Most of the diatoms show movements, swimming slowly or gliding over solid substances; but like the movements of _Oscillaria_ and the desmids, the movements are not satisfactorily understood, although several explanations have been offered. They resemble somewhat the desmids in their reproduction. THE TRUE BROWN ALGÆ. These are all marine forms, many of great size, reaching a length in some cases of a hundred metres or more, and showing a good deal of differentiation in their tissues and organs. [Illustration: FIG. 26.--_A_, a branch of common rock weed (_Fucus_), one-half natural size. _x_, end of a branch bearing conceptacles. _B_, section through a conceptacle containing oögonia (_og._), × 25. _C_, _E_, successive stages in the development of the oögonium, × 150. _F_, _G_, antheridia. In _G_, one of the antheridia has discharged the mass of spermatozoids (_an._), × 150.] One of the commonest forms is the ordinary rock weed (_Fucus_), which covers the rocks of our northeastern coast with a heavy drapery for several feet above low-water mark, so that the plants are completely exposed as the tide recedes. The commonest species, _F. vesiculosus_ (Fig. 26, _A_), is distinguished by the air sacs with which the stems are provided. The plant is attached to the rock by means of a sort of disc or root from which springs a stem of tough, leathery texture, and forking regularly at intervals, so that the ultimate branches are very numerous, and the plant may reach a length of a metre or more. The branches are flattened and leaf-like, the centre traversed by a thickened midrib. The end of the growing branches is occupied by a transversely elongated pit or depression. The growing point is at the bottom of this pit, and by a regular forking of the growing point the symmetrical branching of the plant is brought about. Scattered over the surface are little circular pits through whose openings protrude bunches of fine hairs. When wet the plant is flexible and leathery, but it may become quite dry and hard without suffering, as may be seen when the plants are exposed to the sun at low tide. The air bladders are placed in pairs, for the most part, and buoy up the plant, bringing it up to the surface when covered with water. The interior of the plant is very soft and gelatinous, while the outer part forms a sort of tough rind of much firmer consistence. The ends of some of the branches (Fig. 26, _A_, _x_) are usually much swollen, and the surface covered with little elevations from which may often be seen protruding clusters of hairs like those arising from the other parts of the plant. A section through one of these enlarged ends shows that each elevation corresponds to a cavity situated below it. On some of the plants these cavities are filled with an orange-yellow mass; in others there are a number of roundish olive-brown bodies large enough to be easily seen. The yellow masses are masses of antheridia; the round bodies, the oögonia.