Elements of Structural and Systematic Botany Part 9

The first trace of the plant, visible to the naked eye, is a little downy, white speck, just large enough to be seen. This rapidly increases in size, becoming oblong in shape, and growing finally somewhat darker in color; and by the time it reaches a height of a few millimetres a short stalk becomes perceptible, and presently the whole a.s.sumes the form of a closed umbrella. The top is covered with little prominences, that diminish in number and size toward the bottom. After the cap reaches its full size, the stalk begins to grow, slowly at first, but finally with great rapidity, reaching a height of several centimetres within a few hours. At the same time that the stalk is elongating, the cap spreads out, radial clefts appearing on its upper surface, which flatten out very much as the folds of an umbrella are stretched as it opens, and the s.p.a.ces between the clefts appear as ridges, comparable to the ribs of the umbrella (Fig. 48, _B_). The under side of the cap has a number of ridges running from the centre to the margin, and of a black color, due to the innumerable spores covering their surface (_C_). Almost as soon as the umbrella opens, the spores are shed, and the whole structure shrivels up and dissolves, leaving almost no trace behind.

If we examine microscopically the youngest specimens procurable, freeing from air with alcohol, and mounting in water or dilute glycerine, we find it to be a little, nearly globular ma.s.s of colorless filaments, with numerous cross-walls, the whole arising from similar looser filaments imbedded in the substratum (Fig. 48, _G_). If the specimen is not too young, a denser central portion can be made out, and in still older ones (Fig. 48, _H_) this central ma.s.s has a.s.sumed the form of a short, thick stalk, crowned by a flat cap, the whole invested by a loose ma.s.s of filaments that merge more or less gradually into the central portion. By the time the spore fruit (for this structure corresponds to the spore fruit of the _Ascomycetes_) reaches a height of two or three millimetres, and is plainly visible to the naked eye, the cap grows downward at the margins, so as to almost entirely conceal the stalk. A longitudinal section of such a stage shows the stalk to be composed of a small-celled, close tissue becoming looser in the cap, on whose inner surface the spore-bearing ridges ("gills" or _Lamellae_) have begun to develop. Some of these run completely to the edge of the cap, others only part way. To study their structure, make cross-sections of the cap of a nearly full-grown, but unopened, specimen, and this will give numerous sections of the young gills.

We find them to be flat plates, composed within of loosely interwoven filaments, whose ends stand out at right angles to the surface of the gills, forming a layer of closely-set upright cells (basidia) (Fig. 48, _D_). These are at first all alike, but later some of them become club-shaped, and develop at the end several (usually four) little points, at the end of which spores are formed in exactly the same way as we saw in the germinating teleuto spores of the cedar rust, all the protoplasm of the basidium pa.s.sing into the growing spores (Fig. 48, _E_, _F_). The ripe spores (_E_, _sp._) are oval, and possess a firm, dark outer wall. Occasionally some of the basidia develop into very large sterile cells (E, _x_), projecting far beyond the others, and often reaching the neighboring gill.

Similar in structure and development to _Coprinus_ are all the large and common forms; but they differ much in the position of the spore-bearing tissue, as well as in the form and size of the whole spore fruit. They are sometimes divided, according to the position of the spores, into three orders: the closed-fruited (_Angiocarpous_) forms, the half-closed (_Hemi-angiocarpous_), and the open or naked-fruited forms (_Gymnocarpous_).

[Ill.u.s.tration: FIG. 49.--_Basidiomycetes_. _A_, common puff-ball (_Lycoperdon_). _B_, earth star (_Geaster_). _A_, . _B_, one-half natural size.]

Of the first, the puff-b.a.l.l.s (Fig. 49) are common examples. One species, the giant puff-ball (_Lycoperdon giganteum_), often reaches a diameter of thirty to forty centimetres. The earth stars (_Geaster_) have a double covering to the spore fruit, the outer one splitting at maturity into strips (Fig. 49, _B_). Another pretty and common form is the little birds'-nest fungus (_Cyathus_), growing on rotten wood or soil containing much decaying vegetable matter (Fig. 50).

[Ill.u.s.tration: FIG. 50.--Birds'-nest fungus (_Cyathus_). _A_, young.

_B_, full grown. _C_, section through _B_, showing the "sporangia"

(_sp._). All twice the natural size.]

