The formation of resting-spores of vaucheria is accomplished by means of special organs, _oögonia_ (_o_, Fig. 268) and _antheridia_ (_a_, Fig. 268). These are both specially developed branches from the thallus. The antheridia are nearly cylindrical, and curved toward the oögonia. The upper part of an antheridium is cut off by a cross wall, and within it numerous ciliated _sperm-cells_ are formed. These escape by the ruptured apex of the antheridium. The oögonia are more enlarged than the antheridia, and have a beak-like projection turned a little to one side of the apex. They are separated from the thallus thread by a cross wall, and contain a single large green cell, the _egg-cell_. The apex of the oögonium is dissolved, and through the opening the sperm-cells enter. Fertilization is thus accomplished. After fertilization the egg-cell becomes invested with a thick wall and is thus converted into a resting-spore, the _oöspore_. [Illustration: FIG. 269.—FUCUS. Fruiting branches at _s_, _s_. On the stem are two air-bladders. ] * * * * * _Fucus._—These are rather large specialized algæ belonging to the group known as brown seaweeds and found attached by a disk to the rocks of the seashore just below high tide (Fig. 269). They are firm and strong to resist wave action and are so attached as to avoid being washed ashore. They are very abundant algæ. In shape the plants are long, branched, and multicellular, with either flat or terete branches. They are olive-brown. Propagation is by the breaking off of the branches. No zoöspores are produced, as in many other seaweeds; and reproduction is wholly sexual. The _antheridia_, bearing _sperm-cells_, and the _oögonia_, each bearing eight _egg-cells_, are sunken in pits or _conceptacles_. These pits are aggregated in the swollen lighter coloured tips of some of the branches (_s_, _s_, Fig. 269). The egg-cells and sperm-cells escape from the pits and fertilization takes place in the water. The matured eggs, or spores, reproduce the fucus plant directly. * * * * * [Illustration: FIG. 270.—NITELLA.] _Nitella._—This is a large branched and specialized fresh-water alga found in tufts attached to the bottom in shallow ponds (Fig. 270). Between the whorls of branches are long _internodes consisting of a single cylindrical cell_, which is one of the largest cells known in vegetable tissue. Under the microscope the walls of this cell are found to be lined with a layer of small stationary chloroplastids, within which layer the protoplasm, in favourable circumstances, will be found in motion, moving up one side and down the other (in rotation). Note the clear streak up the side of the cell and its relation to the moving current. FUNGI