The common ferns are the types of the sub-kingdom, and a careful study of any of these will illustrate the principal peculiarities of the group. The whole plant, as we know it, is really nothing but the sporogonium, originating from the egg cell in exactly the same way as the moss sporogonium, and like it gives rise to spores which are formed upon the leaves.
The spores may be collected by placing the spore-bearing leaves on sheets of paper and letting them dry, when the ripe spores will be discharged covering the paper as a fine, brown powder. If these are sown on fine, rather closely packed earth, and kept moist and covered with glass so as to prevent evaporation, within a week or two a fine, green, moss-like growth will make its appearance, and by the end of five or six weeks, if the weather is warm, little, flat, heart-shaped plants of a dark-green color may be seen. These look like small liverworts, and are the sexual plants (prothallia) of our ferns ([Fig. 66], F). Removing one of these carefully, we find on the lower side numerous fine hairs like those on the lower surface of the liverworts, which fasten it firmly to the ground. By and by, if our culture has been successful, we may find attached to some of the larger of these, little fern plants growing from the under side of the prothallia, and attached to the ground by a delicate root. As the little plant becomes larger the prothallium dies, leaving it attached to the ground as an independent plant, which after a time bears the spores.
Fig. 66.—A, spore of the ostrich fern (Onoclea), with the outer coat removed. B, germinating spore, × 150. C, young prothallium, × 50. r, root hair. sp. spore membrane. D, E, older prothallia. a, apical cell, × 150. F, a female prothallium, seen from below, × 12. ar. archegonia. G, H, young archegonia, in optical section, × 150. o, central cell. b, ventral canal cell. c, upper canal cell. I, a ripe archegonium in the act of opening, × 150. o, egg cell. J, a male prothallium, × 50. an. antheridia. K, L, young antheridia, in optical section, × 300. M, ripe antheridium, × 300. sp. sperm cells. N, O, antheridia that have partially discharged their contents, × 300. P, spermatozoids, killed with iodine, × 500. v, vesicle attached to the hinder end.
In choosing spores for germination it is best to select those of large size and containing abundant chlorophyll, as they germinate more readily. Especially favorable for this purpose are the spores of the ostrich fern (Onoclea struthiopteris) ([Fig. 70], I, J), or the sensitive fern (O. sensibilis). Another common and readily grown species is the lady fern (Asplenium filixfœmina) ([Fig. 70], H). The spores of most ferns retain their vitality for many months, and hence can be kept dry until wanted.
The first stages of germination may be readily seen by sowing the spores in water, where, under favorable circumstances, they will begin to grow within three or four days. The outer, dry, brown coat of the spore is first ruptured, and often completely thrown off by the swelling of the spore contents. Below this is a second colorless membrane which is also ruptured, but remains attached to the spore. Through the orifice in the second coat, the inner delicate membrane protrudes in the form of a nearly colorless papilla which rapidly elongates and becomes separated from the body of the spore by a partition, constituting the first root hair ([Fig. 66], B, C, r). The body of the spore containing most of the chlorophyll elongates more slowly, and divides by a series of transverse walls so as to form a short row of cells, resembling in structure some of the simpler algæ (C).
In order to follow the development further, spores must be sown upon earth, as they do not develop normally in water beyond this stage.
In studying plants grown on earth, they should be carefully removed and washed in a drop of water so as to remove, as far as possible, any adherent particles, and then may be mounted in water for microscopic examination.
In most cases, after three or four cross-walls are formed, two walls arise in the end cell so inclined as to enclose a wedge-shaped cell (a) from which are cut off two series of segments by walls directed alternately right and left ([Fig. 66], D, E, a), the apical cell growing to its original dimensions after each pair of segments is cut off. The segments divide by vertical walls in various directions so that the young plant rapidly assumes the form of a flat plate of cells attached to the ground by root hairs developed from the lower surfaces of the cells, and sometimes from the marginal ones. As the division walls are all vertical, the plant is nowhere more than one cell thick. The marginal cells of the young segments divide more rapidly than the inner ones, and soon project beyond the apical cell which thus comes to lie at the bottom of a cleft in the front of the plant which in consequence becomes heart-shaped (E, F). Sooner or later the apical cell ceases to form regular segments and becomes indistinguishable from the other cells.
In the ostrich fern and lady fern the plants are diœcious. The male plants ([Fig. 66], J) are very small, often barely visible to the naked eye, and when growing thickly form dense, moss-like patches. They are variable in form, some irregularly shaped, others simple rows of cells, and some have the heart shape of the larger plants.
The female plants ([Fig. 66], F) are always comparatively large and regularly heart-shaped, occasionally reaching a diameter of nearly or quite one centimetre, so that they are easily recognizable without microscopical examination.
All the cells of the plant except the root hairs contain large and distinct chloroplasts much like those in the leaves of the moss, and like them usually to be found in process of division.
