Section XV.THE SEED.

Fig.379. Akene of Mayweed (no pappus). 380. That of Succory (its pappus a shallow cup). 381. Of Sunflower (pappus of two deciduous scales). 382. Of Sneezeweed (Helenium), with its pappus of five scales. 383. Of Sow-Thistle, with its pappus of delicate downy hairs. 384. Of the Dandelion, its pappus raised on a long beak.

361.A Cremocarp(Fig.385), a name given to the fruit of Umbelliferæ, consists as it were of a pair of akenes united completely in the blossom, but splitting apart when ripe into the two closed carpels. Each of these is aMericarporHemicarp, names seldom used.

362.A Utricleis the same as an akene, but with a thin and bladdery loose pericarp; like that of the Goosefoot or Pigweed (Fig.386). When ripe it may burst open irregularly to discharge the seed; or it may open by a circular line all round, the upper part falling off like a lid; as in the Amaranth (Fig.387).

Fig.385. Fruit (cremocarp) of Osmorrhiza; the two akene-like ripe carpels separating at maturity from a slender axis or carpophore.

Fig.386. Utricle of the common Pigweed (Chenopodium album).Fig.387. Utricle (pyxis) of Amaranth, opening all round (circumscissile).

Fig.386. Utricle of the common Pigweed (Chenopodium album).

Fig.387. Utricle (pyxis) of Amaranth, opening all round (circumscissile).

363.A Caryopsis, or Grain, is like an akene with the seed adhering to the thin pericarp throughout, so that fruit and seed are incorporated into one body; as in wheat, Indian corn, and other kinds of grain.

364.A Nutis a dry and indehiscent fruit, commonly one-celled and one-seeded,with a hard, crustaceous, or bony wall, such as the cocoa-nut, hazelnut, chestnut, and the acorn (Fig.37,388.) Here the involucre, in the form of a cup at the base, is called theCupule. In the Chestnut the cupule forms the bur; in the Hazel, a leafy husk.

Fig.388. Nut (acorn) of the Oak, with its cup or cupule.

365.A Samara, or Key-fruit, is either a nut or an akene, or any other indehiscent fruit, furnished with a wing, like that of Ash (Fig.389), and Elm (Fig.390). The Maple-fruit is a pair of keys (Fig.391).

366. Dehiscent Fruits, or Pods, are of two classes, viz., those of a simple pistil or carpel, and those of a compound pistil. Two common sorts of the first are named as follows:—

367.The Follicleis a fruit of a simple carpel, which dehisces down one side only, i. e. by the inner or ventral suture. The fruits of Marsh Marigold (Fig.392), Pæony, Larkspur, and Milkweed are of this kind.

Fig.389. Samara or key of the White Ash, winged at end. 390. Samara of the American Elm, winged all round.Fig.391. Pair of samaras of Sugar Maple.Fig.392. Follicle of Marsh Marigold (Caltha palustris).Fig.393. Legume of a Sweet Pea, opened.Fig.394. Loment or jointed legume of a Tick-Trefoil (Desmodium).

Fig.389. Samara or key of the White Ash, winged at end. 390. Samara of the American Elm, winged all round.

Fig.391. Pair of samaras of Sugar Maple.

Fig.392. Follicle of Marsh Marigold (Caltha palustris).

Fig.393. Legume of a Sweet Pea, opened.

Fig.394. Loment or jointed legume of a Tick-Trefoil (Desmodium).

368.The Legumeor true Pod, such as the peapod (Fig.393), and the fruit of the Leguminous or Pulse family generally, is one which opens along the dorsal as well as the ventral suture. The two pieces into which it splits are calledValves. ALomentis a legume which is constricted between the seeds, and at length breaks up crosswise into distinct joints, as in Fig.394.

