Fig. 267.—Strand, or Filament of Zygnema, freed from its gelatinous covering.
Vaucheria is another alga common in shallow water and on damp soil. The thallus is much branched, but the threads are not divided by cross walls as in spirogyra. The plants are attached by means of colourless root-like organs which are much like the root-hairs of the higher plants: these are rhizoids. The chlorophyll is in the form of grains scattered through the thread.
Vaucheria has a special mode of asexual reproduction by means of swimming spores or swarm-spores. These are formed singly in a short enlarged lateral branch known as the sporangium. When the sporangium bursts, the entire contents escape, forming a single large swarm-spore, which swims about by means of numerous lashes or cilia on its surface. The swarm-spores are so large that they can be seen with the naked eye. After swimming about for some time they come to rest and germinate, producing a new plant.
Fig. 268.—Thread of Vaucheria with Oögonia and Antheridia.
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.
Fig. 269.—Fucus. Fruiting branches at s, s. On the stem are two air-bladders.