Mesoglia vermicularis ([Fig. 285]) consists of strings of cells cohering and held together by their membranous covering. In the lowly organised plant Vaucheria ([Plate I]., No. 23, V. sessilis)—so named after its discoverer Vaucher, a German botanist—a genus of Siphonaceæ, we have an example of true processes of sexual generation. The branching filaments are often seen to bear at their sides peculiar globular bodies or oval protuberances, nipple-shaped buddings-out of the cell-wall, filled with a dark-coloured endochrome and distributed in pairs, one of which curves round to meet the other, when conjugation is seen to take place. Near these bodies others are found with pointed projections, which have been described as “horns,” but these, Pringshelm says, are “antherids which produce antherozoids in their interior,” while the capsule-like bodies constituting the oospores become, when fertilised, a new generation, which swarm out through a cavity or aperture in the parent cell-wall.
The fruit of fresh-water and most olive-green algals is enclosed in spherical cavities under the epidermis of the frond, termed conceptacles, and may be either male or female. The zoids are bottle-shaped and have flagella; the transparent vesicle in which they are contained is itself enclosed in a second of similar form. In monœcious and diœcious algals the female conceptacles are distinguished from the male by their olive colour. The spores, when developed, are borne on a pedicle emanating from the inner wall of the conceptacle. They rupture the outer wall at its apex; at first the spore appears simple, but soon after a series of changes takes place, consisting in a splitting up of the endochrome into six or eight masses of spheroidal bodies. A budding-out occurs in a few hours’ time, and ultimately elongates into a cylindrical thread. The Vaucheria present a double mode of reproduction, and their fronds consist of branching tubes resembling in their general character that of the Bryophyta, from which indeed they differ only in respect of the arrangement of their green contents. In that most remarkable plant Saprolegnia ferox, which is structurally so closely allied to Vaucheria, though separated from them by the absence of green colouring matter, a corresponding analogy in the processes of development takes place. In the formation of its zoospores, an intermediate step is presented between that of the algæ and a class of plants formally placed among fungi.
The Ulvaceæ.—The typical form of seaweeds is the Ulva lactuca, well known from its fronds of dark-green “laver” on every coast throughout the world. Ulvæ are seen to differ but little from the preceding group of fresh-water algals. The specific difference is that the cells, when multiplied by binary subdivision, not only remain in firm connection with each other but possess a more regular arrangement. The frond plane of the algal is either more simple or lobed, and is formed of a double layer of cells closely packed together and producing zoospores. The whole group is chiefly distinguished from Porphyra by their green colour, the latter being roseate or purple. Ulvæ are mostly marine, with one or two exceptions. One species (U. thermalis) grows in the hot springs of Gastein, in a temperature of about 117° Fahr. The development of Ulvæ is seen in [Fig. 286]. The isolated cells, A, resemble in some points those of the Protococcus; these give rise to successive subdivisions determining the clusters seen at B and C, and by their aggregation to the confervoid filament shown at D. These filaments increase in length and breadth by successive additions, and finally take the form of fronds, or rows of cells.
Fig. 286.—Successive Stages of Development of Ulvæ.
A. Isolated spores; B and C. Clusters of cells; D. Cells in the filamentous stage.
Fig. 287.—Sphacelaria cirrhosa, with spores borne at the sides of the branchlets.
The marine greenish-olive algæ present a general appearance which might at first sight be mistaken for plants of a higher order of cryptogams. Their fronds have no longer the form of a filament, but assume that of a membranous expansion of cells. The cells in which zoospores are found have an increased quantity of coloured protoplasm accumulated towards one point of the cell-wall; while the zoospores are observed to converge with their apices towards the same point. In some algæ, which seem to be closely related in form and structure to the Bryophyta, we notice this important difference, that the zoospores are developed in an organ specially destined for the purpose, presenting peculiarities of form and distinguishing it from other parts of the branching tubular frond. In the genus Derbesia distinct spore cases develop, a young branch of which, when destined to become a spore case, instead of elongating indefinitely, begins, after having arrived at a certain length, to swell out into an ovoid vesicle, in the cavity of which a considerable accumulation of protoplasm takes place. This is separated from the rest of the plant, and becomes an opaque mass, surrounded by a distinct membrane. After a time a division of the mass takes place, and a number of pyriform zoospores, each of which is furnished with flagella, are set free.
Desmidiaceæ, Diatomaceæ, Algæ.