) are marked with cross lines or rings, which are thickened portions of the cell-wall.

_Spiral_ ducts or vessels (Fig. 453-455) have thin walls, strengthened by a spiral fibre adherent within. This is as delicate and as strong as spider-web: when uncoiled by pulling apart, it tears up and annihilates the cell-wall. The uncoiled threads are seen by gently pulling apart many leaves, such as those of Amaryllis, or the stalk of a Strawberry leaflet.

_Laticiferous ducts_, _Vessels of the Latex_, or _Milk-vessels_ are peculiar branching tubes which hold _latex_ or milky juice in certain plants. It is very difficult to see them, and more so to make out their nature. They are peculiar in branching and inosculating, so as to make a net-work of tubes, running in among the cellular tissue; and they are very small, except when gorged and old (Fig. 460, 461).

Sec. 2. CELL-CONTENTS.

414. The living contents of young and active cells are mainly protoplasm with water or watery sap which this has imbibed. Old and effete cells are often empty of solid matter, containing only water with whatever may be dissolved in it, or air, according to the time and circumstances. All the various products which plants in general elaborate, or which particular plants specially elaborate, out of the common food which they derive from the soil and the air, are contained in the cells, and in the cells they are produced.

415. =Sap= is a general name for the principal liquid contents,--_Crude sap_, for that which the plant takes in, _Elaborated sap_ for what it has digested or assimilated. They must be undistinguishably mixed in the cells.

416. Among the solid matters into which cells convert some of their elaborated sap two are general and most important. These are _Chlorophyll_ and _Starch_.

417. =Chlorophyll= (meaning _leaf-green_) is what gives the green color to herbage. It consists of soft grains of rather complex nature, partly wax-like, partly protoplasmic. These abound in the cells of all common leaves and the green rind of plants, wherever exposed to the light. The green color is seen through the transparent skin of the leaf and the walls of the containing cells. Chlorophyll is essential to ordinary assimilation in plants: by its means, under the influence of sunlight, the plant converts crude sap into vegetable matter.

418. Far the largest part of all vegetable matter produced is that which goes to build up the plant's fabric or cellular structure, either directly or indirectly. There is no one good name for this most important product of vegetation. In its final state of cell-walls, the permanent fabric of herb and shrub and tree, it is called _Cellulose_ (408): in its most soluble form it is _Sugar_ of one or another kind; in a less soluble form it is _Dextrine_, a kind of liquefied starch: in the form of solid grains stored up in the cells it is _Starch_. By a series of slight chemical changes (mainly a variation in the water entering into the composition), one of these forms is converted into another.

419. =Starch= (_Farina_ or _Fecula_) is the form in which this common plant material is, as it were, laid by for future use. It consists of solid grains, somewhat different in form in different plants, in size varying from 1/300 to 1/4000 of an inch, partly translucent when wet, and of a pearly lustre. From the concentric lines, which commonly appear under the microscope, the grains seem to be made up of layer over layer. When loose they are commonly oval, as in potato-starch (Fig. 462): when much compacted the grains may become angular (Fig. 463).

420. The starch in a potato was produced in the foliage. In the soluble form of dextrine, or that of sugar, it was conveyed through the cells of the herbage and stalks to a subterranean shoot, and there stored up in the tuber. When the potato sprouts, the starch in the vicinity of developing buds or eyes is changed back again, first into mucilaginous dextrine, then into sugar, dissolved in the sap, and in this form it is made to flow to the growing parts, where it is laid down into cellulose or cell-wall.

421. Besides these cell-contents which are in obvious and essential relation to nutrition, there are others the use of which is problematical. Of such the commonest are

422. =Crystals.= These when slender or needle-shaped are called RHAPHIDES. They are of inorganic matter, usually of oxalate or phosphate or sulphate of lime. Some, at least of the latter, may be direct crystallizations of what is taken in dissolved in the water absorbed, but others must be the result of some elaboration in the plant. Some plants have hardly any; others abound in them, especially in the foliage and bark. In Locust-bark almost every cell holds a crystal; so that in a square inch not thicker than writing-paper there may be over a million and a half of them. When needle-shaped (rhaphides), as in stalks of Calla-Lily, Rhubarb, or Four-o'clock, they are usually packed in sheaf-like bundles. (Fig. 465, 466.)

Sec. 3. ANATOMY OF ROOTS AND STEMS.

