Elements of Structural and Systematic Botany Part 10

The mature capsule is broadly oval in form (Fig. 58, _C_), and provided with a lid that falls off when the spores are ripe. While the capsule is young it is covered by a pointed membranous cap (_B_, _cal._) that finally falls off. When the lid is removed, a fine fringe is seen surrounding the opening of the capsule, and serving the same purpose as the elaters of the liverworts (Fig. 58, _E_).

[Ill.u.s.tration: FIG. 58.--_A_, fruiting plant of a moss (_Funaria_), with young sporogonium (_sp._), 4. B, plant with ripe sporogonium.

_cal_. calyptra, 2. _C_, sporogonium with calyptra removed. _op._ lid, 4. _D_, spores: i, ungerminated; ii-iv, germinating, 300.

_E_, two teeth from the margin of the capsule, 50. _F_, epidermal cells and breathing pore from the surface of the sporogonium, 150.

_G_, longitudinal section of a young sporogonium, 12. _sp._ spore mother cells. _H_, a small portion of _G_, magnified about 300 times.

_sp._ spore mother cells.]

If the lower part of the stem is carefully examined with a lens, we may detect a number of fine green filaments growing from it, looking like the root hairs, except for their color. Sometimes the ground about young patches of the moss is quite covered by a fine film of such threads, and looking carefully over it probably very small moss plants may be seen growing up here and there from it.

[Ill.u.s.tration: FIG. 59.--Longitudinal section through the summit of a small male plant of _Funaria_. _a_, _a'_, antheridia. _p_, paraphysis.

_L_, section of a leaf, 150.]

This moss is dicious. The male plants are smaller than the female, and may be recognized by the bright red antheridia which are formed at the end of the stem in considerable numbers, and surrounded by a circle of leaves so that the whole looks something like a flower.

(This is still more evident in some other mosses. See Figure 65, _E_, _F_.)

The leaves when magnified are seen to be composed of a single layer of cells, except the midrib, which is made up of several thicknesses of elongated cells. Where the leaf is one cell thick, the cells are oblong in form, becoming narrower as they approach the midrib and the margin. They contain numerous chloroplasts imbedded in the layer of protoplasm that lines the wall. The nucleus (Fig. 63, _C_, _n_) may usually be seen without difficulty, especially if the leaf is treated with iodine. This plant is one of the best for studying the division of the chloroplasts, which may usually be found in all stages of division (Fig. 63, _D_). In the chloroplasts, especially if the plant has been exposed to light for several hours, will be found numerous small granules, that a.s.sume a bluish tint on the application of iodine, showing them to be starch grains. If the plant is kept in the dark for a day or two, these will be absent, having been used up; but if exposed to the light again, new ones will be formed, showing that they are formed only under the action of light.

[Ill.u.s.tration: FIG. 60.--_A_, _B_, young antheridia of _Funaria_, optical section, 150. _C_, two sperm cells of _Atrichum_. _D_, spermatozoids of _Sphagnum_, 600.]

Starch is composed of carbon, hydrogen, and oxygen, and so far as is known is only produced by chlorophyll-bearing cells, under the influence of light. The carbon used in the manufacture of starch is taken from the atmosphere in the form of carbonic acid, so that green plants serve to purify the atmosphere by the removal of this substance, which is deleterious to animal life, while at the same time the carbon, an essential part of all living matter, is combined in such form as to make it available for the food of other organisms.

The marginal cells of the leaf are narrow, and some of them prolonged into teeth.

A cross-section of the stem (63, _E_) shows on the outside a single row of epidermal cells, then larger chlorophyll-bearing cells, and in the centre a group of very delicate, small, colorless cells, which in longitudinal section are seen to be elongated, and similar to those forming the midrib of the leaf. These cells probably serve for conducting fluids, much as the similar but more perfectly developed bundles of cells (fibro-vascular bundles) found in the stems and leaves of the higher plants.

The root hairs, fastening the plant to the ground, are rows of cells with brown walls and oblique part.i.tions. They often merge insensibly into the green filaments (protonema) already noticed.

