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 Ba.s.swood, shown in Fig. 444, are only the fifteen-hundredth of an inch wide. Those of b.u.t.tonwood (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 Ba.s.swood, 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 Ba.s.swood 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 princ.i.p.al 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 Ba.s.swood (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.

[Ill.u.s.tration: Fig. 448. Magnified bit of a pine shaving, taken parallel with the silver grain. 449. Separate whole wood-cell, more magnified.

450. Same, still more magnified; both sections represented: _a_, disks in section, _b_, in face.]

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.

[Ill.u.s.tration: Fig. 451, 452. A large and a smaller dotted duct from Grape-Vine.]

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.

[Ill.u.s.tration: Fig. 453, 454. Spiral ducts which uncoil into a single thread. 455. Spiral duct which tears up as a band. 456. An annular duct, with variations above. 457. Loose spiral duct pa.s.sing into annular. 458.

Scalariform ducts of a Fern; part of a bundle, prismatic by pressure.

459. One torn into a band.]

_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.

[Ill.u.s.tration: Fig. 460. Milk Vessels of Dandelion, with cells of the common cellular tissue. 461. Others from the same older and gorged with milky juice. All highly magnified.]

_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).

-- 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 circ.u.mstances. 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 princ.i.p.al liquid contents,--_Crude sap_, for that which the plant takes in, _Elaborated sap_ for what it has digested or a.s.similated. 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 a.s.similation 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 l.u.s.tre. 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).

[Ill.u.s.tration: Fig. 462. Some magnified starch-grains, in two cells of a potato. 463. Some cells of the alb.u.men or floury part of Indian Corn, filled with starch-grains.]

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.

[Ill.u.s.tration: Fig. 464. Four cells from dried Onion-peel, each holding a crystal of different shape, one of them twinned. 465. Some cells from stalk of Rhubarb-plant, three containing chlorophyll; two (one torn across) with rhaphides. 466. Rhaphides in a cell, from Arisaema, with small cells surrounding. 467. Prismatic crystals from the bark of Hickory. 468. Glomerate crystal in a cell, from Beet-root. 469. A few cells of Locust-bark, a crystal in each. 470. A detached cell, with rhaphides being forced out, as happens when put in water.]

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.)

-- 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_.

[Ill.u.s.tration: Fig. 471. Diagram of structure of Palm or Yucca. 472.

Structure of a Corn stalk, in transverse and longitudinal section. 473.

Same of a small Palm stem. The dots on the cross sections represent cut ends of the woody bundles or threads.]

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 circ.u.mference, 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.

[Ill.u.s.tration: Fig. 474. Short piece of stem of Flax, magnified, showing the bark, wood, and pith in a cross section.]

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.

[Ill.u.s.tration: Fig. 475. Diagram of a cross section of a very young exogenous stem, showing six woody bundles or wedges. 476. Same later, with wedges increased to twelve. 477. Still later, the wedges filling the s.p.a.ce, separated only by the thin lines, or medullary rays, running from pith to bark.]

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.

[Ill.u.s.tration: Fig. 478. Piece of a stem of Soft Maple, of a year old, cut crosswise and lengthwise.]

[Ill.u.s.tration: Fig. 479. A portion of the same, magnified.]

[Ill.u.s.tration: Fig. 480. A small piece of the same, taken from one side, reaching from the bark to the pith, and highly magnified: _a_, a small bit of the pith; _b_, spiral ducts of what is called the _medullary sheath_; _c_, the wood; _d_, _d_, dotted ducts in the wood; _e_, _e_, annular ducts; _f_, the liber or inner bark; _g_, the green bark; _h_, the corky layer; _i_, the skin, or epidermis; _j_, one of the medullary rays, or plates of silver grain, seen on the cross-section.]

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 Ba.s.swood 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