Fig. 66.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.
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 alternatelyright 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 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 alternatelyright 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 (KI, 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 (Liii) 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, butthis 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.
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 (KI, 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 (Liii) 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, butthis 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.
Fig. 67.Fig. 67.—A, embryo of the ostrich fern just before breaking through the prothallium, × 50.st.apex of stem.l, first leaf.r, first root.ar.neck of the archegonium.B, young plant, still attached to the prothallium (pr.).C, underground stem of the maiden-hair fern (Adiantum), with one young leaf, and the base of an older one, × 1.D, three cross-sections of a leaf stalk:i, nearest the base;iii, nearest the blade of the leaf, showing the division of the fibro-vascular bundle, × 5.E, part of the blade of the leaf, × ½.F, a single spore-bearing leaflet, showing the edge folded over to cover the sporangia, × 1.G, part of the fibro-vascular bundle of the leaf stalk (cross-section), × 50.x, woody part of the bundle.y, bast.sh.bundle sheath.H, a small portion of the same bundle, × 150.I, stony tissue from the underground stem, × 150.J, sieve tube from the underground stem, × 300.
Fig. 67.—A, embryo of the ostrich fern just before breaking through the prothallium, × 50.st.apex of stem.l, first leaf.r, first root.ar.neck of the archegonium.B, young plant, still attached to the prothallium (pr.).C, underground stem of the maiden-hair fern (Adiantum), with one young leaf, and the base of an older one, × 1.D, three cross-sections of a leaf stalk:i, nearest the base;iii, nearest the blade of the leaf, showing the division of the fibro-vascular bundle, × 5.E, part of the blade of the leaf, × ½.F, a single spore-bearing leaflet, showing the edge folded over to cover the sporangia, × 1.G, part of the fibro-vascular bundle of the leaf stalk (cross-section), × 50.x, woody part of the bundle.y, bast.sh.bundle sheath.H, a small portion of the same bundle, × 150.I, stony tissue from the underground stem, × 150.J, sieve tube from the underground stem, × 300.
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 (compareFig. 67,A, withFig. 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.
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 (compareFig. 67,A, withFig. 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 witha 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, likeRiccia, 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), whosewalls 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.
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), whosewalls 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.Di) 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 placedupon a piece of paper, as it dries the spores are discharged, covering the paper with the spores, which look like fine brown powder.
Fig. 68.Fig. 68.—A, vertical section of the leaf of the maiden-hair fern, which has cut across a vein (f.b.), × 150.B, surface view of the epidermis from the lower surface of a leaf.f, vein.p, breathing pore, × 150.C, longitudinal section of the fibro-vascular bundle of the leaf stalk, showing tracheids with ladder-shaped markings, × 150.D, longitudinal section through the tip of a root, × 150.a, apical cell.Pl.young fibro-vascular bundle.Pb.young ground tissue.E, cross-section of the root, through the region of the apical cell (a), × 150.F, cross-section through a full-grown root, × 25.r, root hairs.G, the fibro-vascular bundle of the same, × 150.
Fig. 68.—A, vertical section of the leaf of the maiden-hair fern, which has cut across a vein (f.b.), × 150.B, surface view of the epidermis from the lower surface of a leaf.f, vein.p, breathing pore, × 150.C, longitudinal section of the fibro-vascular bundle of the leaf stalk, showing tracheids with ladder-shaped markings, × 150.D, longitudinal section through the tip of a root, × 150.a, apical cell.Pl.young fibro-vascular bundle.Pb.young ground tissue.E, cross-section of the root, through the region of the apical cell (a), × 150.F, cross-section through a full-grown root, × 25.r, root hairs.G, the fibro-vascular bundle of the same, × 150.
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 heavywalls. 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.
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 heavywalls. 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.
Fig. 69.Fig. 69.—A, mother cell of the sporangium of the maiden-hair fern, × 300.B, young sporangium, surface view, × 150:i, from the side;ii, from above.C–E, successive stages in the development of the sporangium seen in optical section, × 150.F, nearly ripe sporangium, × 50:i, from in front;ii, from the side.an.ring.st.point of opening.G, group of four spores, × 150.H, a single spore, × 300.
