Elementary Botany

513. We are now ready to take up the more careful study of the moss plant. There are a great many kinds of mosses, and they differ greatly from each other in the finer details of structure. Yet there are certain general resemblances which make it convenient to take for study almost any one of the common species in a neighborhood, which forms abundant fruit. Some, however, are more suited to a first study than others. (Polytrichum and funaria are good mosses to study.)

514. Mnium.—We will select here the plant shown in fig. 280. This is known as a mnium (M. affine), and one or another of the species of mnium can be obtained without much difficulty. The mosses, as we have already learned, possess an axis (stem) and leaf-like expansions, so that they are leafy-stemmed plants also. Certain of the branches of the mnium stand upright, or nearly so, and the leaves are all of the same size at any given point on the stem, as seen in the figure. There are three rows of these leaves, and this is true of most of the mosses.

515. The mnium plants usually form quite extensive and pretty mats of green in shady moist woods or ravines. Here and there among the erect stems are prostrate ones, with two rows of prominent leaves so arranged that it reminds one of some of the leafy-stemmed liverworts. If we examine some of the leaves of the mnium we see that the greater part of the leaf consists of a single layer of green cells, just as is the case in the leafy-stemmed liverworts. But along the middle line is a thicker layer, so that it forms a distinct midrib. This is characteristic of the leaves of mosses, and is one way in which they are separated from the leafy-stemmed liverworts, the latter never having a midrib.

516. The fruiting moss plant.—In fig. 280 is a moss plant “in fruit,” as we say. Above the leafy stem a slender stalk bears the capsule, and in this capsule are borne the spores. The capsule then belongs to the sporophyte phase of the moss plant, and we should inquire whether the entire plant as we see it here is the sporophyte, or whether part of it is gametophyte. If a part of it is gametophyte and a part sporophyte, then where does the one end and the other begin? If we strip off the leaves at the end of the leafy stem, and make a longisection in the middle line, we should find that the stalk which bears the capsule is simply stuck into the end of the leafy stem, and is not organically connected with it. This is the dividing line, then, between the gametophyte and the sporophyte. We shall find that here the archegonium containing the egg is borne, which is a surer way of determining the limits of the two phases of the plant.

517. The male and female moss plants.—The two plants of mnium shown in figs. 281, 282 are quite different, as one can easily see, and yet they belong to the same species. One is a female plant, while the other is a male plant. The sexual organs then in mnium, as in many others of the mosses, are borne on separate plants. The archegonia are borne at the end of the stem, and are protected by somewhat narrower leaves which closely overlap and are wrapped together. They are similar to the archegonia of the liverworts.

The male plants of mnium are easily selected, since the leaves at the end of the stem form a broad rosette with the antheridia, and some sterile threads packed closely together in the center. The ends of the mass of antheridia can be seen with the naked eye, as shown in fig. 282. When the antheridia are ripe, if we make a section through a cluster, or if we merely tease out some from the end with a needle in a drop of water on the slide, then prepare for examination with the microscope, we can see the form of the antheridia. They are somewhat clavate or elliptical in outline, as seen in fig. 284. Between them there stand short threads composed of several cells containing chlorophyll grains. These are sterile threads (paraphyses).

518. Sporogonium.—In fig. 280 we see illustrated a sporogonium of mnium, which is of course developed from the fertilized egg-cell of the archegonium. There is a nearly cylindrical capsule, bent downward, and supported on a long slender stalk. Upon the capsule is a peculiar cap,[29] shaped like a ladle or spatula. This is the remnant of the old archegonium, which, for a time surrounded and protected the young embryo of the sporogonium, just as takes place in the liverworts. In most of the mosses this old remnant of the archegonium is borne aloft on the capsule as a cap, while in the liverworts it is thrown to one side as the sporogonium elongates.

519. Structure of the moss capsule.—At the free end on the moss capsule as shown in the case of mnium in fig. 280, after the remnant of the archegonium falls away, there is seen a conical lid which fits closely over the end. When the capsule is ripe this lid easily falls away, and can be brushed off so that it is necessary to handle the plants with care if it is desired to preserve this for study.

520. When the lid is brushed away as the capsule dries more we see that the end of the capsule covered by the lid appears “frazzled.” If we examine this end with the microscope we see that the tissue of the capsule here is torn with great regularity, so that there are two rows of narrow, sharp teeth which project outward in a ring around the opening. If we blow our “breath” upon these teeth they will be seen to move, and as the moisture disappears and reappears in the teeth, they close and open the mouth of the capsule, so sensitive are they to the changes in the humidity of the air. In this way all of the spores are prevented to some extent from escaping from the capsule at one time.