These latter have usually colorless walls, and more numerous chloroplasts, looking very much like a delicate specimen of _Cladophora_ or some similar alga. If a sufficient number of these filaments is examined, some of them will probably show young moss plants growing from them (Fig. 63, _A_, _k_), and with a little patience the leafy plant can be traced back to a little bud originating as a branch of the filament. Its diameter is at first scarcely greater than that of the filament, but a series of walls, close together, are formed, so placed as to cut off a pyramidal cell at the top, forming the apical cell of the young moss plant. This apical cell has the form of a three-sided pyramid with the base upward. From it are developed three series of cells, cut off in succession from the three sides, and from these cells are derived all the tissues of the plant which soon becomes of sufficient size to be easily recognizable.

The protonemal filaments may be made to grow from almost any part of the plant by keeping it moist, but grow most abundantly from the base of the stem.

The s.e.xual organs are much like those of the liverworts and are borne at the apex of the stems.

The antheridia (Figs. 59, 60) are club-shaped bodies with a short stalk. The upper part consists of a single layer of large chlorophyll-bearing cells, enclosing a ma.s.s of very small, nearly cubical, colorless, sperm cells each of which contains an excessively small spermatozoid.

The young antheridium has an apical cell giving rise to two series of segments (Fig. 60, _A_), which in the earlier stages are very plainly marked.

When ripe the chlorophyll in the outer cells changes color, becoming red, and if a few such antheridia from a plant that has been kept rather dry for a day or two, are teased out in a drop of water, they will quickly open at the apex, the whole ma.s.s of sperm cells being discharged at once.

Among the antheridia are borne peculiar hairs (Fig. 59, _p_) tipped by a large globular cell.

[Ill.u.s.tration: FIG. 61.--_A_, _B_, young; _C_, nearly ripe archegonium of _Funaria_, optical section, 150. _D_, upper part of the neck of _C_, seen from without, showing how it is twisted. _E_, base of a ripe archegonium. _F_, open apex of the same, 150. _o_, egg cell. _b_, ventral ca.n.a.l cell.]

Owing to their small size the spermatozoids are difficult to see satisfactorily and other mosses (_e.g._ peat mosses, Figure 64, the hairy cap moss, Figure 65, _I_), are preferable where obtainable.

The spermatozoids of a peat moss are shown in Figure 60, _D_. Like all of the bryophytes they have but two cilia.

The archegonia (Fig. 61) should be looked for in the younger plants in the neighborhood of those that bear capsules. Like the antheridia they occur in groups. They closely resemble those of the liverworts, but the neck is longer and twisted and the base more ma.s.sive.

Usually but a single one of the group is fertilized.

[Ill.u.s.tration: FIG. 62.--_A_, young embryo of _Funaria_, still enclosed within the base of the archegonium, 300. _B_, an older embryo freed from the archegonium, 150. _a_, the apical cell.]

To study the first division of the embryo, it is usually necessary to render the archegonium transparent, which may be done by using a little caustic potash; or letting it lie for a few hours in dilute glycerine will sometimes suffice. If potash is used it must be thoroughly washed away, by drawing pure water under the cover gla.s.s with a bit of blotting paper, until every trace of the potash is removed. The first wall in the embryo is nearly at right angles to the axis of the archegonium and divides the egg cell into nearly equal parts. This is followed by nearly vertical walls in each cell (Fig. 62, _A_). Very soon a two-sided apical cell (Fig. 62, _B_, _a_) is formed in the upper half of the embryo, which persists until the embryo has reached a considerable size. As in the liverworts the young embryo is completely covered by the growing archegonium wall.

The embryo may be readily removed from the archegonium by adding a little potash to the water in which it is lying, allowing it to remain for a few moments and pressing gently upon the cover gla.s.s with a needle. In this way it can be easily forced out of the archegonium, and then by thoroughly washing away the potash, neutralizing if necessary with a little acetic acid, very beautiful preparations may be made. If desired, these may be mounted permanently in glycerine which, however, must be added very gradually to avoid shrinking the cells.

[Ill.u.s.tration: FIG. 63.--_A_, protonema of _Funaria_, with a bud (_k_), 50. _B_, outline of a leaf, showing also the thickened midrib, 12. _C_, cells of the leaf, 300. _n_, nucleus. _D_, chlorophyll granules undergoing division, 300. _E_, cross-section of the stem, 50.]