Fig. 69.—A, mother cell of the sporangium of the maiden-hair fern, × 300.B, young sporangium, surface view, × 150:i, from the side;ii, from above.C–E, successive stages in the development of the sporangium seen in optical section, × 150.F, nearly ripe sporangium, × 50:i, from in front;ii, from the side.an.ring.st.point of opening.G, group of four spores, × 150.H, a single spore, × 300.
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 inFig. 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 nowconsists 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).
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 inFig. 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 nowconsists 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 inFigure 68,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.
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 inFigure 68,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.
Thereare three well-marked classes of the Pteridophytes: the ferns (Filicinæ); horse-tails (Equisetinæ); and the club mosses (Lycopodinæ).
The ferns constitute by far the greater number of pteridophytes, and their general structure corresponds with that of the maiden-hair fern described. There are three orders, of which two, the true ferns (Filices) and the adder-tongues (Ophioglossaceæ), are represented in the United States. A third order, intermediate in some respects between these two, and called the ringless ferns (Marattiaceæ), has no representatives within our territory.
The classification is at present based largely upon the characters of the sporophyte, the sexual plants being still very imperfectly known in many forms.
The adder-tongues (Ophioglossaceæ) are mostly plants of rather small size, ranging from about ten to fifty centimetres in height. There are two genera in the United States, the true adder-tongues (Ophioglossum) and the grape ferns (Botrychium). They send up but one leaf each year, and this in fruiting specimens (Fig. 70,A) is divided into two portions, the spore bearing (x) and the green vegetative part. InBotrychiumthe leaves are more or less deeply divided, and the sporangia distinct (Fig. 71,B). InOphioglossumthe sterile division of the leaf is usually smooth and undivided, and thespore-bearing division forms a sort of spike, and the sporangia are much less distinct. The sporangia in both differ essentially from those of the true ferns in not being derived from a single epidermal cell, but are developed in part from the ground tissue of the leaf.
Fig. 70.Fig. 70.—Forms of ferns.A, grape fern (Botrychium), × ½.x, fertile part of the leaf.B, sporangia ofBotrychium, × 3.C, flowering fern (Osmunda).x, spore-bearing leaflets, × ½.D, a sporangium ofOsmunda, × 25.r, ring.E,Polypodium, × 1.F, brake (Pteris), × 1.G, shield fern (Aspidium), × 2.H, spleen-wort (Asplenium), × 2.I, ostrich fern (Onoclea), × 1.J, the same, with the incurved edges of the leaflet partially raised so as to show the masses of sporangia beneath, × 2.
Fig. 70.—Forms of ferns.A, grape fern (Botrychium), × ½.x, fertile part of the leaf.B, sporangia ofBotrychium, × 3.C, flowering fern (Osmunda).x, spore-bearing leaflets, × ½.D, a sporangium ofOsmunda, × 25.r, ring.E,Polypodium, × 1.F, brake (Pteris), × 1.G, shield fern (Aspidium), × 2.H, spleen-wort (Asplenium), × 2.I, ostrich fern (Onoclea), × 1.J, the same, with the incurved edges of the leaflet partially raised so as to show the masses of sporangia beneath, × 2.
In the true ferns (Filices), the sporangia resemble those already described, arising in all (unless possiblyOsmunda) from a single epidermal cell.
One group, the water ferns (Rhizocarpeæ), produce two kinds of spores, large and small. The former produce male, the latter female prothallia. In both cases the prothallium issmall, and often scarcely protrudes beyond the spore, and may be reduced to a single archegonium or antheridium (Fig. 71,B,C) with only one or two cells representing the vegetative cells of the prothallium (v). The water ferns are all aquatic or semi-aquatic plants, few in number and scarce or local in their distribution. The commonest are those of the genusMarsilia(Fig. 71,A), looking like a four-leaved clover. Others (Salvinia,Azolla) are floating forms (Fig. 71,D).
