William Edward Hartpole Lecky

If we compare the structure of the sporogonium of a moss or liverwort with the plant bearing the sexual organs, we find that its tissues are better differentiated, and that it is on the whole a more complex structure than the plant that bears it. It, however, remains attached to the parent plant, deriving its nourishment in part through the “foot” by means of which it is attached to the plant.

In the Pteridophytes, however, we find that the sporogonium becomes very much more developed, and finally becomes entirely detached from the sexual plant, developing in most cases roots that fasten it to the ground, after which it may live for many years, and reach a very large size.

The sexual plant, which is here called the “prothallium,” is of very simple structure, resembling the lower liverworts usually, and never reaches more than about a centimetre in diameter, and is often much smaller than this.

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 filixfoemina) (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 dioecious. 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.

Up to this point, all the cells of the embryo are much alike, and the embryo, like that of the bryophytes, is completely surrounded by the enlarged base of the archegonium (compare Fig.67, A, with Fig.55); but before the embryo breaks through the overlying cells a differentiation of the tissues begins. In the axis of each of the four divisions the cells divide lengthwise so as to form a cylindrical mass of narrow cells, not unlike those in the stem of a moss. Here, however, some of the cells undergo a further change; the walls thicken in places, and the cells lose their contents, forming a peculiar conducting tissue (tracheary tissue), found only in the two highest sub-kingdoms. The whole central cylinder is called a “fibro-vascular bundle,” and in its perfect form, at least, is found in no plants below the ferns, which are also the first to develop true roots.

The young root and leaf now rapidly elongate, and burst through the overlying cells, the former growing downward and becoming fastened in the ground, the latter growing upward through the notch in the front of the prothallium, and increasing rapidly in size (Fig.67, B). The leaf is more or less deeply cleft, and traversed by veins which are continuations of the fibro-vascular bundle of the stalk, and themselves fork once or twice. The surface of the leaf is covered with a well-developed epidermis, and the cells occupying the space between the veins contain numerous chloroplasts, so that the little plant is now quite independent of the prothallium, which has hitherto supported it. As soon as the fern is firmly established, the prothallium withers away.

Comparing this now with the development of the sporogonium in the bryophytes, it is evident that the young fern is the equivalent of the sporogonium or spore fruit of the former, being, like it, the direct product of the fertilized egg cell; and the prothallium represents the moss or liverwort, upon which are borne the sexual organs. In the fern, however, the sporogonium becomes entirely independent of the sexual plant, and does not produce spores until it has reached a large size, living many years. The sexual stage, on the other hand, is very much reduced, as we have seen, being so small as to be ordinarily completely overlooked; but its resemblance to the lower liverworts, like Riccia, or the horned liverworts, is obvious. The terms oÖphyte (egg-bearing plant) and sporophyte (spore-bearing plant, or sporogonium) are sometimes used to distinguish between the sexual plant and the spore-bearing one produced from it.

The common maiden-hair fern (Adiantum pedatum) has been selected here for studying the structure of the full-grown sporophyte, but almost any other common fern will answer. The maiden-hair fern is common in rich woods, and may be at once recognized by the form of its leaves. These arise from a creeping, underground stem (Fig.67, C), which is covered with brownish scales, and each leaf consists of a slender stalk, reddish brown or nearly black in color, which divides into two equal branches at the top. Each of these main branches bears a row of smaller ones on the outside, and these have a row of delicate leaflets on each side (Fig.67, E). The stem of the plant is fastened to the ground by means of numerous stout roots. The youngest of these, near the growing point of the stem, are unbranched, but the older ones branch extensively (C).

On breaking the stem across, it is seen to be dark-colored, except in the centre, which is traversed by a woody cylinder (fibro-vascular bundle) of a lighter color. This is sometimes circular in sections, sometimes horse-shoe shaped. Where the stem branches, the bundle of the branch may be traced back to where it joins that of the main stem.