369. The pods or dehiscent fruits belonging to a compound ovary have several technical names: but they all may be regarded as kinds of

370.The Capsule, the dry and dehiscent fruit of any compound pistil. The capsule may discharge its seeds through chinks or pores, as in thePoppy, or burst irregularly in some part, as in Lobelia and the Snapdragon; but commonly it splits open (or isdehiscent) lengthwise into regular pieces, calledValves.

Fig.395. Capsule of Iris, with loculicidal dehiscence; below, cut across.Fig.396. Pod of a Marsh St. John's-wort, with septicidal dehiscence.

Fig.395. Capsule of Iris, with loculicidal dehiscence; below, cut across.

Fig.396. Pod of a Marsh St. John's-wort, with septicidal dehiscence.

371.RegularDehiscencein a capsule takes place in two ways, which are best illustrated in pods of two or three cells. It is either

Loculicidal, or, splitting directly into theloculior cells, that is, down the back (or the dorsal suture) of each cell or carpel, as in Iris (Fig.395); or

Septicidal, that is, splitting through the partitions orsepta, as in St. John's-wort (Fig.396), Rhododendron, etc. This divides the capsule into its component carpels, which then open by their ventral suture.

Fig.397, 398. Diagrams of the two modes.Fig.399. Diagram of septifragal dehiscence of the loculicidal type. 400. Same of the septicidal ormarginicidaltype.

Fig.397, 398. Diagrams of the two modes.

Fig.399. Diagram of septifragal dehiscence of the loculicidal type. 400. Same of the septicidal ormarginicidaltype.

372. In loculicidal dehiscence the valves naturally bear the partitions on their middle; in the septicidal, half the thickness of a partition is borne on the margin of each valve. See the annexed diagrams. A variation of either mode occurs when the valves break away from the partitions, these remaining attached in the axis of the fruit. This is calledSeptifragaldehiscence. One form is seen in the Morning-Glory (Fig.400).

373. The capsules of Rue, Spurge, and some others, are both loculicidal and septicidal, and so split into half-carpellary valves or pieces.

374.The Silique(Fig.401) is the technical name of the peculiar pod of the Mustard family; which is two-celled by a false partition stretched across between two parietal placentæ. It generally opens by two valves from below upward, and the placentæ with the partition are left behind when the valves fall off.

375.A Silicle or Pouchis only a short and broad silique, like that of the Shepherd's Purse, Fig.402, 403.

Fig.401. Silique of a Cadamine or Spring Cress.Fig.402. Silicle of Shepherd's Purse.Fig.403. Same, with one valve removed.Fig.404. Pyxis of Purslane, the lid detaching.

Fig.401. Silique of a Cadamine or Spring Cress.

Fig.402. Silicle of Shepherd's Purse.

Fig.403. Same, with one valve removed.

Fig.404. Pyxis of Purslane, the lid detaching.

376.The Pyxisis a pod which opens by a circular horizontal line, the upper part forming a lid, as in Purslane (Fig.404), the Plantain, Henbane, etc. In these the dehiscence extends all round, or iscircumscissile. So it does in Amaranth (Fig.387), forming a one-seeded utricular pyxis. In Jeffersonia, the line does not separate quite round, but leaves a portion for a hinge to the lid.

377. Of Multiple or Collective Fruits, which are properly masses of fruits aggregated into one body (as is seen in the Mulberry (Fig.408), Pine-apple, etc.), there are two kinds with special names and of peculiar structure.

Fig.405. A fig-fruit when young. 406. Same in section. 407. Magnified portion, a slice, showing some of the flowers.Fig.408. A mulberry. 409. One of the grains younger, enlarged; seen to be a pistillate flower with calyx becoming fleshy. 410. Same, with fleshy calyx cut across.

Fig.405. A fig-fruit when young. 406. Same in section. 407. Magnified portion, a slice, showing some of the flowers.

Fig.408. A mulberry. 409. One of the grains younger, enlarged; seen to be a pistillate flower with calyx becoming fleshy. 410. Same, with fleshy calyx cut across.