Fig. 71.Fig. 71.—A,Marsilia, one of theRhizocarpeæ(after Underwood).sp.the “fruits” containing the sporangia.B, a small spore ofPilularia, with the ripe antheridium protruding, × 180.C, male prothallium removed from the spore, × 180.D,Azolla(after Sprague), × 1.
Fig. 71.—A,Marsilia, one of theRhizocarpeæ(after Underwood).sp.the “fruits” containing the sporangia.B, a small spore ofPilularia, with the ripe antheridium protruding, × 180.C, male prothallium removed from the spore, × 180.D,Azolla(after Sprague), × 1.
Of the true ferns there are a number of families distinguished mainly by the position of the sporangia, as well as by some differences in their structure. Of our common ferns, those differing most widely from the types are the flowering ferns (Osmunda), shown inFigure 70,C,D. In these the sporangia are large and the ring (r) rudimentary. The leaflets bearing the sporangia are more or less contracted and covered completely with the sporangia, sometimes all the leaflets of the spore-bearing leaf being thus changed, sometimes only a few of them, as in the species figured.
Our other common ferns have the sporangia in groups (sori, sing.sorus) on the backs of the leaves. These sori are of different shape in different genera, and are usually protected by a delicate membranous covering (indusium). Illustrationsof some of the commonest genera are shown inFigure 70,E,J.
The second class of the pteridophytes includes the horse-tails (Equisetinæ) of which all living forms belong to a single genus (Equisetum). Formerly they were much more numerous than at present, remains of many different forms being especially abundant in the coal formations.
Fig. 72.Fig. 72.—A, spore-bearing stem of the field horse-tail (Equisetum), × 1.x, the spore-bearing cone.B, sterile stem of the same, × ½.C, underground stem, with tubers (o), × ½.D, cross-section of an aerial stem, × 5.f.b.fibro-vascular bundle.E, a single fibro-vascular bundle, × 150.tr.vessels.F, a single leaf from the cone, × 5.G, the same cut lengthwise, through a spore sac (sp.), × 5.H, a spore, × 50.I, the same, moistened so that the elaters are coiled up, × 150.J, a male prothallium, × 50.an.an antheridium.K, spermatozoids, × 300.
Fig. 72.—A, spore-bearing stem of the field horse-tail (Equisetum), × 1.x, the spore-bearing cone.B, sterile stem of the same, × ½.C, underground stem, with tubers (o), × ½.D, cross-section of an aerial stem, × 5.f.b.fibro-vascular bundle.E, a single fibro-vascular bundle, × 150.tr.vessels.F, a single leaf from the cone, × 5.G, the same cut lengthwise, through a spore sac (sp.), × 5.H, a spore, × 50.I, the same, moistened so that the elaters are coiled up, × 150.J, a male prothallium, × 50.an.an antheridium.K, spermatozoids, × 300.
One of the commonest forms is the field horse-tail (Equisetum arvense), a very abundant and widely distributed species. It grows in low, moist ground, and is often found in great abundance growing in the sand or gravel used as “ballast” for railway tracks.
The plant sends up branches of two kinds from a creeping underground stem that may reach a length of a metre or more. This stem (Fig. 72,C) is distinctly jointed, bearing at each joint a toothed sheath, best seen in the younger portions, as they are apt to be destroyed in the older parts. Sometimes attached to this are small tubers (o) which are much-shortened branches and under favorable circumstances give rise to new stems. They have a hard, brown rind, and are composed within mainly of a firm, white tissue, filled with starch.
The surface of the stem is marked with furrows, and a section across it shows that corresponding to these are as many large air spaces that traverse the stem from joint to joint. From the joints numerous roots, quite like those of the ferns, arise.
If the stem is dug up in the late fall or winter, numerous short branches of a lighter color will be found growing from the joints. These later grow up above ground into branches of two sorts. Those produced first (Fig. 72,A), in April or May, are stouter than the others, and nearly destitute of chlorophyll. They are usually twenty to thirty centimetres in height, of a light reddish brown color, and, like all the stems, distinctly jointed. The sheaths about the joints (L) are much larger than in the others, and have from ten to twelve large black teeth at the top. These sheaths are the leaves. At the top of the branch the joints are very close together, and the leaves of different form, and closely set so as to form a compact cone (x).
