In this article we will discuss about the process of reproduction in sphagnum with the help of diagrams.
Sphagnum can be dioecious as well as monoecious; but antheridia occur on different branches. Initially, the sexual branches are in the capitulum but due to elongation of the main stem they are isolated.
The antheridial branches (Fig. 4.5B) are specialized divergent branches in which the upper region bears stalked axillary antheridia (Fig. 4.7A). The leaves of antheridial branches are similar to that of vegetative leaves but these branches have a characteristic shade—deep red to brown, dark green to yellow. A plant bears many antheridial branches. After the formation of antheridia a branch can resume its vegetative function.
The archegonial branches are stumpy bud-like (Fig. 4.5B), the perichaetial leaves are large and bright green. The apical cell of an archegonial branch is used up in the formation of first archegonium—primary archegonium. Secondary archegonia may arise from segments cut off by the apical cell. Usually there are three archegonia (Fig. 4.7B) without paraphyses. There are several archegonial branches on a shoot.
Sphagnum is intermediate between mosses and liverworts in structure and development of its sex organs.
Sphagnum, in its mode of development of antheridium is similar to moss Funaria, However, its antheridia are similar to hepatics, as against clavate in mosses. This is because the apical cell discontinues its activity rather early.
An antheridium dehisces, splitting at its apex to form several revolute lobes, which allow an easy escape of antherozoids. A mature archegonium lacks a marked differentiation into neck and venter. Moreover, the terminal portion of the archegonium is, however, not sharply differentiated, as in the archegonia of liverworts.
The sporophyte has a massive foot and a bulbous capsule of 4-cell thick layer, the jacket. The upper portion of the jacket is operculum, which delimits the lid of capsule. Within the capsule is columella and overaching the columella cells is the spore sac.
The seta is represented by a constriction between capsule and foot. The function of seta is performed by the elongation of female shoot termed as pseudopodium, which remains short till the maturity of sporophyte. In a mature sporophyte the pseudopodium is an elongate structure (Fig. 4.8A).
Dehiscence of capsule is due to the dual effect of hot weather and rapid growth of pseudopodium. On drying, the cells of columella shrivell and the space is replaced by air. The capsule tends to be cylindrical and due to the compression of enclosed air the operculum flicks away and brings about a rupture of spore sac, dispersing the spores as yellow powder. In the entire process of dehiscence of capsule and dispersal of spores, an audible sound is produced.
In the species growing submerged, the capsule ruptures, allowing an escape for spores. The spores remain viable for a few months and germinate (Fig. 4.8C) readily in favourable conditions to produce a germ tube which forms a filament of cells. The terminal cell of this filament differentiates into an apical cell of two cutting faces. Due to the activity of this apical cell a flat plate of cells—protonema (Fig. 4.8D) is formed.
Later, due to the loss of control by the apical cell, on this plate appear localized areas of growth towards the margin, and rhizoids (Fig. 4.8E) appear at the posterior end. Any marginal cell can also regenerate into a filament which ends up into a thallose protonema.
In this way vegetative multiplication of thallose protonema is possible. The marginal cell of this protonema gives rise to a shoot-bud. One plant is produced by an individual protonema. The first-formed leaves are of uniform hexagonal chlorophyllous cells. Leaves formed later differentiate into a network of hyaline and green cells.