378.The Syconium or Fig-fruit(Fig.405, 406) is a fleshy axis or summit of stem, hollowed out, and lined within by a multitude of minute flowers, the whole becoming pulpy, and in the common fig, luscious.

379.The Strobile or Cone(Fig.411), is the peculiar multiple fruit of Pines, Cypresses, and the like; hence namedConiferæ, viz. cone-bearingplants. As already shown (313), these cones areopen pistils, mostly in the form of flat scales, regularly overlying each other, and pressed together in a spike or head. Each scale bears one or two naked seeds on its inner face. When ripe and dry, the scales turn back or diverge, and in the Pine the seed peels off and falls, generally carrying with it a wing, a part of the lining of the scale, which facilitates the dispersion of the seeds by the wind (Fig.412, 413). In Arbor-Vitæ, the scales of the small cone are few, and not very unlike the leaves. In Cypress they are very thick at the top and narrow at the base, so as to make a peculiar sort of closed cone. In Juniper and Red Cedar, the few scales of the very small cone become fleshy, and ripen into a fruit which closely resembles a berry.

Fig.411. Cone of a common Pitch Pine. 412. Inside view of a separated scale or open carpel; one seed in place: 413, the other seed.

380. Seeds are the final product of the flower, to which all its parts and offices are subservient. Like the ovule from which it originates, a seed consists of coats and kernel.

Fig.414. Seed of a Linden or Basswood cut through lengthwise, and magnified, the parts lettered:a, the hilum or scar;b, the outer coat;c, the inner;d, the albumen;e, the embryo.

381.The Seed-coatsare commonly two (320), the outer and the inner. Fig.414shows the two, in a seed cut through lengthwise. The outer coat is often hard or crustaceous, whence it is called theTesta, or shell of the seed; the inner is almost always thin and delicate.

Fig.415. A winged seed of the Trumpet-Creeper.Fig.416. One of Catalpa, the kernel cut to show the embryo.

Fig.415. A winged seed of the Trumpet-Creeper.

Fig.416. One of Catalpa, the kernel cut to show the embryo.

Fig.417. Seed of Milkweed, with aComaor tuft of long silky hairs at one end.

382. The shape and the markings, so various in different seeds, depend mostly on the outer coat. Sometimes this fits the kernel closely; sometimes it is expanded into awing, as in the Trumpet-Creeper (Fig.415), and occasionally this wing is cut up into shreds or tufts, as in the Catalpa (Fig.416); or instead of a wing it may bear aComa, or tuft of long and soft hairs, as in the Milkweed or Silkweed (Fig.417). The use of wings, or downy tufts is to render the seeds buoyantfor dispersion by the winds. This is clear, not only from their evident adaptation to this purpose, but also from the fact that winged and tufted seeds are found only in fruits that split open at maturity, never in those that remain closed. The coat of some seeds is beset with long hairs or wool.Cotton, one of the most important vegetable products, since it forms the principal clothing of the larger part of the human race, consists of the long and woolly hairs which thickly cover the whole surface of the seed. There are also crests or other appendages of various sorts on certain seeds. A few seeds have an additional, but more or less incomplete covering, outside of the real seed-coats called an

383.Aril, or Arillus.The loose and transparent bag which encloses the seed of the White Water-Lily (Fig.418) is of this kind. So is themaceof the nutmeg; and also the scarlet pulp around the seeds of the Waxwork (Celastrus) and Strawberry-bush (Euonymus). The aril is a growth from the extremity of the seed-stalk, or from the placenta when there is no seed-stalk.

Fig.418. Seed of White Water Lily, enclosed in its aril.

384. A short and thickish appendage at or close to the hilum in certain seeds is called aCaruncleorStrophiole(Fig.419).

Fig.419. Seed of Ricinus or Castor oil plant, with caruncle.