423. This is so nearly the same that an account of the internal structure of stems may serve for the root also.

424. At the beginning, either in the embryo or in an incipient shoot from a bud, the whole stem is of tender cellular tissue or parenchyma. But wood (consisting of wood-cells and ducts or vessels) begins to be formed in the earliest growth; and is from the first arranged in two ways, making two general kinds of wood. The difference is obvious even in herbs, but is more conspicuous in the enduring stems of shrubs and trees.

425. On one or the other of these two types the stems of all phanerogamous plants are constructed. In one, the wood is made up of separate threads, scattered here and there throughout the whole diameter of the stem. In the other, the wood is all collected to form a layer (in a slice across the stem appearing as a ring) between a central cellular part which has none in it, the _Pith_, and an outer cellular part, the _Bark_.

426. An Asparagus-shoot and a Corn-stalk for herbs, and a rattan for a woody kind, represent the first kind. To it belong all plants with monocotyledonous embryo (40). A Bean-stalk and the stem of any common shrub or tree represent the second; and to it belong all plants with dicotyledonous or polycotyledonous embryo. The first has been called, not very properly, _Endogenous_, which means inside-growing; the second, properly enough, _Exogenous_, or outside-growing.

427. =Endogenous Stems=, those of Monocotyls (40), attain their greatest size and most characteristic development in Palms and Dragon-trees, therefore chiefly in warm climates, although the Palmetto and some Yuccas become trees along the southern borders of the United States. In such stems the woody bundles are more numerous and crowded toward the circumference, and so the harder wood is outside; while in an exogenous stem the oldest and hardest wood is toward the centre. An endogenous stem has no clear distinction of pith, bark, and wood, concentrically arranged, no silver grain, no annual layers, no bark that peels off clean from the wood. Yet old stems of Yuccas and the like, that continue to increase in diameter, do form a sort of layers and a kind of scaly bark when old. Yuccas show well the curving of the woody bundles (Fig. 471) which below taper out and are lost at the rind.

428. =Exogenous Stems=, those of Dicotyls (37), or of plants coming from dicotyledonous and also polycotyledonous embryos, have a structure which is familiar in the wood of our ordinary trees and shrubs. It is the same in an herbaceous shoot (such as a Flax-stem, Fig. 474) as in a Maple-stem of the first year's growth, except that the woody layer is commonly thinner or perhaps reduced to a circle of bundles. It was so in the tree-stem at the beginning. The wood all forms in a cylinder,--in cross section a ring--around a central cellular part, dividing the cellular core within, the pith, from a cellular bark without. As the wood-bundles increase in number and in size, they press upon each other and become wedge-shaped in the cross section; and they continue to grow from the outside, next the bark, so that they become very thin wedges or plates. Between the plates or wedges are very thin plates (in cross section lines) of much compressed cellular tissue, which connect the pith with the bark. The plan of a one-year-old woody stem of this kind is exhibited in the figures, which are essentially diagrams.

429. When such a stem grows on from year to year, it adds annually a layer of wood outside the preceding one, between that and the bark. This is exogenous growth, or outside-growing, as the name denotes.

430. Some new bark is formed every year, as well as new wood, the former inside, as the latter is outside of that of the year preceding. The ring or zone of tender forming tissue between the bark and the wood has been called the _Cambium Layer_. _Cambium_ is an old name of the physiologists for nutritive juice. And this thin layer is so gorged with rich nutritive sap when spring growth is renewed, that the bark then seems to be loose from the wood and a layer of viscid sap (or _cambium_) to be poured out between the two. But there is all the while a connection of the bark and the wood by delicate cells, rapidly multiplying and growing.

431. =The Bark= of a year-old stem consists of three parts, more or less distinct, namely,--beginning next the wood,--

1. The LIBER or FIBROUS BARK, the _Inner Bark_. This contains some wood-cells, or their equivalent, commonly in the form of bast or bast-cells (411, Fig. 444), such as those of Basswood or Linden, and among herbs those of flax and hemp, which are spun and woven or made into cordage. It also contains cells which are named _sieve_-cells, on account of numerous slits and pores in their walls, by which the protoplasm of contiguous cells communicates. In woody stems, whenever a new layer of wood is formed, some new liber or inner bark is also formed outside of it.

2. The GREEN BARK or _Middle Bark_. This consists of cellular tissue only, and contains the same green matter (_chlorophyll_, 417) as the leaves. In woody stems, before the season's growth is completed, it becomes covered by

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.

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_.

Sec. 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 spaces or air passages, 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.