There are two families of the Characeæ, the Chareæ, of which Chara is the type, and the Nitelleæ, represented by various species of Nitella and Tolypella. The second family have the internodes without any cortex—that is, consisting of a single long cell; and the crown at the top of the oögonium is composed of ten cells instead of five. They are also destitute of the limy coating of the Chareæ.
Both as regards the structure of the plant itself, as well as the reproductive organs, especially the very complex antheridium, the Characeæ are very widely separated from any other group of plants, either above or below them.
CHAPTER VI.
THE BROWN ALGÆ (Phæophyceæ).
Fig. 24.—Forms of diatoms. A, Pinnularia. i, seen from above; ii, from the side. B, Fragillaria (?). C, Navicula. D, F, Eunotia. E, Gomphonema. G, Cocconeis. H, Diatoma. All × 300.
These plants are all characterized by the presence of a brown pigment, in addition to the chlorophyll, which almost entirely conceals the latter, giving the plants a brownish color, ranging from a light yellowish brown to nearly black. One order of plants that possibly belongs here (Diatomaceæ) are single celled, but the others are for the most part large seaweeds. The diatoms, which are placed in this class simply on account of the color, are probably not closely related to the other brown algæ, but just where they should be placed is difficult to say. In some respects they approach quite closely the desmids, and are not infrequently regarded as related to them. They are among the commonest of organisms occurring everywhere in stagnant and running water, both fresh and salt, forming usually, slimy, yellowish coatings on stones, mud, aquatic plants, etc. Like the desmids they may be single or united into filaments, and not infrequently are attached by means of a delicate gelatinous stalk ([Fig. 25]).
