Fig. 21.—A, C, successive stages in the development of the sexual organs of a green felt (Vaucheria). an. antheridium. og. oögonium. D, a ripe oögonium. E, the same after it has opened. o, the egg cell. F, a ripe spore. G, a species in which the sexual organs are borne separately on the main filament. A, F, × 150. G, × 50. cl. chloroplasts. ol. oil.
When the filaments are growing upon the ground, or at the bottom of shallow water, the lower end is colorless, and forms a more or less branching root-like structure, fastening it to the earth. These rootlets, like the rest of the filament, are undivided by walls.
One of the commonest and at the same time most characteristic species is Vaucheria racemosa ([Fig. 21], A, F). The plant multiplies non-sexually by branches pinched off by a constriction at the point where they join the main filament, or by the filament itself becoming constricted and separating into several parts, each one constituting a new individual.
The sexual organs are formed on special branches, and their arrangement is such as to make the species instantly recognizable.
The first sign of their development is the formation of a short branch ([Fig. 21], A) growing out at right angles to the main filament. This branch becomes club-shaped, and the end somewhat pointed and more slender, and curves over. This slender, curved portion is almost colorless, and is soon shut off from the rest of the branch. It is called an “antheridium,” and within are produced, by internal division, numerous excessively small spermatozoids.
As the branch grows, its contents become very dense, the oil drops especially increasing in number and size. About the time that the antheridium becomes shut off, a circle of buds appears about its base ([Fig. 21], B, og.). These are the young oögonia, which rapidly increase in size, assuming an oval form, and become separated by walls from the main branch (C). Unlike the antheridium, the oögonia contain a great deal of chlorophyll, appearing deep green.
When ripe, the antheridium opens at the end and discharges the spermatozoids, which are, however, so very small as scarcely to be visible except with the strongest lenses. They are little oval bodies with two cilia, which may sometimes be rendered visible by staining with iodine.
Fig. 22.—A, non-sexual reproduction in Vaucheria sessilis. B, non-sexual spore of V. geminata, × 50.
The oögonia, which at first are uniformly colored, just before maturity show a colorless space at the top, from which the chloroplasts and oil drops have disappeared (D), and at the same time this portion pushes out in the form of a short beak. Soon after the wall is absorbed at this point, and a portion of the contents is forced out, leaving an opening, and at the same time the remaining contents contract to form a round mass, the germ or egg cell ([Fig. 21], E, o). Almost as soon as the oögonium opens, the spermatozoids collect about it and enter; but, on account of their minuteness, it is almost impossible to follow them into the egg cell, or to determine whether several or only one enter. The fertilized egg cell becomes almost at once surrounded by a wall, which rapidly thickens, and forms a resting spore. As the spore ripens, it loses its green color, becoming colorless, with a few reddish brown specks scattered through it (F).
In some species the sexual organs are borne directly on the filament ([Fig. 21], G).
Large zoöspores are formed in some of the green felts ([Fig. 22], A), and are produced singly in the ends of branches that become swollen, dark green, and filled with very dense protoplasm. This end becomes separated by a wall from the rest of the branch, the end opens, and the contents escape as a very large zoöspore, covered with numerous short cilia (A ii). After a short period of activity, this loses its cilia, develops a wall, and begins to grow (III, IV). Other species (B) produce similar spores, which, however, are not motile, and remain within the mother cell until they are set free by the decay of its wall.
Order V.—Characeæ.
The Characeæ, or stone-worts, as some of them are called, are so very different from the other green algæ that it is highly probable that they should be separated from them.
The type of the order is the genus Chara ([Fig. 23]), called stone-worts from the coating of carbonate of lime found in most of them, giving them a harsh, stony texture. Several species are common growing upon the bottom of ponds and slow streams, and range in size from a few centimetres to a metre or more in height.