385. The various terms which define the position or direction of the ovule (erect, ascending, etc.) apply equally to the seed: so also the terms anatropous, orthotropous, campylotropous, etc., as already defined (320,321), and such terms as

Hilum, orScarleft where the seed-stalk or funiculus falls away, or where the seed was attached directly to the placenta when there is no seed-stalk.

Rhaphe, the line or ridge which runs from the hilum to the chalaza in anatropous and amphitropous seeds.

Chalaza, the place where the seed-coats and the kernel or nucleus are organically connected,—at the hilum in orthotropous and campylotropous seeds, at the extremity of the rhaphe or tip of the seed in other kinds.

Micropyle, answering to theForamenor orifice of the ovule. Compare the accompanying figures and those of the ovules, Fig.341-355.

Fig.420. Seed of a Violet (anatropous):a, hilum;b, rhaphe;c, chalaza.Fig.421. Seed of a Larkspur (also anatropous); the parts lettered as in the last.Fig.422. The same, cut through lengthwise:a, the hilum;c, chalaza;d, outer seed coat;e, inner seed-coat;f, the albumen;g, the minute embryo.Fig.423. Seed of a St. John's-wort, divided lengthwise; here the whole kernel is embryo.

Fig.420. Seed of a Violet (anatropous):a, hilum;b, rhaphe;c, chalaza.

Fig.421. Seed of a Larkspur (also anatropous); the parts lettered as in the last.

Fig.422. The same, cut through lengthwise:a, the hilum;c, chalaza;d, outer seed coat;e, inner seed-coat;f, the albumen;g, the minute embryo.

Fig.423. Seed of a St. John's-wort, divided lengthwise; here the whole kernel is embryo.

386.The Kernel, or Nucleus, is the whole body of the seed within the coats. In many seeds the kernel is allEmbryo; in others a large part of it is theAlbumen. For example, in Fig.423, it is wholly embryo; in Fig.422, all but the small speck (g) is albumen.

387.The Albumen or Endospermof the seed is sufficiently characterized and its office explained in Sect. III.,31-35.

388.The EmbryoorGerm, which is the rudimentary plantlet and the final result of blossoming, and its development in germination have been extensively illustrated in SectionsII.andIII.Its essential parts are theRadicleand theCotyledons.

389.Its Radicle or Caulicle(the former is the term long and generally used in botanical descriptions, but the latter is the more correct one, for it is the initial stem, which merely gives origin to the root), as to its position in the seed, always points to and lies near the micropyle. In relation to the pericarp it is

Superior, when it points to the apex of the fruit or cell, and

Inferior, when it points to its base, or downward.

Fig.424. Embryo of Calycanthus; upper part cut away, to show the convolute cotyledons.

390.The Cotyledonshave already been illustrated as respects their number,—giving the important distinction ofDicotyledonous,PolycotyledonousandMonocotyledonousembryos (36-43),—also as regards their thickness, whetherfoliaceousorfleshy; and some of the very various shapes and adaptations to the seed have been figured. They may be straight, or folded, or rolled up. In the latter case the cotyledons may be rolled up as it were from one margin, as in Calycanthus (Fig.424), or from apex to base in a flat spiral, or they may be both folded (plicate) and rolled up (convolute), as in Sugar Maple (Fig.11.) In one very natural family, the Cruciferæ, two different modes prevail in the way the two cotyledons are brought round against the radicle. In one series they are

Accumbent, that is, the edges of the flat cotyledons lie against the radicle, as in Fig.425, 426. In another they are

Fig.425. Seed of Bitter Cress, Barbarea, cut across to show the accumbent cotyledons. 426. Embryo of same, whole.

Incumbent, or with the plane of the cotyledons brought up in the opposite direction, so that the back of one of them lies against the radicle, as shown in Fig.427, 428.

Fig.427. Seed of a Sisymbrium, cut across to show the incumbent cotyledons. 428. Embryo of the same, detached whole.

