Fig. 41.—Zygnema insigne, with zygote.
Fig. 42.—Germinating zygote of Spirogyra jugalis: the young plant is still unicellular; the end which is still in the wall of the zygote is elongated and root-like; the chromatophore divides and forms the spiral band.
The conjugation among the Zygnemaceæ takes place in the following manner: the cells of two filaments, lying side by side, or two cells, the one being situated above the other in the same filament (Fig. [41]), push out small protuberances opposite each other (Fig. [39] A, a, b); these finally meet, and the dividing wall is absorbed so that a tube is formed connecting one cell with the other; the protoplasmic contents round off, and the whole of these contents of one of the cells glides through the conjugation-tube and coalesces with that of the other (Fig. [39] B), the aggregate mass then rounds off, surrounds itself with a cell-wall, and becomes a zygote. A distinct difference may be found between the cells in the two filaments, those in the one whose protoplasmic contents pass over being cylindrical, while those of the recipient one are more barrel-shaped, and of a larger diameter. The former may be regarded as a male, the latter as a female plant. The zygote germinates after a period of rest, and grows out into a new filament (Fig. [42]).
Order 3. Mesocarpaceæ. The cell-walls are glabrous, unconstricted in the centre, and united into simple unbranched filaments. The chromatophore consists of an axial chlorophyll-plate, with several pyrenoids. The zygote is formed by the coalescence of two cells (Fig. [43]) (sometimes three or four), but the whole protoplasmic contents of the cells do not take part in this process, a portion always remaining behind; the aplanogametes coalesce in the conjugation-canal. The zygote thus formed appears incapable of germination until after 3–5 divisions. Of the cells so formed, only one is fertile, the sterile cells, according to Pringsheim, constituting a rudimentary sporocarp. The germinating cells grow out into a new filament. In this order, conjugation has been observed between two cells of the same filament. The Mesocarpaceæ thrive best in water which contains lime.
Fig. 43.—Mougeotia calcarea. Cells showing various modes of conjugation: at m tripartition; pg quadripartition; s quinquipartilion of the zygote.
Class 6. Chlorophyceæ (Green Algæ).
These Algæ are coloured green by chlorophyll, seldom in combination with other colouring matter, and then especially with red. The product of assimilation is frequently starch, which generally accumulates round certain specially formed portions of protoplasm termed pyrenoids. The thallus is uni- or multicellular; in the higher forms (certain Siphoneæ) the organs of vegetation attain differentiation into stem and leaf. The asexual reproduction takes place in various ways; the sexual reproduction is effected by conjugation of motile gametes, or by oogamous fertilisation. The swarm-cells (zoospores, gametes, and spermatozoids) are constructed symetrically, and have true protoplasmic cilia, these generally being attached to the front end of the swarm-cells. Most of these Algæ live in water (fresh or salt); some are found upon damp soil, stones, or tree-stems, and some live enclosed in other plants.