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 pa.s.sed through the apex, it will show the structure shown in Figure 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.

CHAPTER XIII.

CLa.s.sIFICATION OF THE PTERIDOPHYTES.

There are three well-marked cla.s.ses of the Pteridophytes: the ferns (_Filicinae_); horse-tails (_Equisetinae_); and the club mosses (_Lycopodinae_).

CLa.s.s I.--FERNS (_Filicinae_).

The ferns const.i.tute 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 (_Ophioglossaceae_), are represented in the United States. A third order, intermediate in some respects between these two, and called the ringless ferns (_Marattiaceae_), has no representatives within our territory.

The cla.s.sification is at present based largely upon the characters of the sporophyte, the s.e.xual plants being still very imperfectly known in many forms.

The adder-tongues (_Ophioglossaceae_) 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. In _Botrychium_ the leaves are more or less deeply divided, and the sporangia distinct (Fig. 71, _B_). In _Ophioglossum_ the sterile division of the leaf is usually smooth and undivided, and the spore-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.

[Ill.u.s.tration: FIG. 70.--Forms of ferns. _A_, grape fern (_Botrychium_), . _x_, fertile part of the leaf. _B_, sporangia of _Botrychium_, 3. _C_, flowering fern (_Osmunda_). _x_, spore-bearing leaflets, . _D_, a sporangium of _Osmunda_, 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 ma.s.ses of sporangia beneath, 2.]

In the true ferns (_Filices_), the sporangia resemble those already described, arising in all (unless possibly _Osmunda_) from a single epidermal cell.

One group, the water ferns (_Rhizocarpeae_), produce two kinds of spores, large and small. The former produce male, the latter female prothallia. In both cases the prothallium is small, 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 genus _Marsilia_ (Fig. 71, _A_), looking like a four-leaved clover. Others (_Salvinia_, _Azolla_) are floating forms (Fig. 71, _D_).

[Ill.u.s.tration: FIG. 71.--_A_, _Marsilia_, one of the _Rhizocarpeae_ (after Underwood). _sp._ the "fruits" containing the sporangia. _B_, a small spore of _Pilularia_, 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 in Figure 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). Ill.u.s.trations of some of the commonest genera are shown in Figure 70, _E_, _J_.

CLa.s.s II.--HORSE-TAILS (_Equisetinae_).

The second cla.s.s of the pteridophytes includes the horse-tails (_Equisetinae_) 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.

[Ill.u.s.tration: FIG. 72.--_A_, spore-bearing stem of the field horse-tail (_Equisetum_), 1. _x_, the spore-bearing cone. _B_, sterile stem of the same, . _C_, underground stem, with tubers (_o_), . _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 circ.u.mstances 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 s.p.a.ces 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 dest.i.tute 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 s.p.a.ces is larger, and in addition there is a large central cavity. The fibro-vascular bundles (_f.b._) are arranged in a circle, alternating with the air channels, and each one has running through it a small air pa.s.sage.

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 ma.s.s 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 quant.i.ties 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 s.p.a.ce. 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 of the fern bundle. There is no individual bundle sheath, but the whole circle of bundles has a common outer sheath.

[9] A vessel differs from a tracheid in being composed of several cells placed end to end, the part.i.tions being wholly or partially absorbed, so as to throw the cells into close communication.

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 of _Equisetum_. 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.

CLa.s.s III.--THE CLUB MOSSES (_Lycopodinae_).

The last cla.s.s 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 (_Lycopodiaceae_) 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 quant.i.ties 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 ma.s.s 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 ma.s.s of ground tissue of firm, woody texture surrounding the central oval or circular fibro-vascular cylinder. 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 s.p.a.ces. Examined in longitudinal sections they are long and pointed, belonging to the cla.s.s of cells known as "fibres."

[Ill.u.s.tration: FIG. 73.--_A_, a club moss (_Lycopodium_), ?. _x_, cone. _r_, root. _B_, a cone, 1. _C_, single scale with sporangium (_sp._). _D_, spores: i, from above; ii, from below, 325. _E_, cross section of stem, 8. _f.b._ fibro-vascular bundle. _F_, portion of the fibro-vascular bundle, 150. _G_, cells of the ground tissue, 150.]

The xylem (_F_, _xy._) of the fibro-vascular bundle is composed of tracheids, much like those of the ferns; the phloem is composed of narrow cells, pretty much all alike.

The spores (_D_) are dest.i.tute of chlorophyll and have upon the outside a network of ridges, except on one side where three straight lines converge, the spore being slightly flattened between them.

Almost nothing is known of the prothallia of our native species.