Bryophyta, the botanical name of the second great subdivision of the vegetable kingdom, which includes the mosses and liverworts. They are all plants of small, often minute, size, and, as the absence of popular names indicates, the different kinds are not commonly recognized. Even the distinction between liverworts and mosses is not clearly made, not only the former but other small plants of higher groups being popularly called mosses. A little careful observation soon shows, however, that the Bryophytes form a well-defined class, including several subordinate groups. Though their study necessarily involves minute observation they possess many features of interest. The adaptations they show to their conditions of life are often very perfect and present interesting analogies with the adaptive characters of the higher plants. They are of great scientific interest not only as representing a special type of life-history and organization, but because in several of the subordinate groups series of forms can be traced, which enable the general course of their evolution to be inferred even in the practical absence of fossil remains of any antiquity.
Bryophytes are very generally distributed over the earth, and those of a single country, such as Britain, afford examples of all the chief natural groups. Sometimes, as is the case with the bog-mosses and some arctic mosses, they may cover considerable tracts. As a rule, however, they occupy a subordinate place in the vegetation, and the different kinds require to be carefully looked for. Covering, as they often do, what would otherwise be bare ground, they are of value in assisting to retain moisture in the soil and in preparing the way for its colonization by higher plants. Although many forms are capable of withstanding periods of drought they succeed best in relatively moist climates and localities. This is shown both by their unequal abundance in different localities of one country and in their scarcity in certain geographical regions as compared with their luxuriance in others.
The external appearance and general organization show great variety. In all mosses and many liverworts (figs. 8, 11) the plant consists of a stem bearing small leaves. In a number of liverworts (figs. 2, 7), on the other hand, it presents no distinction of stem and leaf, but is a flat, dorsiventral body usually closely applied to the substratum on which it grows. This, in contradistinction to the leafy shoot, is termed a thallus. True roots are never present, the plants being attached to the soil by rhizoids, which resemble the root-hairs of higher plants.
Fig. 1. - Archegonia of Marchantia polymorpha. (After Sachs.)
1. Mature but unopened archegonium. e, Ovum; b, ventral-canal cell; d, lid-cells of neck.
2. Archegonium ready for fertilization; a passage leads down to the rounded ovum e.
3. Archegonium after fertilization; the fertilized ovum is developing into a sporogonium f; d, perianth.
The reproductive organs borne by the thallus or plant are called antheridia and archegonia, and serve for sexual reproduction. The antheridium (figs. 5, 15) has a longer or shorter stalk and consists of a wall formed of a single layer of flat cells enclosing a mass of minute cells from which the spermatozoids are developed. In the cases which have been most carefully investigated two spermatozoids have been found to arise from each of the small cubical cells of the central tissue. When mature the antheridium opens on being moistened and the spermatozoids become free in the water by the dissolution of the mucilaginous cell-walls enclosing them. Each has the form (fig. 5, D) of a more or less spirally twisted, club-shaped body, bearing at the pointed anterior end two long cilia by means of which it moves through the water. The archegonium (fig. 1) has the form of a narrow flask with a long neck. It usually has a short stalk and consists of a central row of cells enclosed by a layer of cells forming the wall. The egg-cell or ovum lies within the wider basal region or venter, and above it come the ventral canal-cell and canal-cells within the neck of the archegonium.
When the archegonium opens by the separation of the cells at the tip, the disorganized canal-cells escape, leaving a narrow tubular passage leading down to the ovum. Each antheridium or archegonium arises from a single cell, and while the mature structure is similar in the two groups, the development presents differences in liverworts and mosses. Without entering into details it may be mentioned that in the mosses it proceeds both in the archegonium and antheridium by the segmentation of an apical cell, while this is not the case in the liverworts. Fertilization is effected by the passage of a spermatozoid, attracted probably by means of a chemical stimulus, down the passage of the archegonial neck and its fusion with the ovum. It thus, as in other cases of sexual reproduction, involves the union of 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 Pteridophyta). 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.