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_). [Illustration: 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). [Illustration: FIG. 25.--Diatoms attached by a gelatinous stalk. × 150] They are at once distinguished from the desmids by their color, which is always some shade of yellowish or reddish brown. The commonest forms, _e.g._ _Navicula_ (Fig. 24, _C_), are boat-shaped when seen from above, but there is great variety in this respect. The cell wall is always impregnated with large amounts of flint, so that after the cell dies its shape is perfectly preserved, the flint making a perfect cast of it, looking like glass. These flinty shells exhibit wonderfully beautiful and delicate markings which are sometimes so fine as to test the best lenses to make them out. This shell is composed of two parts, one shutting over the other like a pill box and its cover. This arrangement is best seen in such large forms as _Pinnularia_ (Fig. 24, _A_ ii).