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Prothallus of Ferns

(except in the Moonwort group) is a flattened, green, expanded body, which grows in damp places, e.g., on damp bricks. It is thin, and consists of a single layer of cells, except in the middle, where it reaches a thickness of several layers. The cells contain an abundant supply of chlorophyll bodies, which give the Prothallus its colour. In general outline, the Prothallus, when full-grown, usually resembles the conventional figure of a heart, having one end narrowed, and a notch in the broader margin (see Fig. 291). It seldom exceeds 1/4in. in breadth. On the lower surface are numerous root-hairs, each made up of a row of cells. On the lower surface also, and along the edges, are formed the antheridia. The archegonia are situated in the middle of the lower surface. The antheridia originate as out-growths of cells of the epidermis; each outgrowth is cut off, as a new cell, by a cell wall. In some Prothalli, the contents of the cell thus formed break up into a number of small, rounded cells, called "parent cells." In each of these there is formed an antherozoid, slender, but coiled spirally in two or three turns, and provided with a tuft of fine hairs, or cilia, at one end (see Fig. 292). In most Prothalli, however, a process of cell-division goes on in the young antheridium, whereby it is finally made up of a layer of cells surrounding a central cell (see Fig. 293), and, in this latter, the parent cells are developed, and produce antherozoids. The outer coat, formed by the layer of cells, has to aid in expelling the antherozoids when ripe, and the cells do this by absorbing water rapidly, swelling, and compressing the contents of the central cell till its apex, which is not covered by the layer, is burst (see Fig. 294), and the "parent cells" are expelled, and, soon rupturing, set free the antherozoids. The latter move about actively in water, e.g., in a drop of dew or of rain.

The archegonia are situated on the lower surface, behind the notch already mentioned (see Fig. 291). Each originates, like the antheridia, from a cell of the epidermis, from which it grows out, in a hemispherical form. A cell wall forms, and cuts it off from the cell of the epidermis. It increases in size, and becomes further divided into three layers; and these are again sub-divided by cell walls. The result is that a structure is formed in the shape of a flask with a long, narrow neck. The hollow of the flask is occupied by a large cell, the oosphere, rich in protoplasm (see Fig. 295). The tube of the neck is at first filled with a narrow cell, the canal-cell, the cell wall of which becomes mucilaginous, swells, and is expelled from the outer opening of the tube, leaving a passage for the antherozoid down the tube to the oosphere, when the latter is ripe to be acted on by it (see Fig. 296). The antherozoids are caught in the mucilage while moving over the moist Prothallus; they wriggle down the tube, reach the oosphere, and fertilise it. The latter very soon begins to grow; and the final result is the development of the oospore into the leafy plant or Fern. It may be mentioned that the oospore, at a very early period, divides into eight cells, in two layers. Of these cells, four lie next the base, and four next the front margin of the Prothallus. Of the latter, the two farthest from the neck of the archegonium give origin to the first leaf or frond; one, near the neck, to the growing point of the stem; and the fourth to hairs. Of the other four cells, one, opposite to the stem, develops into the root, one ultimately disappears, and the other two form the "foot," a structure that remains sunk in the archegonium, which has grown so as still to surround the foot (see Figs. 297 and 298). By means of this organ, the young plant absorbs nourishment from the Prothallus, which, for a time, increases in size, but is gradually used up, and withers away, and afterwards the young Fern is able to nourish itself by its own roots and leaves. Two departures from this mode of reproduction have been detected in Ferns within recent years. The one of these, called "apogamy" (from apo, afar, and gamos, marriage) by Professor de Bary, was detected, in Pteris cretica, by Professor Farlow, and is now known to occur in a few other Ferns, including Nephrodium Filix-mas cristatum. In this process, the young Fern is produced as a bud from certain parts of the Prothallus, without the formation of sexual organs. The sexual process in this case is abolished, as the name indicates. In 1884, Mr. Druery stated, in the Linnaean Society, the discovery that, in certain Ferns, the Prothalli are produced as outgrowths from the pinnules of the Fern fronds, and not from the spores. This process has been called "apospory" (from apo, afar, and spora, a spore or seed). It has been investigated and described by Professor Bower in examples supplied by Mr. Druery, of Athyrium Filix-faemina clarissima, and Polystichum angulare pulcherrimum. In this departure, the production of spores is suppressed; the Prothalli in the former being modified sporangia, while, in the latter Fern, no trace of the sporangium even can be detected. The sexual reproduction is not affected, and the leafy Ferns are developed from the Prothalli in the usual way.

