Mosses (Bryophyta)
If we have followed carefully the development of marchantia, the study of one of the mosses will be comparatively easy. The mosses are more familiar plants than the liverworts. They grow on trees, stones, and on the soil in both wet and dry places. One of the common larger mosses, known as Polytrichum commune, may serve as an example, Fig. 295. This plant grows on rather dry knolls, mostly in the borders of open woods, where it forms large beds. In dry weather these beds have a reddish brown appearance, but when moist they form beautiful green cushions. This colour is due, in the first instance, to the colour of the old stems and leaves, and, in the second instance, to the peculiar action of the green living leaves under the influence of changing moisture-conditions. The inner or upper surface of the leaf is covered with thin, longitudinal ridges of delicate cells which contain chlorophyll. These cells are shown in cross-section in Fig. 296, as dots or granules. All the other tissue of the leaf consists of thick-walled, corky cells which do not allow moisture to penetrate. When the air is moist the green leaves spread out, exposing the chlorophyll cells to the air, but in dry weather the margins of the leaves roll inward, and the leaves fold closely against the stem, thus protecting the delicate assimilating tissue.
The antheridia and archegonia of polytrichum are borne in groups at the ends of the branches on different plants (many mosses bear both organs on the same branch). They are surrounded by involucres of characteristic leaves termed perichætia or perichætal leaves. Multicellular hairs known as paraphyses are scattered among the archegonia and antheridia. The involucres with the organs borne within them are called receptacles, or, less appropriately, “moss flowers.” As in marchantia, the organs are very minute and must be highly magnified to be studied.
The antheridia are borne in broad cup-like receptacles on the antheridial plants (Fig. 297). They are much like the antheridia of marchantia, but they stand free among the paraphyses and are not sunk in cavities. At maturity they burst and allow the sperm-cells or spermatozoids to escape. In polytrichum, when the receptacles have fulfilled their function, the stem continues to grow from the centre of the cup (m, Fig. 295). The archegonia are borne in other receptacles on different plants. They are like the archegonia of marchantia except that they stand erect on the end of the branch.
The sporogonium which develops from the fertilized egg is shown in a, b, Fig. 295. It consists of a long, brown stalk bearing the spore-case at its summit. The base of the stalk is imbedded in the end of the moss stem by which it is nourished. The capsule is entirely inclosed by a hairy cap, the calyptra, b. The calyptra is really the remnant of the archegonium, which, for a time, increases in size to accommodate and protect the young growing capsule. It is finally torn loose and carried up on the spore-case. The mouth of the capsule is closed by a circular lid, the operculum, having a conical projection at the centre.
The operculum soon drops, or it may be removed, displaying a fringe of sixty-four teeth guarding the mouth of the capsule. This ring of teeth is known as the peristome. In most mosses the teeth exhibit peculiar hygroscopic movements; i.e. when moist they bend outwards, and upon drying curve in toward the mouth of the capsule. This motion, it will be seen, serves to disperse these spores gradually over a long period of time.
Not the entire capsule is filled with spores. There are no elaters, but the centre of the capsule is occupied by a columnar strand of tissue, the columella, which expands at the mouth into a thin, membranous disk, closing the entire mouth of the capsule except the narrow annular chink guarded by the teeth. In this moss the points of the teeth are attached to the margin of the membrane, allowing the spores to sift out through the spaces between them.
When the spores germinate they form a green, branched thread, the protonema. This gives rise directly to moss plants, which appear as little buds on the thread. When the moss plants have sent their little rhizoids into the earth, the protonema dies, for it is no longer necessary for the support of the little plants, and the moss plants grow independently.
Funaria is a moss very common on damp, open soil. It forms green patches of small fine leaves from which arise long brown stalks terminated by curved capsules (Fig. 298). The structure is similar to that of polytrichum, except the absence of plates on the under side of the leaves, the continuous growth of the stem, the curved capsule, double peristome, monœcious rather than diœcious receptacles, and nearly glabrous unsymmetrical calyptra.
