Page:EB1911 - Volume 04.djvu/724

 two cells, and the vegetative plant, since it bears the sexual organs, is called the sexual generation or gametophyte.

From the fertilized ovum another and very different stage arises, which remains attached to the sexual plant and has thus the appearance of a fruit borne on it. It consists of a capsule usually borne on a longer or shorter stalk or seta, the base of which is inserted into the tissues of the gametophyte. This basal region, which serves to absorb nourishment, is called the foot. Within the capsule numerous reproductive cells, the spores, are developed. In contrast to the sexual generation this stage is called the spore-bearing generation (sporogonium, sporophyte). The examination of any moss “in fruit” (fig. 11, B) will show the readily detachable sporogonium borne on the leafy sexual plant, and the relation existing between the two generations will be evident from figs. 2, 3, 9, and 16. In liverworts (with one or two exceptions) the mature capsule is filled with spores mingled with sterile cells or elaters and opens by splitting into valves. In mosses (fig. 11, C) the sporogonium is more highly organized; a central column of sterile tissue (the columella) is found in the capsule, which opens by the removal of a lid or operculum, and there are no elaters among the spores. By the opening of the capsule the spores are set free, and under suitable conditions germinate and give rise to the sexual generation. In mosses (fig. 12) a filamentous growth, the protonema, is first formed, and the leafy plants arise upon this. In liverworts this preliminary phase of the sexual generation is as a rule ill-marked or absent, and the plant may be said to develop directly from the spore.

It will be evident that the two generations exhibit a regular succession or alternation in the life-history of all Bryophytes. The gametophyte is developed from the spore and bears the sexual organs; the sporogonium is developed from the fertilized egg and produces spores. An important cytological difference between the two generations can only be mentioned here. By the union of the nuclei of the spermatozoid and ovum in fertilization the number of chromosomes in the resulting nucleus is doubled, and this double number is maintained throughout all the cell-divisions of the sporogonium. On the development of the spores, which takes place by the division of each spore-mother-cell into four, the number of chromosomes becomes one half of what it has been in all the nuclei of the sporogonium. This reduced number is maintained throughout the development of the sexual generation. Thus in Pellia the nuclei of the gametophyte have eight chromosomes and those of the sporophyte sixteen. The relation in which the two generations stand to one another is the most important common characteristic of the Bryophyta. The gametophyte is always the independently living individual upon which the spore-bearing generation is throughout its life dependent. In all plants higher than the Bryophyta the sporophyte becomes an independently rooted plant and is the conspicuous stage in the life-history. Thus in the fern the sexual generation is the small prothallus developed from the spore, while the familiar fern-plant is the spore-bearing generation (see ). On the other hand a corresponding alternation of generations is only indicated in the lower plants (Thallophyta).

The Bryophyta are divided into the Hepaticae (liverworts) and Musci (mosses). In the Hepaticae we can recognize three subordinate groups—the Marchantiales, Jungermanniales and Anthocerotales; and in the Musci also three groups—the Sphagnales, Andreaeales and Bryales. Since these series of forms differ considerably among themselves, it is difficult to express in a definition the distinction between a liverwort and a moss which is readily made in practice. We may therefore leave it to the description of the several groups of Hepaticae and Musci to supplement the differences mentioned above and to bring out the exceptions which exist.

Hepaticae (Liverworts).

The range of form and structure of both generations in the liverworts is so great that no one form can be taken as a satisfactory type. It will, however, be of use to preface the more general description by a brief account of a particular example, and we may take for this purpose a very common and easily recognized thalloid liverwort belonging to the Jungermanniales.

Pellia epiphylla (fig. 2) can be found at any season growing in large patches on the damp soil of woods, banks, &c. The broad flat thallus is green and may be a couple of inches long. It is sparingly branched, the branching being apparently dichotomous; the growing point is situated in a depression at the anterior end of each branch. The wing-like lateral portions of the thallus gradually thin out from the midrib; from the projecting lower surface of this numerous rhizoids spring. These are elongated superficial cells, and serve to fix the thallus to the soil and obtain water and salts from it. No leaf-like appendages are borne on the thallus, but short glandular hairs occur behind the apex. The plant is composed throughout of very similar living cells, the more superficial ones containing numerous chlorophyll grains, while starch is stored in the internal cells of the midrib. The cells contain a number of oil-bodies the function of which is imperfectly understood. The growth of the thallus proceeds by the regular segmentation of a single apical cell. The sexual organs are borne on the upper surface, and both antheridia and archegonia occur on the same branch (fig. 3, A). The antheridia (an) are scattered over the middle region of the thallus, and each is surrounded by a tubular upgrowth from the surface. The archegonia (ar) are developed in a group behind the apex, and the latter continues to grow for a time after their formation, so that they come to be seated in a depression of the upper surface. They are further protected by the growth of the hinder margin of the depression to form a scale-like involucre (in). Fertilization takes place about June, and the sporogonium is fully developed by the winter. The embryo developed from the fertilized ovum consists at first of a number of tiers of cells. Its terminal tier gives rise to the capsule, the first divisions in the four cells of the tier marking off the wall of the capsule from the cells destined to produce the spores. In fig. 4, C, which represents a longitudinal section of a young embryo of Pellia, these archesporial cells are shaded. The tiers below give rise to the seta and foot. The mature sporogonium (fig. 3, B) consists of the foot embedded in the tissue of the thallus, the seta, which remains short until just before the shedding of the spores, and the spherical capsule. It remains for long enclosed within the calyptra formed by the further development of the archegonial wall and surmounted by the neck of the archegonium. The calyptra is ultimately burst through, and in early spring the seta elongates rapidly, raising the dark-coloured capsule (fig. 2). In the young condition the wall of the capsule, which consists of two layers of cells, encloses a mass of similar cells developed from the archesporium. Some of these become spore-mother-cells and give rise by cell division to four spores, while others remain undivided and become the elaters. The latter are elongated spindle-shaped cells with thick brown spiral bands on the inside of their thin walls. They radiate out from a small plug of sterile cells projecting into the base of the capsule, and some are attached to this, while others lie free among the spores. The latter are large, and at first are unicellular; but in Pellia, which in this respect is exceptional, they commence their further development within the capsule, and thus consist of several cells when shed.