In the second order the spores are at first protected, as we have seen in _Coprinus_, which belongs to this order, but finally become exposed. Here belong the toadstools and mushrooms (Fig. 51, _B_), the large shelf-shaped fungi (_Polyporus_), so common on tree trunks and rotten logs (Fig. 51, _C_, _D_, _E_), and the p.r.i.c.kly fungus (_Hydnum_) (Fig. 51, _G_).

[Ill.u.s.tration: FIG. 51.--Forms of _Basidiomycetes_. _A_, _Tremella_, one-half natural size. _B_, _Agaricus_, natural size. _C_, _E_, _Polyporus_: _C_, ; _E_, . _D_, part of the under surface of _D_, natural size. _F_, _Clavaria_, a small piece, natural size. _G_, _Hydnum_, a piece of the natural size.]

Of the last, or naked-fruited forms, the commonest belong to the genus _Clavaria_ (Fig. 51, _F_), smooth-branching forms, usually of a brownish color, bearing the spores directly upon the surface of the branches.

CHAPTER XI.

SUB-KINGDOM IV.

BRYOPHYTA.

The Bryophytes, or mosses, are for the most part land plants, though a few are aquatic, and with very few exceptions are richly supplied with chlorophyll. They are for the most part small plants, few of them being over a few centimetres in height; but, nevertheless, compared with the plants that we have heretofore studied, quite complex in their structure. The lowest members of the group are flattened, creeping plants, or a few of them floating aquatics, without distinct stem and leaves; but the higher ones have a pretty well-developed central axis or stem, with simple leaves attached.

There are two cla.s.ses--I. Liverworts (_Hepaticae_), and II. Mosses (_Musci_).

CLa.s.s I.--THE LIVERWORTS.

One of the commonest of this cla.s.s, and to be had at any time, is named _Madotheca_. It is one of the highest of the cla.s.s, having distinct stem and leaves. It grows most commonly on the shady side of tree trunks, being most luxuriant near the ground, where the supply of moisture is most constant. It also occurs on stones and rocks in moist places. It closely resembles a true moss in general appearance, and from the scale-like arrangement of its leaves is sometimes called "scale moss."

The leaves (Fig. 52, _A_, _B_) are rounded in outline unequally, two-lobed, and arranged in two rows on the upper side of the stem, so closely overlapping as to conceal it entirely. On the under side are similar but smaller leaves, less regularly disposed. The stems branch at intervals, the branches spreading out laterally so that the whole plant is decidedly flattened. On the under side are fine, whitish hairs, that fasten it to the substratum. If we examine a number of specimens, especially early in the spring, a difference will be observed in the plants. Some of them will be found to bear peculiar structures (Fig. 52, _C_, _D_), in which the spores are produced.

These are called "sporogonia." They are at first globular, but when ripe open by means of four valves, and discharge a greenish brown ma.s.s of spores. An examination of the younger parts of the same plants will probably show small buds (Fig. 54, _H_), which contain the female reproductive organs, from which the sporogonia arise.

[Ill.u.s.tration: FIG. 52.--_A_, part of a plant of a leafy liverwort (_Madotheca_), 2. _B_, part of the same, seen from below, 4. _C_, a branch with two open sporogonia (_sp._), 4. _D_, a single sporogonium, 8.]

On other plants may be found numerous short side branches (Fig. 53, _B_), with very closely set leaves. If these are carefully separated, the antheridia can just be seen as minute whitish globules, barely visible to the naked eye. Plants that, like this one, have the male and female reproductive organs on distinct plants, are said to be "dicious."

A microscopical examination of the stem and leaves shows their structure to be very simple. The former is cylindrical, and composed of nearly uniform elongated cells, with straight cross-walls. The leaves consist of a single layer of small, roundish cells, which, like those of the stem, contain numerous rounded chloroplasts, to which is due their dark green color.

The tissues are developed from a single apical cell, but it is difficult to obtain good sections through it.

The antheridia are borne singly at the bases of the leaves on the special branches already described (Fig. 53, _A_, _an._). By carefully dissecting with needles such a branch in a drop of water, some of the antheridia will usually be detached uninjured, and may be readily studied, the full-grown ones being just large enough to be seen with the naked eye. They are globular bodies, attached by a stalk composed of two rows of cells. The globular portion consists of a wall of chlorophyll-bearing cells, composed of two layers below, but single above (Fig. 53, _C_). Within is a ma.s.s of excessively small cells, each of which contains a spermatozoid. In the young antheridium (_A_, _an._) the wall is single throughout, and the central cells few in number. To study them in their natural position, thin longitudinal sections of the antheridial branch should be made.

