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The Development of Organs

Protonema.--The large spores germinate in water to form a thread-like protonema; or, on land, to form a flat plate of cells, from which the moss-plant develops. Several stages from spore to adult plant are shown in diagrams 1, 2, 3, 4, 5, 6 and 7. (See also diagrams on pages 118 and 120.)

Plant (gametophyte).--The cells of the protonema by division at one point form the plant--stem, leaves and rhizoids--then the protonema disappears. Upon the moss plant are developed the male flowers (antheridia) and the female flowers (archegonia). In Sphagnum acutifolium the male and female flowers mature in late autumn and in winter and may often be found by digging under the snow.

Antheridium.--An antheridium has its origin in a cell of the outer wall of a branch. This cell divides to make two, one of which divides to form a stem or pedicel, while the second divides to form the globular part of the antheridium. The mature antheridium bursts at the apex, the margins roll back and a cloud of flattened membranous sacs (vesicles), each containing one spirally coiled antherosoid, are thrown out. The antherosoids are soon set free by the breaking down of the vesicle wall. Each antherosoid is a spirally coiled cell, club-shaped, with two vibratile lashes at the attenuated end.

Archegonium.--The archegonium is developed by cell-division; it is similar to a tiny flask, at the base of which is an egg-cell (ovum) which, after fertilisation, is to become a spore-case (1, 2, 3). Fertilisation occurs early in the spring, that is, sperm-cells (antherosoids) from antheridia, swim over in water to a mature egg-cell of an archegonium, coalesce with it and make possible the development of an embryo spore-case (4).

The Spore-case.--The first embryos may be found late in February. A study of their development shows that the fertilised egg-cell divides into four cells and then by repeated division of cells takes the form of the several diagrams in order, A, B, C, and D. The embryo which has been formed as a result of fertilisation is divided into two regions, the three upper segments with the apical cell give rise to spores, while the lower segments with the basal cell form a "foot." The rudimentary spore-case is at first sessile, but later is raised by the lengthening of the apex of the branch upon which it is borne.

Veil (calyptra).--The veil is the fragmentary remains of the old archegonium wall which was burst by the enlarging spore-case within and left at the base, or carried up on the lid.

Pedicel (seta).--The "foot" of the Sphagnums is homologous with the pedicel of other mosses, as it is due to a development of the base of the archegonium. The so-called pedicel (pseudopodium) is the result of a lengthening upward of the apex of the branch which bears the archegonium. This branch so enlarges just below the developing spore-case as to completely envelop the "foot."

Lid (operculum).--When the spore-case is mature enough to open, the upper portion separates from the lower by the breaking down of the walls of a zone of cells. This zone of weak cells is first noticeable in a young spore-case as a groove. This groove is due to one zone of cells growing less rapidly than the zones of cells on either side. The breaking along the groove is due to the thinner walls of the groove cells. One zone of thicker-walled cells forms a rim to the spore-case and the other zone of thicker-walled cells forms the rim of the lid.

The character of the leaves and the manner of branching, both of which are visible to the naked eye, enable one to separate the species with some accuracy into eight groups, which are helpful to a beginner, in that they gave him some definite points of difference to look for in a genus of which to the novice all species appear alike.