Part 191. The bryophyte division, mossworts (Bryophyta) comprises only the two classes liverworts (Hepaticae) and true mosses (Musci) which in general are often called mossworts.

Mossworts show us possibly how green earth-plants first stood upright. The occasion for their vertical development may have arisen when certain flat algae more or less like Coleochaete, became stranded and had to form spore-cases as best they could before the mud completely dried. If some of them were able to make a small globular capsule this might be fed entirely by the thallus, but being immersed within it could not ordinarily scatter the spores very far. Their descendants perhaps give us Riccia. Others we may suppose, hit upon the plan of elongating the capsule upward, giving it some chlorophyll to utilize the sunshine, and thus enable it to make more spores and scatter them farther-all with much less dependence upon the slender resources of the little nurse. The result would be a liverwort of the Anthoceros type which solves the problem of uplifting its spores in the simplest way. Various more or less complicated expedients were adopted by plants akin, and the outcome is seen in such liverworts as Marchantia where small capsules are made to hang from the branches of vertical thallus-lobes, or in mosses like Sphagnum where a similar though erect capsule is borne on an even more elaborately developed vertical branch of the nurse-plant, which may live for many years. The most complicated ways of securing elevation are found in the mosses typified by Funaria, where both the nurse-plant and the spore-plant develop vertically as far as they can-the latter, as it were, standing upon the shoulders of the former-and by photosynthesis making food for the spores. But the utmost height attained by these methods is only a few inches; the foundation is weak. Growth which has to be accomplished during a short season of moisture or by improving brief periods of wet weather, must naturally for the most part be limited to rather soft tissue and small organs. Mosses often grow crowded together like Sphagnum and thereby give mutual support and store a supply of water for use in common; but although axes of considerable height may be built in this way, the offspring is not much benefited, for the crowded tops of the axes form virtually a new surface above which is the only height effective for scattering the spores. It is plain that effectiveness is not always favored by complexity.

The view suggested above that mossworts have evolved directly from algae akin to Coleochaete, although regarded as probable by many botanists, receives no support from the study of fossil plants; and is by no means the only view consistent with what is known of the plants of to-day. Thus, it is quite possible that our mossworts may be the more or less simplified descendants of larger plants widely different from any we know, which themselves were descended from seaweeds very unlike Colechaete and of which we have now no trace. Not a few facts point to this conclusion; but the truth is we are much at a loss as to what to believe regarding the origin of mossworts, and the question seems likely to remain long a puzzle. Meanwhile, the hypothesis of direct algal origin may help us to imagine something of the nature of the problems which had to be faced by the earliest land-plants, whatever these plants may have been; and may suggest, at least by analogy, something of the means that may have proved most effective.

When we remember that Bryophytes have had to depend almost entirely upon superficial moisture it is not a little remarkable how much they have been able to accomplish for the welfare of their offspring. In spite of serious difficulties attending the use on land of reproductive arrangements adapted to aquatic life, these little plants very commonly achieve the benefits of cross-fertilization, and of a considerable period of nursing for their young. All this is made possible by the formation of archegonia which not only protect the protoplast of the egg, but by further development shield the young spore-plant all through its time of special tenderness.

Finally, in a considerable variety of ways means are provided for scattering the spores as far as possible and under the most favorable conditions for giving the new plants a good fair start.

Bryophyta are distinguished by having archegonia on lobed or pseudo-leafy gametophytes which bear sporophytes lacking true roots, stems, and leaves.