[Ill.u.s.tration: FIG. 53.--_A_, end of a branch from a male plant of _Madotheca_. The small side branchlets bear the antheridia, 2. _B_, two young antheridia (_an._), the upper one seen in optical section, the lower one from without, 150. _C_, a ripe antheridium, optical section, 50. _D_, sperm cells with young spermatozoids. _E_, ripe spermatozoids, 600.]

When ripe, if brought into water, the antheridium bursts at the top into a number of irregular lobes that curl back and allow the ma.s.s of sperm cells to escape. The spermatozoids, which are derived princ.i.p.ally from the nucleus of the sperm cells (53, _D_) are so small as to make a satisfactory examination possible only with very powerful lenses. The ripe spermatozoid is coiled in a flat spiral (53, _E_), and has two excessively delicate cilia, visible only under the most favorable circ.u.mstances.

The female organ in the bryophytes is called an "archegonium," and differs considerably from anything we have yet studied, but recalls somewhat the structure of the oogonium of _Chara_. They are found in groups, contained in little bud-like branches (54, _H_). In order to study them, a plant should be chosen that has numbers of such buds, and the smallest that can be found should be used. Those containing the young archegonia are very small; but after one has been fertilized, the leaves enclosing it grow much larger, and the bud becomes quite conspicuous, being surrounded by two or three comparatively large leaves. By dissecting the young buds, archegonia in all stages of growth may be found.

[Ill.u.s.tration: FIG. 54.--_A-D_, development of the archegonium of _Madotheca_. _B_, surface view, the others in optical section. _o_, egg cell, 150. _E_, base of a fertilized archegonium, containing a young embryo (_em._), 150. _F_, margin of one of the leaves surrounding the archegonia. _G_, young sporogonium still surrounded by the much enlarged base of the archegonium. _h_, neck of the archegonium. _ar._ abortive archegonia, 12. _H_, short branch containing the young sporogonium, 4.]

When very young the archegonium is composed of an axial row of three cells, surrounded by a single outer layer of cells, the upper ones forming five or six regular rows, which are somewhat twisted (Fig. 54, _A_, _B_). As it becomes older, the lower part enlarges slightly, the whole looking something like a long-necked flask (_C_, _D_). The centre of the neck is occupied by a single row of cells (ca.n.a.l cells), with more granular contents than the outer cells, the lowest cell of the row being somewhat larger than the others (Fig. 54, _C_, _o_). When nearly ripe, the division walls of the ca.n.a.l cells are absorbed, and the protoplasm of the lowest cell contracts and forms a globular naked cell, the egg cell (_D_, _o_).

If a ripe archegonium is placed in water, it soon opens at the top, and the contents of the ca.n.a.l cells are forced out, leaving a clear channel down to the egg cell. If the latter is not fertilized, the inner walls of the neck cells turn brown, and the egg cell dies; but if a spermatozoid penetrates to the egg cell, the latter develops a wall and begins to grow, forming the embryo or young sporogonium.

[Ill.u.s.tration: FIG. 55.--Longitudinal section of a nearly full-grown sporogonium of _Madotheca_, which has not, however, broken through the overlying cells, 25. _sp._ cavity in which the spores are formed.

_ar._ abortive archegonium.]

The first division wall to be formed in the embryo is transverse, and is followed by vertical ones (Fig. 54, _E_, _em._). As the embryo enlarges, the walls of the basal part of the archegonium grow rapidly, so that the embryo remains enclosed in the archegonium until it is nearly full-grown (Fig. 55). As it increases in size, it becomes differentiated into three parts: a wedge-shaped base or "foot" penetrating downward into the upper part of the plant, and serving to supply the embryo with nourishment; second, a stalk supporting the third part, the capsule or spore-bearing portion of the fruit. The capsule is further differentiated into a wall, which later becomes dark colored, and a central cavity, in which are developed special cells, some of which by further division into four parts produce the spores, while the others, elongating enormously, give rise to special cells, called elaters (Fig. 56, _B_).

[Ill.u.s.tration: FIG. 56.--Spore (_A_) and two elaters (_B_) of _Madotheca_, 300.]

The ripe spores are nearly globular, contain chlorophyll and drops of oil, and the outer wall is brown and covered with fine points (Fig. 56, _A_). The elaters are long-pointed cells, having on the inner surface of the wall a single or double dark brown spiral band.

