=308. Botrydium granulatum.=—An example of one of the simpler members of the Siphoneæ is Botrydium granulatum. It is found sometimes in abundance on wet ground which is colored green or red by its presence, according to the stage of development. The plant body is long pear-shaped, the smaller end attached to the ground by slender branched rhizoids (Fig. 143). The protoplasm contains many nuclei and lines the inside of the wall. When multiplication takes place large numbers of small zoospores with one cilium each are formed in the protoplasm, and escape at free end. Reproduction takes place by two-ciliated gametes, which fuse in pairs to form zygospores. In dry seasons the protoplasm in the pear-shaped plant passes down into the rhizoids and forms small rounded _planospores_. All the stages of development are too complicated to describe here. CHAPTER XVI. ŒDOGONIUM. =309.= Œdogonium is also an alga. The plant is sometimes associated with spirogyra, and occurs in similar situations. Our attention was called to it in the study of chlorophyll bodies. These we recollect are, in this plant, small oval disks, and thus differ from those in spirogyra. =310. Form of œdogonium.=—Like spirogyra, œdogonium forms simple threads which are made up of cylindrical cells placed end to end. But the plant is very different from any member of the group to which spirogyra belongs. In the first place each cell is not the equivalent of an individual plant as in spirogyra. Growth is localized or confined to certain cells of the thread which divide at one end in such a way as to leave a peculiar overlapping of the cell walls in the form of a series of shallow caps or vessels (fig. 144), and this is one of the characteristics of this genus. Other differences we find in the manner of reproduction. =311. Fruiting stage of œdogonium.=—Material in the fruiting stage is quite easily obtainable, and may be preserved for study in formalin if there is any doubt about obtaining it at the time we need it for study. This condition of the plant is easily detected because of the swollen condition of some of the cells, or by the presence of brown bodies with a thick wall in some of the cells. =312. Sexual organs of œdogonium. Oogonium and egg.=—The enlarged cell is the oogonium, the wall of the cell being the wall of the oogonium. (See fig. 145.) The protoplasm inside, before fertilization, is the egg-cell. In those cases where the brown body with a thick wall is present fertilization has taken place, and this body is the _fertilized egg_, or _oospore_. It contains large quantities of an oily substance, and, like the fertilized egg of spirogyra and vaucheria, is able to withstand greater changes in temperature than the vegetative stage, and can endure drying and freezing for some time without injury. [Illustration: Fig. 144. Portion of thread of œdogonium, showing chlorophyll grains, and peculiar cap cell walls.]