The archegonia arise from cells of the lower surface, just behind the notch in front ([Fig. 66], F, ar.). Previous to their formation the cells at this point divide by walls parallel to the surface of the plant, so as to form several layers of cells, and from the lowest layer of cells the archegonia arise. They resemble those of the liverworts but are shorter, and the lower part is completely sunk within the tissues of the plant ([Fig. 66], G, I). They arise as single surface cells, this first dividing into three by walls parallel to the outer surface. The lower cell undergoes one or two divisions, but undergoes no further change; the second cell (C, o), becomes the egg cell, and from it is cut off another cell (c), the canal cell of the neck; the uppermost of the three becomes the neck. There are four rows of neck cells, the two forward ones being longer than the others, so that the neck is bent backward. In the full-grown archegonium, there are two canal cells, the lower one (H, b) called the ventral canal cell, being smaller than the other.
Shortly before the archegonium opens, the canal cells become disorganized in the same way as in the bryophytes, and the protoplasm of the central cell contracts to form the egg cell which shows a large, central nucleus, and in favorable cases, a clear space at the top called the “receptive spot,” as it is here that the spermatozoid enters. When ripe, if placed in water, the neck cells become very much distended and finally open widely at the top, the upper ones not infrequently being detached, and the remains of the neck cells are forced out ([Fig. 66], I).
The antheridia ([Fig. 66]. J, M) arise as simple hemispherical cells, in which two walls are formed (K I, II), the lower funnel-shaped, the upper hemispherical and meeting the lower one so as to enclose a central cell (shaded in the figure), from which the sperm cells arise. Finally, a ring-shaped wall (L iii) is formed, cutting off a sort of cap cell, so that the antheridium at this stage consists of a central cell, surrounded by three other cells, the two lower ring-shaped, the upper disc-shaped. The central cell, which contains dense, glistening protoplasm, is destitute of chlorophyll, but the outer cells have a few small chloroplasts. The former divides repeatedly, until a mass of about thirty-two sperm cells is formed, each giving rise to a large spirally-coiled spermatozoid. When ripe, the mass of sperm cells crowds so upon the outer cells as to render them almost invisible, and as they ripen they separate by a partial dissolving of the division walls. When brought into water, the outer cells of the antheridium swell strongly, and the matter derived from the dissolved walls of the sperm cells also absorbs water, so that finally the pressure becomes so great that the wall of the antheridium breaks, and the sperm cells are forced out by the swelling up of the wall cells (N, O). After lying a few moments in the water, the wall of each sperm cell becomes completely dissolved, and the spermatozoids are released, and swim rapidly away with a twisting movement. They may be killed with a little iodine, when each is seen to be a somewhat flattened band, coiled several times. At the forward end, the coils are smaller, and there are numerous very long and delicate cilia. At the hinder end may generally be seen a delicate sac (P, v), containing a few small granules, some of which usually show the reaction of starch, turning blue when iodine is applied.
In studying the development of the antheridia, it is only necessary to mount the plants in water and examine them directly; but the study of the archegonia requires careful longitudinal sections of the prothallium. To make these, the prothallium should be placed between small pieces of pith, and the razor must be very sharp. It may be necessary to use a little potash to make the sections transparent enough to see the structure, but this must be used cautiously on account of the great delicacy of the tissues.
If a plant with ripe archegonia is placed in a drop of water, with the lower surface uppermost, and at the same time male plants are put with it, and the whole covered with a cover glass, the archegonia and antheridia will open simultaneously; and, if examined with the microscope, we shall see the spermatozoids collect about the open archegonia, to which they are attracted by the substance forced out when it opens. With a little patience, one or more may be seen to enter the open neck through which it forces itself, by a slow twisting movement, down to the egg cell. In order to make the experiment successful, the plants should be allowed to become a little dry, care being taken that no water is poured over them for a day or two beforehand.
The first divisions of the fertilized egg cell resemble those in the moss embryo, except that the first wall is parallel with the archegonium axis, instead of at right angles to it. Very soon, however, the embryo becomes very different, four growing points being established instead of the single one found in the moss embryo. The two growing points on the side of the embryo nearest the archegonium neck grow faster than the others, one of these outstripping the other, and soon becoming recognizable as the first leaf of the embryo ([Fig. 67], A, L). The other (r) is peculiar, in having its growing point covered by several layers of cells, cut off from its outer face, a peculiarity which we shall find is characteristic of the roots of all the higher plants, and, indeed, this is the first root of the young fern. Of the other two growing points, the one next the leaf grows slowly, forming a blunt cone (st.), and is the apex of the stem. The other (f) has no definite form, and serves merely as an organ of absorption, by means of which nourishment is supplied to the embryo from the prothallium; it is known as the foot.