A thin cross-section of the stem shows, when magnified, three regions. First, an outer row of cells, often absent in the older portions; this is the epidermis. Second, within the epidermis are several rows of cells similar to the epidermal cells, but somewhat larger, and like them having dark-brown walls. These merge gradually into larger cells, with thicker golden brown walls (Fig.67, I). The latter, if sufficiently magnified, show distinct striation of the walls, which are often penetrated by deep narrow depressions or “pits.” This thick-walled tissue is called “stony tissue” (schlerenchyma). All the cells contain numerous granules, which the iodine test shows to be starch. All of this second region lying between the epidermis and the fibro-vascular bundle is known as the ground tissue. The third region (fibro-vascular) is, as we have seen without the microscope, circular or horse-shoe shaped. It is sharply separated from the ground tissue by a row of small cells, called the “bundle sheath.” The cross-section of the bundle of the leaf stalk resembles, almost exactly, that of the stem; and, as it is much easier to cut, it is to be preferred in studying the arrangement of the tissues of the bundle (Fig.67, G). Within the bundle sheath (sh.) there are two well-marked regions, a central band (x) of large empty cells, with somewhat angular outlines, and distinctly separated walls; and an outer portion (y) filling up the space between these central cells and the bundle sheath. The central tissue (x) is called the woody tissue (xylem); the outer, the bast (phloem). The latter is composed of smaller cells of variable form, and with softer walls than the wood cells.

A longitudinal section of either the stem or leaf stalk shows that all the cells are decidedly elongated, especially those of the fibro-vascular bundle. The xylem (Fig.68, C, x) is made up principally of large empty cells, with pointed ends, whose walls are marked with closely set, narrow, transverse pits, giving them the appearance of little ladders, whence they are called “scalariform,” or ladder-shaped markings. These empty cells are known as “tracheids,” and tissue composed of such empty cells, “tracheary tissue.” Besides the tracheids, there are a few small cells with oblique ends, and with some granular contents.

The phloem is composed of cells similar to the latter, but there may also be found, especially in the stem, other larger ones (Fig.67, J), whose walls are marked with shallow depressions, whose bottoms are finely pitted. These are the so-called “sieve tubes.”

For microscopical examination, either fresh or alcoholic material may be used, the sections being mounted in water. Potash will be found useful in rendering opaque sections transparent.

The leaves, when young, are coiled up (Fig.67, C), owing to growth in the earlier stages being greater on the lower than on the upper side. As the leaf unfolds, the stalk straightens, and the upper portion (blade) becomes flat.

The general structure of the leaf stalk may be understood by making a series of cross-sections at different heights, and examining them with a hand lens. The arrangement is essentially the same as in the stem. The epidermis and immediately underlying ground tissue are dark-colored, but the inner ground tissue is light-colored, and much softer than the corresponding part of the stem; and some of the outer cells show a greenish color, due to the presence of chlorophyll.

The section of the fibro-vascular bundle differs at different heights. Near the base of the stalk (Fig.D i) it is horseshoe-shaped; but, if examined higher up, it is found to divide (II, III), one part going to each of the main branches of the leaf. These secondary bundles divide further, forming the veins of the leaflets.

The leaflets (E, F) are one-sided, the principal vein running close to the lower edge, and the others branching from it, and forking as they approach the upper margin, which is deeply lobed, the lobes being again divided into teeth. The leaflets are very thin and delicate, with extremely smooth surface, which sheds water perfectly. If the plant is a large one, some of the leaves will probably bear spores. The spore-bearing leaves are at once distinguished by having the middle of each lobe of the leaflets folded over upon the lower side (F). On lifting one of these flaps, numerous little rounded bodies (spore cases) are seen, whitish when young, but becoming brown as they ripen. If a leaf with ripe spore cases is placed upon a piece of paper, as it dries the spores are discharged, covering the paper with the spores, which look like fine brown powder.

A microscopical examination of the leaf stalk shows the tissues to be almost exactly like those of the stem, except the inner ground tissue, whose cells are thin-walled and colorless (soft tissue or “parenchyma”) instead of stony tissue. The structure of the blade of the leaf, however, shows a number of peculiarities. Stripping off a little of the epidermis with a needle, or shaving off a thin slice with a razor, it may be examined in water, removing the air if necessary with alcohol. It is composed of a single layer of cells, of very irregular outline, except where it overlies a vein (Fig.68, B, f). Here the cells are long and narrow, with heavy walls. The epidermal cells contain numerous chloroplasts, and on the under surface of the leaf breathing pores (stomata, sing. stoma), not unlike those on the capsules of some of the bryophytes. Each breathing pore consists of two special crescent-shaped epidermal cells (guard cells), enclosing a central opening or pore communicating with an air space below. They arise from cells of the young epidermis that divide by a longitudinal wall, that separates in the middle, leaving the space between.