521. Note. If we make a section longitudinal of the capsule of mnium, or some other moss, we find that the tissue which develops the spores is much more restricted than in the capsule of the liverworts which we have studied. The spore-bearing tissue is confined to a single layer which extends around the capsule some distance from the outside of the wall, so that a central cylinder is left of sterile tissue. This is the columella, and is present in nearly all the mosses. Each of the cells of the fertile layer divides into four spores.

522. Development of the sporogonium.—The egg-cell after fertilization divides by a wall crosswise to the axis of the archegonium. Each of these cells continues to divide for a time, so that a cylinder pointed at both ends is formed. The lower end of this cylinder of tissue wedges its way down through the base of the archegonium into the tissue of the end of the moss stem as shown in fig. 285. This forms the foot through which the nutrient materials are passed from the gametophyte to the sporogonium. The upper part continues to grow, and finally the upper end differentiates into the mature capsule.

523. Protonema of the moss.—When the spores of a moss germinate they form a thread-like body, with chlorophyll. This thread becomes branched, and sometimes quite extended tangles of these threads are formed. This is called the protonema, that is first thread. The older threads become finally brown, while the later ones are green. From this protonema at certain points buds appear which divide by close oblique walls. From these buds the leafy stem of the moss plant grows. Threads similar to these protonemal threads now grow out from the leafy stem, to form the rhizoids. These supply the moss plant with nutriment, and now the protonema usually dies, though in some few species it persists for long periods.

Classification of the Mosses.

CLASS MUSCINEÆ (MUSCI).

524. Order Sphagnales.[30]—This order includes the peat mosses. There is but one family (Sphagnaceæ) and but a single genus (Sphagnum). The peat mosses are widely distributed over the globe, chiefly occurring in moors, or “bogs,” usually low ground around the shores of lakes, ponds, or along streams, but they often occur on wet dripping rocks in cool shady places. Small ponds are sometimes filled in by their growth. As the sphagnum growing in such an abundance of water only partially decays, “ground” is built up rather rapidly, and the sphagnum remains are known as “peat.” This “ground”-building peculiarity of sphagnum sometimes enables the plant (often in conjunction with others) to fill in ponds completely. (See Atoll Moor, Chapter LV.)

The gametophyte of sphagnum, like that of all the mosses, is dimorphic, but the first part (or protonema) which develops from the spores is thalloid, and therefore more like the thallose liverworts. The leafy axis (or gametophore) which develops from the thalloid form is very characteristic (see Chapter LV).

The archegonia are borne on the free end of the main axis, while the antheridia are borne on short branches which are brightly colored, red, yellow, etc. The sporophyte (sporogonium) is globose and possesses a broad foot anchored in the end of a naked prolongation of the end of the leafy gametophore. This naked prolongation of the gametophore looks like the stalk of the sporogonium, but a study of its connection with the sporogonium shows that it is part of the gametophyte, which is only developed after the fertilization of the egg in the archegonium. In the sporogonium there is a short columella, and the archesporium is in the form of an inverted cup.

525. Order Andreæales.[31]—This order includes the single genus Andreæa. The plants are small but form extensive mats, growing on rocks in arctic or alpine regions usually. They are sometimes found in great abundance on bare, rather dry rocks on mountains. The protonema is somewhat thalloid. The sporogonium opens by splitting longitudinally into four valves. An elongated columella is present so that the archesporium is shaped like an inverted test tube.

526. Order Archidiales.[32]—This order contains the single genus Archidium, and by some is placed as an aberrant genus in the Bryales. There is no columella in the simple sporogonium. The archesporium occupies all the internal part of the sporogonium, some cells being fertile and others sterile.

527. Order Bryales.[33]—These include the higher mosses, and a very large number of genera and species. The protonema is filamentous and branched except in a few forms where it is partly thalloid as in Tetraphis (= Georgia). (Tetraphis pellucida is a common moss on very rotten logs. The capsule has four prominent teeth.) In a few of the lower genera (Phascum, Pleuridium, etc.) the capsule opens irregularly, but in the larger number the capsule opens by a lid (operculum). A cylindrical columella is present, and the archesporium is in the form of a tube open at both ends. (Examples: Polytrichum, Bryum, Mnium, Hypnum, etc.)

528.

TABLE SHOWING RELATION OF GAMETOPHYTE AND
SPOROPHYTE IN THE LIVERWORTS AND MOSSES.