The bast-tissue consists of very long, thin cells, drawn out like threads, and very tough. The wood-tissue is likewise. composed of long cells, which fit into each other closely, but the cells not nearly so long as those of the bast. The bast-tissue supplies the bast of the gardener, hemp, flax, and Russia-matting.

FIG. 29.

W. wood-tissue.
B. bast-tissue.

The cambium zone is a narrow band of small, thinwalled cells, which retain their power of growth and division. It lies between the bast and the wood, and is continually producing bast-tissue on the outer side, and wood-tissue on the inner.

Some cells which are united by their ends absorb. the cell-walls which separate them from the cell. immediately above or below them, and thus form a long, open tube. Cells which have been thus modified are called ducts and vessels.

In these a process of thickening the cell-wall goes on, but instead of the new cellulose being deposited equally over the whole inner surface of the cell, cer

tain portions are left unthickened, and the result is to make them appear dotted or pitted; these are known as dotted cells or vessels (fig. 30); in others it is de

posited in the form of a spiral thread, and these are called spiral vessels (fig. 31). Another method in which the thickening takes place is in the form of rings, and such cells are known as annular vessels (fig. 32). In others, again, which are four or five sided, the new deposits take place in the form of thin bars across the sides, and as they thus bear a resemblance to the "rounds" of a ladder, they are termed scalariform vessels (fig. 33).

All these tissues have a definite place in the structure (excepting the lowest forms) of plants. No

matter how they may be arranged inside a plant, the outside of it is always invested by a layer of flat, close-fitting, colourless cells, called the Epidermis. Dotted over this epidermis are a number of little holes, each surrounded by two kidney-shaped cells (fig. 34). These orifices are termed Stomates, or mouths, because they are used for breathing purposes. Each stomate communicates with the intercellular spaces we spoke about just now, and through them carbonic acid gas is absorbed from the atmosphere.

FIG. 34.

This is the internal structure of a plant; with its external characters, we suspect, most of our readers are tolerably familiar, yet for those who are not, we will give a brief account of it.

Fig. 35 represents the ideal plant. We observe that it consists of a main stem, more or less branched at its two ends. The branches at the upper portion of the stem bear leaves and flowers,-those at the

Stg

Co...t

Sta

...Sty

Ov...

Rec

Ca..

lower end of the stem bear neither, and are called roots. These latter serve a double purpose, they fix the plant in the earth, and also imbibe moisture from it. They are never green, and they avoid the light. The stem, on the contrary, is ever seeking the light, and usually green. The leaf is a thin, flat plate attached by a stalk to the stem or branch. Fig. 36 represents a thin slice cut through

the thickness of a leaf. Ep shows the colourless epidermic cells bounding the upper and lower surfaces of the leaf. Beneath them is the green-celled parenchyma, and between these layers of parenchyma, which are closely packed, there are others loosely arranged (LP) to form the intercellular spaces (IS).

L

FIG. 35.

.Stem

--Root

Within this are seen the fibro-vascular bundles (Fv).

St marks the presence of the stomates.

The food of a plant is of two kinds-liquid and

tain iron, nitrogen, potash, phosphorus, and sulphur; they abound in most soils, but cannot be taken up by the plant in a solid form, hence, if a plant be kept without water, it cannot obtain any mineral salts. The gaseous food is obtained from the atmosphere through the stomates of the leaf. It consists of carbonic acid gas, which is composed of carbon and oxygen. The chlorophyll-bearing cells have the power of retaining the carbon and setting the oxygen free; but it can only do so when under the influence of sunshine. On entering the stomates, the gas circulates through the intercellular spaces to the chlorophyll cells, and through the spiral vessels to the fibro-vascular bundles.

As the roots absorb the liquid food (sap) from the ground, it passes from cell to cell, and through certain vessels of the fibro-vascular bundle until it reaches the leaves. The warmth of the sun causes the water in the leaf-cells to evaporate-that is, to fly off through the intercellular spaces and the stomates in the form of a light vapour. If a plant exposed to the sun be not well supplied with water at the roots, it will quickly fade and die, in consequence of this great evaporation robbing the cells of their moisture. As the water evaporates, it is perfectly pure, all the mineral substances it previously contained having been left in the cells. The carbon absorbed from the air is combined with the cell-sap, and forms a substance called starch. Of this starch cellulose for the cell-walls is formed, and it is also changed by the protoplasm into sugar and fat. By the addition of nitrogen and sulphur (taken up in water by the

roots) to the constituent parts of starch, protoplasm has the power of forming albuminoids, of which protoplasm itself is formed, and dependent upon for growth and increase.

This process of manufacturing various substances out of these simple materials is termed assimilation. The substances formed by assimilation are stored up in the cells for future use in nourishing the plant. Thus in the potato, which is a part of the stem, the protoplasm of the cells is thickly dotted with grains of starch, it being laid up in the tuber to provide nourishment for the new shoots (eyes). In the wheat, oat, rice, pea, and bean we find it stored up in the seeds for the nourishment of the young plant or embryo, whilst it is developing its roots and leaves. The presence of starch in a plant may always be detected by the application of a slight quantity of iodine to the cell, when, if it contains starch, it will be stained blue. Oils and fats are also stored up as food for the plant in the same manner as starch; they are specially abundant in such plants as the Flax (from the seeds of which linseed-oil is obtained), Cocoa-nut, Olive, and the Castor-oil plant. Sugar, unlike starch, exists in a liquid state, and abounds chiefly in the stem of the sugar-cane and the taproots of parsnip and beet. It is manufactured by the plant from starch. A variety of substances, also formed in the cells, are known under the general term of alkaloids. Many of them have exceedingly valuable properties, and form important objects of commerce. Some of them are used in medicine, as morphia from the opium poppy, aconite from monks