By holding a leaflet between two pieces of pith, and using a very sharp razor, cross-sections can be made. Such a section is shown in Fig.68, A. The epidermis (e) bounds the upper and lower surfaces, and if a vein (f.b.) is cut across its structure is found to be like that of the fibro-vascular bundle of the leaf stalk, but much simplified.

The ground tissue of the leaf is composed of very loose, thin-walled cells, containing numerous chloroplasts. Between them are large and numerous intercellular spaces, filled with air, and communicating with the breathing pores. These are the principal assimilating cells of the plant; i.e. they are principally concerned in the absorption and decomposition of carbonic acid from the atmosphere, and the manufacture of starch.

The spore cases, or sporangia (Fig.69), are at first little papillÆ (A), arising from the epidermal cells, from which they are early cut off by a cross-wall. In the upper cell several walls next arise, forming a short stalk, composed of three rows of cells, and an upper nearly spherical cell—the sporangium proper. The latter now divides by four walls (B, C, i–iv), into a central tetrahedral cell, and four outer ones. The central cell, whose contents are much denser than the outer ones, divides again by walls parallel to those first formed, so that the young sporangium now consists of a central cell, surrounded by two outer layers of cells. From the central cell a group of cells is formed by further divisions (D), which finally become entirely separated from each other. The outer cells of the spore case divide only by walls, at right angles to their outer surface, so that the wall is never more than two cells thick. Later, the inner of these two layers becomes disorganized, so that the central mass of cells floats free in the cavity of the sporangium, which is now surrounded by but a single layer of cells (E).

Each of the central cells divides into four spores, precisely as in the bryophytes. The young spores (G, H) are nearly colorless and are tetrahedral (like a three-sided pyramid) in form. As they ripen, chlorophyll is formed in them, and some oil. The wall becomes differentiated into three layers, the outer opaque and brown, the two inner more delicate and colorless.

Running around the outside of the ripe spore case is a single row of cells (an.), differing from the others in shape, and having their inner walls thickened. Near the bottom, two (sometimes four) of these cells are wider than the others, and their walls are more strongly thickened. It is at this place (st.) that the spore case opens. When the ripe sporangium becomes dry, the ring of thickened cells (an.) contracts more strongly than the others, and acts like a spring pulling the sporangium open and shaking out the spores, which germinate readily under favorable conditions, and form after a time the sexual plants (prothallia).

The roots of the sporophyte arise in large numbers, the youngest being always nearest the growing point of the stem or larger roots (Fig.67, C). The growing roots are pointed at the end which is also light-colored, the older parts becoming dark brown. A cross-section of the older portions shows a dark-brown ground tissue with a central, light-colored, circular, fibro-vascular bundle (Fig.68, F). Growing from its outer surface are numerous brown root hairs (r).

When magnified the walls of all the outer cells (epidermis and ground tissue) are found to be dark-colored but not very thick, and the cells are usually filled with starch. There is a bundle sheath of much-flattened cells separating the fibro-vascular bundle from the ground tissue. The bundle (Fig.68, G) shows a band of tracheary tissue in the centre surrounded by colorless cells, all about alike.

All of the organs of the fern grow from a definite apical cell, but it is difficult to study except in the root.

Selecting a fresh, pretty large root, a series of thin longitudinal sections should be made either holding the root directly in the fingers or placing it between pieces of pith. In order to avoid drying of the sections, as is indeed true in cutting any delicate tissue, it is a good plan to wet the blade of the razor. If the section has passed through the apex, it will show the structure shown in Figure68, D. The apical cell (a) is large and distinct, irregularly triangular in outline. It is really a triangular pyramid (tetrahedron) with the base upward, which is shown by making a series of cross-sections through the root tip, and comparing them with the longitudinal sections. The cross-section of the apical cell (Fig.L) appears also triangular, showing all its faces to be triangles. Regular series of segments are cut off in succession from each of the four faces of the apical cell. These segments undergo regular divisions also, so that very early a differentiation of the tissues is evident, and the three tissue systems (epidermal, ground, and fibro-vascular) may be traced almost to the apex of the root (68, D). From the outer series of segments is derived the peculiar structure (root cap) covering the delicate growing point and protecting it from injury.

The apices of the stem and leaves, being otherwise protected, develop segments only from the sides of the apical cell, the outer face never having segments cut off from it.