Equisetums, or Horsetails (Pteridophyta)
There are about twenty-five species of equisetum, constituting the only genus of the unique family Equisetaceæ. Among these E. arvense (Fig. 299) is common on clayey and sandy soils.
In this species the work of nutrition and that of spore production are performed by separate shoots from an underground rhizome. The fertile branches appear early in spring. The stem, which is 3 to 6 inches high, consists of a number of cylindrical, furrowed internodes, each sheathed at the base by a circle of scale leaves. The shoots are of a pale yellow colour. They contain no chlorophyll, and are nourished by the food stored in the rhizome (Fig. 299).
The spores are formed on specially developed fertile leaves or sporophylls which are collected into a spike or cone at the end of the stalk (a, Fig. 299). A single sporophyll is shown at b. It consists of a short stalk expanded into a broad, mushroom-like head. Several large sporangia are borne on its under side. The spores formed in the sporangia are very interesting and beautiful objects when examined under the microscope (× about 200). They are spherical, green bodies, each surrounded by two spiral bands attached to the spore at their intersection, s. These bands exhibit hygroscopic movements by means of which the spores become entangled, and are held together. This is of advantage to the plant, as we shall see. All the spores are alike, but some of the prothallia grow to a greater size than the others. The large prothallia produce only archegonia while the smaller ones produce antheridia. Both these organs are much like those of the ferns, and fertilization is accomplished in the same way. Since the prothallia are usually diœcious, the special advantage of the spiral bands, holding the spores together so that both kinds of prothallia may be in close proximity, will be easily understood. As in the fern, the fertilized egg-cell develops into an equisetum plant.
The sterile shoots (st, Fig. 299) appear much later in the season. They give rise to repeated whorls of angular or furrowed branches. The leaves are very much reduced scales, situated at the internodes. The stems are provided with chlorophyll and act as assimilating tissue, nourishing the rhizome and the fertile shoots. Nutriment is also stored in special tubers developed on the rhizome.
Other species of equisetum have only one kind of shoot—a tall, hard, leafless, green shoot with the spike at its summit. Equisetum stems are full of silex, and they are sometimes used for scouring floors and utensils; hence the common name “scouring rush.”
Isoëtes (Pteridophyta)
Isoëtes or quillwort is usually found in water or damp soil on the edges of ponds and lakes. The general habit of the plant is seen in Fig. 300, a. It consists of a short, perennial stem bearing numerous erect, quill-like leaves with broad sheathing bases. The plants are commonly mistaken for young grasses.
Isoëtes bears two kinds of spores, large roughened ones, the macrospores, and small ones or microspores. Both kinds are formed in sporangia borne in an excavation in the expanded base of the leaf. The macrospores are formed on the outer and the microspores on the inner leaves. A sporangium in the base of a leaf is shown at b. It is partially covered by a thin membrane, the velum. The minute triangular appendage at the upper end of the sporangium is called the ligule.
The spores are liberated by the decay of the sporangia. They form rudimentary prothallia of two kinds. The microspores produce prothallia with antheridia, while the macrospores produce prothallia with archegonia. Fertilization takes place as in the mosses or liverworts, and the fertilized egg-cell, by continued growth, gives rise again to the isoëtes plant.
Club-Mosses (Pteridophyta)
The club-mosses are low trailing plants of moss-like looks and habit, although more closely allied to ferns than to true mosses. Except one genus in Florida, all the club-mosses belong to the genus Lycopodium. They grow mostly in woods, having 1-nerved evergreen leaves arranged in four or more ranks. Some of them make long strands, as the ground pine, and are much used for Christmas decorations. The spores are all of one kind or form, borne in 1-celled sporangia that open on the margin into two valves. The sporangia are borne in some species (Fig. 301) as small yellow bodies in the axils of the ordinary leaves near the tip of the shoot; in other species (Fig. 302) they are borne in the axils of small scales that form a catkin-like spike. The spores are very numerous, and they contain an oil that makes them inflammable. About 100 species of lycopodium are known. The plants grown by florists under the name of lycopodium are of the genus Selaginella, more closely allied to isoëtes, bearing two kinds of spores (microspores and macrospores).