These bands are susceptible to changes in moisture, and by their movements probably a.s.sist in scattering the spores after the sporogonium opens.

Just before the spores are ripe, the stalk of the sporogonium elongates rapidly, carrying up the capsule, which breaks through the archegonium wall, and finally splits into four valves, and discharges the spores.

There are four orders of the liverworts represented in the United States, three of which differ from the one we have studied in being flattened plants, without distinct stems and leaves,--at least, the leaves when present are reduced to little scales upon the lower surface.

The first order (_Ricciaceae_) are small aquatic forms, or grow on damp ground or rotten logs. They are not common forms, and not likely to be encountered by the student. One of the floating species is shown in figure 57, _A_.

The second order, the horned liverworts (_Anthoceroteae_), are sometimes to be met with in late summer and autumn, forms growing mostly on damp ground, and at once recognizable by their long-pointed sporogonia, which open when ripe by two valves, like a bean pod (Fig. 57, _B_).

The third order (_Marchantiaceae_) includes the most conspicuous members of the whole cla.s.s. Some of them, like the common liverwort (_Marchantia_), shown in Figure 57, _F_, _K_, and the giant liverwort (Fig. 57, _D_), are large and common forms, growing on the ground in shady places, the former being often found also in greenhouses. They are fastened to the ground by numerous fine, silky hairs, and the tissues are well differentiated, the upper surface of the plant having a well-marked epidermis, with peculiar breathing pores, large enough to be seen with the naked eye (Fig. 57, _E_, _J_, _K_) Each of these is situated in the centre of a little area (Fig. 57, _E_), and beneath it is a large air s.p.a.ce, into which the chlorophyll-bearing cells (_cl._) of the plant project (_J_).

The s.e.xual organs are often produced in these forms upon special branches (_G_), or the antheridia may be sunk in discs on the upper side of the stem (_D_, _an._).

[Ill.u.s.tration: FIG. 57.--Forms of liverworts. _A_, _Riccia_, natural size. _B_, _Anthoceros_ (horned liverwort), natural size. _sp._ sporogonia. _C_, _Lunularia_, natural size, _x_, buds. _D_, giant liverwort (_Conocephalus_), natural size. _an._ antheridial disc. _E_, small piece of the epidermis, showing the breathing pores, 2. _F_, common liverwort (_Marchantia_), 2. _x_, cups containing buds. _G_, archegonial branch of common liverwort, natural size. _H_, two young buds from the common liverwort, 150. _I_, a full-grown bud, 25.

_J_, vertical section through the body of _Marchantia_, cutting through a breathing pore (_s_), 50. _K_, surface view of a breathing pore, 150. _L_, a leafy liverwort (_Jungermannia_). _sp._ sporogonium, 2.]

Some forms, like _Marchantia_ and _Lunularia_ (Fig. 57, _C_), produce little cups (_x_), circular in the first, semicircular in the second, in which special buds (_H_, _I_) are formed that fall off and produce new plants.

The highest of the liverworts (_Jungermanniaceae_) are, for the most part, leafy forms like _Madotheca_, and represented by a great many common forms, growing usually on tree trunks, etc. They are much like _Madotheca_ in general appearance, but usually very small and inconspicuous, so as to be easily overlooked, especially as their color is apt to be brownish, and not unlike that of the bark on which they grow (Fig. 57, _L_).

CLa.s.s II.--THE TRUE MOSSES.

The true mosses (_Musci_) resemble in many respects the higher liverworts, such as _Madotheca_ or _Jungermannia_, all of them having well-marked stems and leaves. The spore fruit is more highly developed than in the liverworts, but never contains elaters.

A good idea of the general structure of the higher mosses may be had from a study of almost any common species. One of the most convenient, as well as common, forms (_Funaria_) is to be had almost the year round, and fruits at almost all seasons, except midwinter. It grows in close patches on the ground in fields, at the bases of walls, sometimes in the crevices between the bricks of sidewalks, etc. If fruiting, it may be recognized by the nodding capsule on a long stalk, that is often more or less twisted, being sensitive to changes in the moisture of the atmosphere. The plant (Fig. 58, _A_, _B_) has a short stem, thickly set with relatively large leaves. These are oblong and pointed, and the centre is traversed by a delicate midrib. The base of the stem is attached to the ground by numerous fine brown hairs.