Though of very great scientific interest, the development of Prothalli, and of the sexual organs on them, is of less practical importance to gardeners in the other groups of Vascular Cryptogams than it is in the true Ferns; but an outline of the chief points of difference in these groups may be given. In the small group Ophioglosseae, represented in the British Flora by the Moonwort and Adder's Tongue Ferns, the Prothallus is formed underground, is destitute of chlorophyll, and is usually formed of a mass of cells. It produces sexual organs, which resemble those of Ferns in the main. The Equisetineae, or Horsetails, resemble Ferns in the Prothalli being green, flattened layers of cells, growing on damp surfaces; but they become branched into long, narrow lobes, and may reach 1/2in. in length. They are diaecious, i.e., each produces only antheridia or archegonia. The former are produced near the tips of the lobes of the male Prothalli; the latter usually in the clefts between the fleshy lobes of the female Prothalli. The development of the sexual organs, and of the "leafy plant" (if an Equisetum deserves this designation), calls for no special comment here, as it agrees in the main points with that in Ferns.

The Club-mosses fall into two groups, of which one, the Lycopodieae, much require to have their development worked out. So far as is known, their Prothalli are irregularly-lobed masses of cellular tissue, and bear both antheridia and archegonia; and the young, leafy plant continues to draw nourishment from it for a time, as in Ferns. There is only one form of spores in this group, and the Prothalli are, therefore, all alike in each species. The second group, Selaginelleae, is largely cultivated in greenhouses, and the cycle of development has been fully studied. In this group, spores of two kinds are produced in sporangia, in the axils of the leaves, near the tips of branches of the leafy plants. The two kinds are the microspores and the macrospores, which produce male and female Prothalli respectively. The microspores (from mikros, small, and spora, a spore) are much smaller than the macrospores (from makros, large, and spora). The Prothalli developed from both are very much reduced in size, as compared with the Prothalli already described; indeed, the greater part, or even the whole, of their development, goes on inside the spores. The peculiarities of development of these Prothalli have been very carefully investigated, and described in detail, by Millardet, and by Pfeffer. The male Prothallus is developed entirely in the interior of the microspore. In this, a small part (the vegetative cell) is first cut off, and the remaining contents are divided by cell walls into six or eight cells, and these (or only certain of them in some species) divide still further to form the parent cells of the antherozoids. In each of these, a long, slender, spiral antherozoid, with cilia at one end, is produced. The macrospores, while still in the sporangium, in Selaginella, show a mass of small-celled tissue, like a cap, at one end, covering a very large cell, which occupies the greater part of the spore. After the spore has been for some time out of the sporangium, this large cell becomes filled with a mass of cells of comparatively large size, individually, which Pfeffer regards as analogous to the endosperm in the seeds of angiosperm flowering plants. The cap above this mass is the Prothallus, and this increases in size, and archegonia form in it, beginning at the apex, and gradually forming at a greater distance from the apex. The coats of the spore burst above the Prothallus, which projects a little. The structure of the archegonium and of the oosphere, and the mode of fertilisation, are similar, in the important points, to those above described as occurring in Ferns; and so, moreover, is the development of the leafy plant. In the nearly allied genus Isaetes the development is much like that in Selaginella, but no endosperm is formed in the macrospore. The Rhizocarpeae agree, to a considerable extent, with the Selaginelleae. The great interest of the Prothallus in Selaginelleae and the allied forms rests in the light the study of it throws on the processes of reproduction in