391. As to the situation of the embryo with respect to the albumen of the seed, when this is present in any quantity, the embryo may beAxile, that is occupying the axis or centre, either for most of its length, as in Violet (Fig.429), Barberry (Fig.48), and Pine (Fig.56); and in these it is straight. But it may be variously curved or coiled in the albumen, as in Helianthemum (Fig.430), in a Potato-seed (Fig.50), or Onion-seed (Fig.60), and Linden (Fig.414); or it may be coiled around the outside of the albumen, partly or into a circle, as in Chickweed (Fig.431, 432) and in Mirabilis (Fig.52). The latter mode prevails in Campylotropous seeds. In the cereal grains, such as Indian Corn (Fig.67) and Rice (Fig.430a), and in all other Grasses, the embryo is straight and applied to the outside of the abundant albumen.

Fig.429. Section of seed of Violet; anatropous with straight axile embryo in the albumen. 430. Section of seed of Rock Rose, Helianthemum Canadense; orthotropous, with curved embryo in the albumen. 430a. Section of a grain of Rice, lengthwise, showing the embryo outside the albumen, which forms the principal bulk.

Fig.431. Seed of a Chickweed, campylotropous. 432. Section of same, showing slender embryo coiled around the outside of the albumen of the kernel.

392. The matured seed, with embryo ready to germinate and reproduce the kind, completes the cycle of the vegetable life in a phanerogamous plant, the account of which began with the seed and seedling.

393. The following simple outlines of the anatomy and physiology of plants (3) are added to the preceding structural part for the better preparation of students in descriptive and systematic botany; also to give to all learners some general idea of the life, growth, intimate structure, and action of the beings which compose so large a part of organic nature. Those who would extend and verify the facts and principles here outlined will use the Physiological Botany of the "Botanical Text Book," by Professor Goodale, or some similar book.

394.Growthis the increase of a living thing in size and substance. It appears so natural that plants and animals should grow, that one rarely thinks of it as requiring explanation. It seems enough to say that a thing is so because it grew so. Growth from the seed, the germination and development of an embryo into a plantlet, and at length into a mature plant (as illustrated in SectionsII.andIII.), can be followed by ordinary observation. But the embryo is already a miniature plantlet, sometimes with hardly any visible distinction of parts, but often one which has already made very considerable growth in the seed. To investigate the formation and growth of the embryo itself requires well-trained eyes and hands, and the expert use of a good compound microscope. So this is beyond the reach of a beginner.

395. Moreover, although observation may show that a seedling, weighing only two or three grains, may double its bulk and weight every week of its early growth, and may in time produce a huge amount of vegetable matter, it is still to be asked what this vegetable matter is, where it came from, and by what means plants are able to increase and accumulate it, and build it up into the fabric of herbs and shrubs and lofty trees.

396.Protoplasm.All this fabric was built up under life, but only a small portion of it is at any one time alive. As growth proceeds, life is passed on from the old to the new parts, much as it has passed on from parent to offspring, from generation to generation in unbroken continuity.Protoplasmis the common name of that plant-stuff in which life essentially resides. All growth depends upon it; for it has the peculiar power of growing and multiplying and building up a living structure,—the animal no less than the vegetable structure, for it is essentially the same in both. Indeed, all the animal protoplasm comes primarily from the vegetable, which has the prerogative of producing it; and the protoplasm of plants furnishes all that portion of the food of animals which forms their flesh and living fabric.

397. The very simplest plants (if such may specifically be called plants rather than animals, or one may say, the simplest living things) are mere particles, or pellets, or threads, or even indefinite masses of protoplasm of vague form, which possess powers of motion or of changing their shape, of imbibing water, air, and even other matters, and of assimilating these into plant-stuff for their own growth and multiplication. Their growth is increase in substance by incorporation of that which they take in and assimilate. Their multiplication is by spontaneous division of their substance or body into two or more, each capable of continuing the process.