A cross-section of the stem (D) shows much the same structure as the underground stem, but the number of air spaces is larger, and in addition there is a large central cavity. Thefibro-vascular bundles (f.b.) are arranged in a circle, alternating with the air channels, and each one has running through it a small air passage.
The cone at the top of the branch is made up of closely set, shield-shaped leaves, which are mostly six-sided, on account of the pressure. These leaves (F,G) have short stalks, and are arranged in circles about the stem. Each one has a number of spore cases hanging down from the edge, and opening by a cleft on the inner side (G,sp.). They are filled with a mass of greenish spores that shake out at the slightest jar when ripe.
The sterile branches (B) are more slender than the spore-bearing ones, and the sheaths shorter. Surrounding the joints, apparently just below the sheaths, but really breaking through their bases, are circles of slender branches resembling the main branch, but more slender. The sterile branches grow to a height of forty to fifty centimetres, and from their bushy form the popular name of the plant, “horse-tail,” is taken. The surface of the plant is hard and rough, due to the presence of great quantities of flint in the epidermis,—a peculiarity common to all the species.
The stem is mainly composed of large, thin-walled cells, becoming smaller as they approach the epidermis. The outer cells of the ground tissue in the green branches contain chlorophyll, and the walls of some of them are thickened. The fibro-vascular bundles differ entirely from those of the ferns. Each bundle is nearly triangular in section (E), with the point inward, and the inner end occupied by a large air space. The tracheary tissue is only slightly developed, being represented by a few vessels[9](tr.) at the outer angles of the bundle, and one or two smaller ones close to the air channel. The rest of the bundle is made up of nearly uniform, rather thin-walled, colorless cells, some of which, however, are larger, and have perforated cross-walls, representing the sieve tubes ofthe fern bundle. There is no individual bundle sheath, but the whole circle of bundles has a common outer sheath.The epidermis is composed of elongated cells whose walls present a peculiar beaded appearance, due to the deposition of flint within them. The breathing pores are arranged in vertical lines, and resemble in general appearance those of the ferns, though differing in some minor details. Like the other epidermal cells the guard cells have heavy deposits of flint, which here are in the form of thick transverse bars.The spore cases have thin walls whose cells, shortly before maturity, develop thickenings upon their walls, which have to do with the opening of the spore case. The spores (H,I) are round cells containing much chlorophyll and provided with four peculiar appendages called elaters. The elaters are extremely sensitive to changes in moisture, coiling up tightly when moistened (I), but quickly springing out again when dry (H). By dusting a few dry spores upon a slide, and putting it under the microscope without any water, the movement may be easily examined. Lightly breathing upon them will cause the elaters to contract, but in a moment, as soon as the moisture of the breath has evaporated, they will uncoil with a quick jerk, causing the spores to move about considerably.The fresh spores begin to germinate within about twenty-four hours, and the early stages, which closely resemble those of the ferns, may be easily followed by sowing the spores in water. With care it is possible to get the mature prothallia, which should be treated as described for the fern prothallia. Under favorable conditions, the first antheridia are ripe in about five weeks; the archegonia, which are borne on separate plants, a few weeks later. The antheridia (Fig. 72,J,an.) are larger than those of the ferns, and the spermatozoids (K) are thicker and with fewer coils, but otherwise much like fern spermatozoids.The archegonia have a shorter neck than those of the ferns, and the neck is straight.Both male and female prothallia are much branched and very irregular in shape.
The stem is mainly composed of large, thin-walled cells, becoming smaller as they approach the epidermis. The outer cells of the ground tissue in the green branches contain chlorophyll, and the walls of some of them are thickened. The fibro-vascular bundles differ entirely from those of the ferns. Each bundle is nearly triangular in section (E), with the point inward, and the inner end occupied by a large air space. The tracheary tissue is only slightly developed, being represented by a few vessels[9](tr.) at the outer angles of the bundle, and one or two smaller ones close to the air channel. The rest of the bundle is made up of nearly uniform, rather thin-walled, colorless cells, some of which, however, are larger, and have perforated cross-walls, representing the sieve tubes ofthe fern bundle. There is no individual bundle sheath, but the whole circle of bundles has a common outer sheath.
