4.

b.

Their structure. Each is composed of a layer of transparent cells containing no chlorophyll, arranged in four rows, and surrounding a central cavity which extends into the cushion formed by the thickened part of the prothallus (a. 1. b. a). In this cavity lies, in young specimens, a large nucleated granular basal cell, with two or three smaller granular cells (neck-cells) above it in the narrow upper part of the cavity; in older specimens this upper part is empty, forming a canal leading down to the basal cell.

Examine young Fern in connection with its prothallus.

VIII.

THE BEAN-PLANT (Vicia faba).

In this, which is selected as a convenient example of a Flowering Plant, the same parts are to be distinguished as in the Fern; but the axis is erect and consists of a root imbedded in the earth and a stem which rises into the air. The appendages of the stem are leaves, developed from the opposite sides of successive nodes; and the internodes become shorter and shorter towards the summit of the stem, which ends in a terminal bud. Buds are also developed in the axils of the leaves, and some of them grow into branches, which repeat the characters of the stem; but others, when the plant attains its full development, grow into stalks which support the flowers; each of which consists of a calyx, a corolla, a staminal tube and a central pistil; the latter is terminated by a style, the free end of which is the stigma.

The staminal tube ends in ten filaments, four of which are rather shorter than the rest; and the filaments bear oval bodies, the anthers, which, when ripe, give exit to a fine powder, made up of minute pollen grains. The pistil is hollow; and, attached by short stalks along the ventral side of it, or that turned towards the axis, is a longitudinal series of minute bodies, the ovules. Each ovule consists of a central conical nucleus, invested by two coats, an outer and an inner. Opposite the summit of the nucleus, these coats are perforated by a canal, the micropyle, which leads down to the

The

nucleus. The nucleus contains a sac, the embryo sac, in which certain cells, one of which is the embryo cell, and the rest endosperm cells, are developed. A pollen grain deposited on the stigma, sends out a hypha-like prolongation, the pollen tube, which elongates, passes down the style, and eventually reaches the micropyle of an ovule. Traversing the micropyle, the end of the pollen tube penetrates the nucleus, and comes into close contact with the embryo sac. This is the process of impregnation, and the result of it is that the embryo cell divides and give rise to a cellular embryo. This becomes a minute Bean-plant, consisting of a radicle or primary root; of two, relatively large, primary leaves, the cotyledons; and of a short stem, the plumule, on which rudimentary leaves soon appear. The cotyledons now increase in size, out of all proportion to the rest of the embryonic plant; and the cells of which they are composed become filled with starch and other nutritious matter. nucleus and coats of the ovule grow to accommodate the enlarging embryo, but, at the same time, become merged into an envelope which constitutes the coat of the seed. The pistil enlarges and becomes the pod; this, when it has attained its full size, dries and readily bursts along its edges, or decays, setting the seeds free. Each seed, when placed in proper conditions of warmth and moisture, then germinates. The cotyledons of the contained embryo swell, burst the seed coat, and, becoming green, emerge as the fleshy seed leaves. The nutritious matters which they contain are absorbed by the plumule and radicle, the latter of which descends into the earth and becomes the root, while the former ascends and becomes the stem of the young bean-plant. The apex of the stem retains, throughout life, the simply cellular structure which is, at first, characteristic of the whole embryo; and the growth in length of the stem, so far as it

depends on the addition of new cells, takes place chiefly, if not exclusively, in this part.

The apex of the root, on the other hand, gives rise to a root-sheath, as in the Fern.

The leaves cease to grow by cell multiplication at their apices, when these are once formed, the addition of new cells taking place at their bases.

The tissues which compose the body of the Bean-plant are similar, in their general characters, to those found in the Fern, but they differ in the manner of their arrangement. The surface is bounded by a layer of epidermic cells, within which, rounded or polygonal cells make up the groundsubstance, or parenchyma, of the plant, extending to its very centre in the younger parts of the stem and in the root; while, in the older parts of the stem, the centre is occupied by a more or less considerable cavity, full of air. This cavity results from the central parenchyma becoming torn asunder, after it has ceased to grow, by the enlargement of the peripheral parts of the stem. Nearer to the circumference than to the centre, lies a ring of woody and vascular tissue, which, in transverse sections, is seen to be broken up into wedge-shaped bundles, by narrow bands of parenchymatous tissue, which extend from the parenchyma within the circle of woody and vascular tissue (medulla or pith) to that which lies outside it. Moreover, each bundle of woody and vascular tissue is divided into two parts, an outer and an inner, by a thin layer of small and very thin-walled cells, termed the cambium layer. What lies outside this layer belongs to the bark and epidermis; what lies inside it, to the wood and pith.

The great morphological distinction between the axis of the Bean and that of the Fern lies in the presence of this cambium layer. The cells composing it, in fact, retain

their power of multiplication, and divide by septa parallel with the length of the stem, or root, as well as transverse to it. Thus new cells are continually being added, on the inner side of the cambium layer, to the thickness of the wood, and on the outer side of it, to the thickness of the bark; and the axis of the plant continually increases in diameter, so long as this process goes on. Plants in which this constant addition to the outer face of the wood and the inner face of the bark takes place, are termed exogens.

At the apex of the stem, and at that of the root, the cambium layer is continuous with the cells, which retain the capacity of dividing in these localities. As the plant is thickest at the junction of the stem and root, and diminishes thence to the free ends, or apices, of these two structures, the cambium layer may be said to have the form of a double And it is the special peculiarity of an exogen to possess this doubly conical layer of constantly dividing cells, the upper end of which is free, at the growing point of the terminal bud of the stem, while its lower end is covered by the root-cap of the ultimate termination of the principal root.

cone.

The most characteristic tissues of the wood are dotted ducts and spiral vessels, the spiral vessels being particularly abundant close to the pith. The bark contains elongated liber or bast cells; but there are no scalariform vessels such as are found in the Fern.

Stomates are absent in the epidermis of the root: they are to be found, here and there, in the epidermis of all the green parts of the stem and its appendages, but, as in the Fern, they are most abundant in the epidermis of the under side of the leaves. As in the Fern, they communicate with intercellular passages, which are widest in the leaves, but extend thence throughout the whole plant.