398. The embryo of a phanerogamous plant at its beginning (344) is essentially such a globule of protoplasm, which soon constricts itself into two and more such globules, which hold together inseparably in a row; then the last of the row divides without separation in the two other planes, toform a compound mass, each grain or globule of which goes on to double itself as it grows; and the definite shaping of this still increasing mass builds up the embryo into its form.

Fig.433-436. Figures to illustrate the earlier stages in the formation of an embryo; a single mass of protoplasm (Fig. 433) dividing into two, three, and then into more incipient cells, which by continued multiplication build up an embryo.

399.Cell-walls.While this growth was going on, each grain of the forming structure formed and clothed itself with a coat, thin and transparent, of something different from protoplasm,—something which hardly and only transiently, if at all, partakes of the life and action. The protoplasm forms the living organism; the coat is a kind of protective covering or shell. The protoplasm, like the flesh of animals which it gives rise to, is composed of four chemical elements: Carbon, Hydrogen, Oxygen, and Nitrogen. The coating is of the nature of wood (is, indeed, that which makes wood), and has only the three elements, Carbon, Hydrogen, and Oxygen, in its composition.

Fig.437. Magnified view of some of a simple fresh water Alga, the Tetraspora lubrica, each sphere of which may answer to an individual plant.

400. Although the forming structure of an embryo in the fertilized ovule is very minute and difficult to see, there are many simple plants of lowest grade, abounding in pools of water, which more readily show the earlier stages or simplest states of plant-growth. One of these, which is common in early spring, requires only moderate magnifying power to bring to view what is shown in Fig.437. In a slimy mass which holds all loosely together, little spheres of green vegetable matter are seen, assembled in fours, and these fours themselves in clusters of fours. A transient inspection shows, what prolonged watching would confirm, that each sphere divides first in one plane, then in the other, to make four, soon acquiring the size of the original, and so on, producing successive groups of fours. These pellets each form on their surface a transparent wall, like that just described. The delicate wall is for some time capable of expansive growth, but is from the first much firmer than the protoplasm within; through it the latter imbibes surrounding moisture, which becomes a watery sap, occupying vacuities in the protoplasmic mass which enlarge or run together as the periphery increases and distends. When full grown the protoplasm may become a mere lining to the wall, or some of it central, as a nucleus, this usually connected with the wall-lining by delicate threads of the same substance. So, when full grown, the wall with its lining—a vesicle, containing liquid or somesolid matters and in age mostly air—naturally came to be named a Cell. But the name was suggested by, and first used only for, cells in combination or built up into a fabric, much as a wall is built of bricks, that is, into a

401.Cellular Structure or Tissue.Suppose numerous cells like those of Fig.437to be heaped up like a pile of cannon-balls, and as they grew, to be compacted together while soft and yielding; they would flatten where they touched, and each sphere, being touched by twelve surrounding ones would become twelve-sided. Fig.438would represent one of them. Suppose the contiguous faces to be united into one wall or partition between adjacent cavities, and acellular structurewould be formed, like that shown in Fig.439. Roots, stems, leaves, and the whole of phanerogamous plants are a fabric of countless numbers of such cells. No such exact regularity in size and shape is ever actually found; but a nearly truthful magnified view of a small portion of a slice of the flower-stalk of a Calla Lily (Fig.440) shows a fairly corresponding structure; except that, owing to the great air-spaces of the interior, the fabric may be likened rather to a stack of chimneys than to a solid fabric. In young and partly transparent parts one may discern the cellular structure by looking down directly on the surface, as of a forming root. (Fig.82,441, 442).

Fig.438. Diagram of a vegetable cell, such as it would be if when spherical it were equally pressed by similar surrounding cells in a heap.Fig.439. Ideal construction of cellular tissue so formed, in section.

Fig.438. Diagram of a vegetable cell, such as it would be if when spherical it were equally pressed by similar surrounding cells in a heap.

