3.

C.

marked ring (annulus) round the edge of the sporangium.

Their mode of dehiscence (look out for one that has opened) by a cleft running transversely across the sporangium.

Burst open some sporangia by pressing on the coverglass examine, with obj., the spores which are set free.

a.

b.

Their size: measure.

Their form: somewhat triangular.

[c. Their structure: a thick outer coat, a thin inner coat, protoplasm, and a nucleus: crush some by pressure on the cover-glass.]

B. THE PROTHALLUS; SEXUAL GENERATION.

Prothalli may be obtained by sowing some spores on peat and keeping them warm and very moist for about three months. They are small deep green leaf-like bodies.

I.

a. The Prothallus.

Transfer a prothallus to a slide, and mount it in water with its under surface uppermost. Examine with 1 inch obj.

a. Its form: a thin kidney-shaped expansion from which, especially towards its convex border, a number of slender filaments (root-hairs) arise. Its structure.

b.

a.

The leafy expansion: it consists throughout most of its extent of a single layer of polyhedral chlorophyll-containing cells, but at a part (the cushion) a little behind the depression marking the growing point it is several cells

C.

B.

The root-hairs: each composed of a single cell which contains no chlorophyll.

The antheridia and archegonia: the former can just be seen with an inch objective as minute eminences on the under surface of the older parts of the prothallus especially among the root-hairs; the latter are partly imbedded in the cushion.

b. The reproductive organs.

These are to be found by examining the under surface of the prothallus with obj.

I.

2.

The antheridia. Most numerous near and among the root-hairs.

b.

a. Their form: small hemispherical eminences. Their structure: made up of an outer layer of cells containing a few chlorophyll-granules, through which can be seen, according to the stage of development, either a single central cell, or a number of smaller cells (mother-cells of spermatozoids) resulting from its division: in the latter cells, in ripe antheridia, spirally coiled bodies (spermatozoids) can be indistinctly seen.

The spermatozoids.

Some of these are sure to be found swimming about in the water if a number of ripe prothalli are examined.

a.

Small bodies, coiled like a corkscrew, thick at one end, and tapering towards the other, which has a number of cilia attached to it. To the thicker end of the spermatozoid is often attached a rounded mass containing colourless granules.

3.

4.

b. Treat with iodine; this stains them and stops their movements, so that their form can be more distinctly seen.

The archegonia. Make vertical sections of the prothallus passing through the cushion; this is best done while holding it between two pieces of pith. Note in the archegonia

a. Their form: chimney-shaped eminences with a small aperture at the apex.

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 central cell, with two or three smaller granular cells (neck canal-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 central cell. The latter has now divided into two; the upper smaller cell resulting from its division forming the ventral canal-cell, which becomes disorganized, while the larger lower cell is the ovum or oosphere. Examine a young fern-plant in connection with its prothallus, to the lower side of which it is attached by means of the foot.

XVI.

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, ten stamens and a central pistil; the latter is terminated by a style, the free end of which is the stigma.

The flower-stalks are modified branches, and the flower itself consists of several whorls of modified leaves.

The stamens form a tube which 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, each of which is a single cell. 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 nucellus,

invested by two coats, an outer and an inner. Opposite the summit of the nucellus, these coats are perforated by a canal, the micropyle, which leads down to the nucellus. One of the cells of the nucellus is very much larger than the rest; this is called the embryo-sac. In its interior several smaller cells are developed, the most important of which is the ovum or oosphere, which lies at the end of the embryosac towards the micropyle. When the pollen is deposited on the stigma the grains germinate. Each sends out a long filament, 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 nucellus, and comes into close contact with the embryo sac. The original nucleus of the pollen-grain has in the mean time divided into two, and one of the daughter nuclei passes down the tube, reaches the embryo-sac, and then fuses with the nucleus of the ovum. This is the process of impregnation, and the result of it is that the ovum divides and gives 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. nucellus 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