For some time the embryo has a nearly cylindrical form, but as it approaches maturity the differentiation into stalk and capsule becomes apparent. The latter increases rapidly in diameter, a.s.suming gradually the oval shape of the full-grown capsule. A longitudinal section of the nearly ripe capsule (Fig. 58, _G_) shows two distinct portions; an outer wall of two layers of cells, and an inner ma.s.s of cells in some of which the spores are produced. This inner ma.s.s of cells is continuous with the upper part of the capsule, but connected with the side walls and bottom by means of slender, branching filaments of chlorophyll-bearing cells.

The spores arise from a single layer of cells near the outside of the inner ma.s.s of cells (_G_, _sp._). These cells (_H_, _sp._) are filled with glistening, granular protoplasm; have a large and distinct nucleus, and no chlorophyll. They finally become entirely separated and each one gives rise to four spores which closely resemble those of the liverworts but are smaller.

Near the base of the capsule, on the outside, are formed breathing pores (Fig. 58, _F_) quite similar to those of the higher plants.

If the spores are kept in water for a few days they will germinate, bursting the outer brown coat, and the contents protruding through the opening surrounded by the colorless inner spore membrane. The protuberance grows rapidly in length and soon becomes separated from the body of the spore by a wall, and lengthening, more and more, gives rise to a green filament like those we found attached to the base of the full-grown plant, and like those giving rise to buds that develop into leafy plants.

CLa.s.sIFICATION OF THE MOSSES.

The mosses may be divided into four orders: I. The peat mosses (_Sphagnaceae_); II. _Andreaeaceae_; III. _Phascaceae_; IV. The common mosses (_Bryaceae_).

[Ill.u.s.tration: FIG. 64.--_A_, a peat moss (_Sphagnum_), . _B_, a sporogonium of the same, 3. _C_, a portion of a leaf, 150. The narrow, chlorophyll-bearing cells form meshes, enclosing the large, colorless empty cells, whose walls are marked with thickened bars, and contain round openings (_o_).]

The peat mosses (Fig. 64) are large pale-green mosses, growing often in enormous ma.s.ses, forming the foundation of peat-bogs. They are of a peculiar spongy texture, very light when dry, and capable of absorbing a great amount of water. They branch (Fig. 64, _A_), the branches being closely crowded at the top, where the stems continue to grow, dying away below.

[Ill.u.s.tration: FIG. 65.--Forms of mosses. _A_, plant of _Phasc.u.m_, 3. _B_, fruiting plant of _Atrichum_, 2. _C_, young capsule of hairy-cap moss (_Polytrichum_), covered by the large, hairy calyptra.

_D_, capsules of _Bartramia_: i, with; ii, without the calyptra. _E_, upper part of a male plant of _Atrichum_, showing the flower, 2.

_F_, a male plant of _Mnium_, 4. _G_, pine-tree moss (_Clemacium_), 1. _H_, _Hypnum_, 1. _I_, ripe capsules of hairy-cap moss: i, with; ii, without calyptra.]

The s.e.xual organs are rarely met with, but should be looked for late in autumn or early spring. The antheridial branches are often bright-colored, red or yellow, so as to be very conspicuous. The capsules, which are not often found, are larger than in most of the common mosses, and quite dest.i.tute of a stalk, the apparent stalk being a prolongation of the axis of the plant in the top of which the base of the sporogonium is imbedded. The capsule is nearly globular, opening by a lid at the top (Fig. 64, _B_).

A microscopical examination of the leaves, which are quite dest.i.tute of a midrib, shows them to be composed of a network of narrow chlorophyll-bearing cells surrounding much larger empty ones whose walls are marked with transverse thickenings, and perforated here and there with large, round holes (Fig. 64, _C_). It is to the presence of these empty cells that the plant owes its peculiar spongy texture, the growing plants being fairly saturated with water.

The _Andreaeaceae_ are very small, and not at all common. The capsule splits into four valves, something like a liverwort.

The _Phascaceae_ are small mosses growing on the ground or low down on the trunks of trees, etc. They differ princ.i.p.ally from the common mosses in having the capsule open irregularly and not by a lid. The commonest forms belong to the genus _Phasc.u.m_ (Fig. 65, _A_).

The vast majority of the mosses the student is likely to meet with belong to the last order, and agree in the main with the one described. Some of the commoner forms are shown in Figure 65.

CHAPTER XII.

SUB-KINGDOM V.

PTERIDOPHYTES.

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