Fig.439. Ideal construction of cellular tissue so formed, in section.

Fig.440. Magnified view of a portion of a transverse slice of stem of Calla Lily. The great spaces are tubular air-channels built up by the cells.

402.The substance of which cell-walls are mainly composed is calledCellulose. It is essentially the same in the stem of a delicate leaf or petal and in the wood of an Oak, except that in the latter the walls aremuch thickened and the calibre small. The protoplasm of each living cell appears to be completely shut up and isolated in its shell of cellulose; but microscopic investigation has brought to view, in many cases, minute threads of protoplasm which here and there traverse the cell-wall through minute pores, thus connecting the living portion of one cell with that of adjacent cells. (See Fig.447, &c.)

Fig.441. Much magnified small portion of young root of a seedling Maple (such as of Fig.82); and 442, a few cells of same more magnified. The prolongations from the back of some of the cells are root hairs.

403. The hairs of plants are cells formed on the surface; either elongated single cells (like the root-hairs of Fig.441, 442), or a row of shorter cells. Cotton fibres are long and simple cells growing from the surface of the seed.

404. The size of the cells of which common plants are made up varies from about the thirtieth to the thousandth of an inch in diameter. An ordinary size of short or roundish cells is from 1/300 to 1/500 of an inch; so that there may generally be from 27 to 125 millions of cells in the compass of a cubic inch!

405. Some parts are built up as a compact structure; in others cells are arranged so as to build up regular air-channels, as in the stems of aquatic and other water-loving plants (Fig.440), or to leave irregular spaces, as in the lower part of most leaves, where the cells only here and there come into close contact (Fig.443).

Fig.443. Magnified section through the thickness of a leaf of Florida Star-Anise.

406. All such soft cellular tissue, like this of leaves, that of pith, and of the green bark, is calledParenchyma, while fibrous and woody parts are composed ofProsenchyma, that is, of peculiarly transformed

407.Strengthening Cells.Common cellular tissue, which makes up the whole structure of all very young plants, and the whole of Mosses and other vegetables of the lowest grade, even when full grown, is too tender or too brittle to give needful strength and toughness for plants which are to rise to any considerable height and support themselves. In these needful strength is imparted, and the conveyance of sap through the plant is facilitated, by the change, as they are formed, of some cells into thicker-walled and tougher tubes, and by the running together of some ofthese, or the prolongation of others, into hollow fibres or tubes of various size. Two sorts of such transformed cells go together, and essentially form the

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 properWoody FibreorWood-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 calledDucts, or sometimesVessels. 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.

Fig.444. Magnified wood-cells of the bark (bast-cells) of Basswood, one and part of another. 445. Some wood cells from the wood (and below part of a duct); and 446, a detached wood-cell of the same; equally magnified.Fig.447. Some wood cells from Buttonwood, Platanus, highly magnified, a whole cell and lower end of another on the left; a cell cut half away lengthwise, and half of another on the right; some pores or pits (a) seen on the left; whileb bmark sections through these on the cut surface. When living and young the protoplasm extends into these and by minuter perforations connects across them. In age the pits become open passages, facilitating the passage of sap and air.

Fig.447. Some wood cells from Buttonwood, Platanus, highly magnified, a whole cell and lower end of another on the left; a cell cut half away lengthwise, and half of another on the right; some pores or pits (a) seen on the left; whileb bmark sections through these on the cut surface. When living and young the protoplasm extends into these and by minuter perforations connects across them. In age the pits become open passages, facilitating the passage of sap and air.

409. The proper cellular tissue, orparenchyma, 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.446represents a cell of the wood of Basswood of average length, and Fig.444one (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, orbast, they are namedBast-cellsorBast-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.

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.

Fig.451, 452. A large and a smaller dotted duct from Grape-Vine.

413.Ducts, also calledVessels, 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

Dottedducts (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.

Scalariformducts (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.

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

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