The Elements of Botany Part 20

387. =The Alb.u.men or Endosperm= of the seed is sufficiently characterized and its office explained in Sect. III., 31-35.

388. =The Embryo= or _Germ_, which is the rudimentary plantlet and the final result of blossoming, and its development in germination have been extensively ill.u.s.trated in Sections II. and III. Its essential parts are the _Radicle_ and the _Cotyledons_.

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.

[Ill.u.s.tration: Fig. 424. Embryo of Calycanthus; upper part cut away, to show the convolute cotyledons.]

390. =The Cotyledons= have already been ill.u.s.trated as respects their number,--giving the important distinction of _Dicotyledonous_, _Polycotyledonous_ and _Monocotyledonous_ embryos (36-43),--also as regards their thickness, whether _foliaceous_ or _fleshy_; 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 Cruciferae, two different modes prevail in the way the two cotyledons are brought round against the radicle. In one series they are

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

[Ill.u.s.tration: Fig. 425. Seed of Bitter Cress, Barbarea, cut across to show the acc.u.mbent cotyledons. 426. Embryo of same, whole.]

_Inc.u.mbent_, 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.

[Ill.u.s.tration: Fig. 427. Seed of a Sisymbrium, cut across to show the inc.u.mbent cotyledons. 428. Embryo of the same, detached whole.]

391. As to the situation of the embryo with respect to the alb.u.men of the seed, when this is present in any quant.i.ty, the embryo may be _Axile_, 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 alb.u.men, 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 alb.u.men, 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 Gra.s.ses, the embryo is straight and applied to the outside of the abundant alb.u.men.

[Ill.u.s.tration: Fig. 429. Section of seed of Violet; anatropous with straight axile embryo in the alb.u.men. 430. Section of seed of Rock Rose, Helianthemum Canadense; orthotropous, with curved embryo in the alb.u.men.

430a. Section of a grain of Rice, lengthwise, showing the embryo outside the alb.u.men, which forms the princ.i.p.al bulk.]

[Ill.u.s.tration: Fig. 431. Seed of a Chickweed, campylotropous. 432.

Section of same, showing slender embryo coiled around the outside of the alb.u.men 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.

Section XVI. VEGETABLE LIFE AND WORK.

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.

-- 1. ANATOMICAL STRUCTURE AND GROWTH.

394. =Growth= _is 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 ill.u.s.trated in Sections II. and III.), 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 acc.u.mulate 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 pa.s.sed on from the old to the new parts, much as it has pa.s.sed on from parent to offspring, from generation to generation in unbroken continuity. _Protoplasm_ is 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 ma.s.ses 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 a.s.similating 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 a.s.similate. 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, to form a compound ma.s.s, each grain or globule of which goes on to double itself as it grows; and the definite shaping of this still increasing ma.s.s builds up the embryo into its form.

[Ill.u.s.tration: Fig. 433-436. Figures to ill.u.s.trate the earlier stages in the formation of an embryo; a single ma.s.s 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 sh.e.l.l. 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.

[Ill.u.s.tration: 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 ma.s.s which holds all loosely together, little spheres of green vegetable matter are seen, a.s.sembled in fours, and these fours themselves in cl.u.s.ters 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 ma.s.s 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 some solid 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. 437 to be heaped up like a pile of cannon-b.a.l.l.s, 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. 438 would represent one of them. Suppose the contiguous faces to be united into one wall or part.i.tion between adjacent cavities, and a _cellular structure_ would 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-s.p.a.ces 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).

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: Fig. 439. Ideal construction of cellular tissue so formed, in section.]

[Ill.u.s.tration: Fig. 440. Magnified view of a portion of a transverse slice of stem of Calla Lily. The great s.p.a.ces are tubular air-channels built up by the cells.]

402. The substance of which cell-walls are mainly composed is called CELLULOSE. 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 are much thickened and the calibre small. The protoplasm of each living cell appears to be completely shut up and isolated in its sh.e.l.l 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.)

[Ill.u.s.tration: 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 compa.s.s 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 s.p.a.ces, as in the lower part of most leaves, where the cells only here and there come into close contact (Fig. 443).

[Ill.u.s.tration: 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 called PARENCHYMA, while fibrous and woody parts are composed of PROSENCHYMA, 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 of these, 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 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 part.i.tions 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.

[Ill.u.s.tration: Fig. 444. Magnified wood-cells of the bark (bast-cells) of Ba.s.swood, 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.]

[Ill.u.s.tration: Fig. 447. Some wood cells from b.u.t.tonwood, Plata.n.u.s, 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; while _b b_ mark 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 pa.s.sages, facilitating the pa.s.sage of sap and air.]

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 ma.s.s 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.