The epidermis is composed of elongated cells whose walls present a peculiar beaded appearance, due to the deposition of flint within them. The breathing pores are arranged in vertical lines, and resemble in general appearance those of the ferns, though differing in some minor details. Like the other epidermal cells the guard cells have heavy deposits of flint, which here are in the form of thick transverse bars.
The spore cases have thin walls whose cells, shortly before maturity, develop thickenings upon their walls, which have to do with the opening of the spore case. The spores (H,I) are round cells containing much chlorophyll and provided with four peculiar appendages called elaters. The elaters are extremely sensitive to changes in moisture, coiling up tightly when moistened (I), but quickly springing out again when dry (H). By dusting a few dry spores upon a slide, and putting it under the microscope without any water, the movement may be easily examined. Lightly breathing upon them will cause the elaters to contract, but in a moment, as soon as the moisture of the breath has evaporated, they will uncoil with a quick jerk, causing the spores to move about considerably.
The fresh spores begin to germinate within about twenty-four hours, and the early stages, which closely resemble those of the ferns, may be easily followed by sowing the spores in water. With care it is possible to get the mature prothallia, which should be treated as described for the fern prothallia. Under favorable conditions, the first antheridia are ripe in about five weeks; the archegonia, which are borne on separate plants, a few weeks later. The antheridia (Fig. 72,J,an.) are larger than those of the ferns, and the spermatozoids (K) are thicker and with fewer coils, but otherwise much like fern spermatozoids.
The archegonia have a shorter neck than those of the ferns, and the neck is straight.
Both male and female prothallia are much branched and very irregular in shape.
There are a number of common species ofEquisetum. Some of them, like the common scouring rush (E. hiemale), are unbranched, and the spores borne at the top of ordinary green branches; others have all the stems branching like the sterile stems of the field horse-tail, but produce a spore-bearing cone at the top of some of them.
The last class of the pteridophytes includes the ground pines, club mosses, etc., and among cultivated plants numerous species of the smaller club mosses (Selaginella).
Two orders are generally recognized, although there is some doubt as to the relationship of the members of the second order. The first order, the larger club mosses (Lycopodiaceæ) is represented in the northern states by a single genus (Lycopodium), of which the common ground pine (L. dendroideum) (Fig. 73) is a familiar species. The plant grows in the evergreen forests of the northern United States as well as in the mountains further south, and in the larger northern cities is often sold in large quantities at the holidays for decorating. It sends up from a creeping, woody, subterranean stem, numerous smaller stems which branch extensively, and are thickly set with small moss-like leaves, the whole looking much like a little tree. At the ends of some of the branches are small cones (A,x,B) composed of closely overlapping, scale-like leaves, much as in a fir cone. Near the base, on the inner surface of each of these scales, is a kidney-shaped capsule (C,sp.) opening by a cleft along the upper edge and filled with a mass of fine yellow powder. These capsules are the spore cases.
The bases of the upright stems are almost bare, but become covered with leaves higher up. The leaves are in shape like those of a moss, but are thicker. The spore-bearing leaves are broader and when slightly magnified show a toothed margin.
The stem is traversed by a central fibro-vascular cylinder that separates easily from the surrounding tissue, owing to the rupture of the cells of the bundle sheath, this being particularly frequent in dried specimens. When slightly magnified the arrangement of the tissues may be seen (Fig. 73,E). Within the epidermis is a mass of ground tissue of firm, woody texture surrounding the central oval or circular fibro-vascularcylinder. This shows a number of white bars (xylem) surrounded by a more delicate tissue (phloem).
On magnifying the section more strongly, the cells of the ground tissue (G) are seen to be oval in outline, with thick striated walls and small intercellular spaces. Examined in longitudinal sections they are long and pointed, belonging to the class of cells known as “fibres.”
On magnifying the section more strongly, the cells of the ground tissue (G) are seen to be oval in outline, with thick striated walls and small intercellular spaces. Examined in longitudinal sections they are long and pointed, belonging to the class of cells known as “fibres.”