The _Conjugatæ_, while in some respects approaching the _Confervaceæ_ in structure, yet differ from them to such an extent in some respects that their close relationship is doubtful. They are very common and familiar plants, some of them forming great floating masses upon the surface of every stagnant pond and ditch, being commonly known as "pond scum." The commonest of these pond scums belong to the genus _Spirogyra_, and one of these will illustrate the characteristics of the order. When in active growth these masses are of a vivid green, and owing to the presence of a gelatinous coating feel slimy, slipping through the hands when one attempts to lift them from the water. Spread out in water, the masses are seen to be composed of slender threads, often many centimetres in length, and showing no sign of branching. [Illustration: FIG. 18.--_A_, a filament of a common pond scum (_Spirogyra_) separating into two parts. _B_, a cell undergoing division. The cell is seen in optical section, and the chlorophyll bands are omitted, _n_, _nʹ_, the two nuclei. _C_, a complete cell. _n_, nucleus. _py._ pyrenoid. _D_, _E_, successive stages in the process of conjugation. _G_, a ripe spore. _H_, a form in which conjugation takes place between the cells of the same filament. All × 150.] For microscopical examination the larger species are preferable. When one of these is magnified (Fig. 18, _A_, _C_), the unbranched filament is shown to be made up of perfectly cylindrical cells, with rather delicate walls. The protoplasm is confined to a thin layer lining the walls, except for numerous fine filaments that radiate from the centrally placed nucleus (_n_), which thus appears suspended in the middle of the cell. The nucleus is large and distinct in the larger species, and has a noticeably large and conspicuous nucleolus. The most noticeable thing about the cell is the green spiral bands running around it. These are the chloroplasts, which in all the _Conjugatæ_ are of very peculiar forms. The number of these bands varies much in different species of _Spirogyra_, but is commonly two or three. These chloroplasts, like those of other plants, are not noticeably different in structure from the ordinary protoplasm, as is shown by extracting the chlorophyll, which may be done by placing the plants in alcohol for a short time. This extracts the chlorophyll, but a microscopic examination of the decolored cells shows that the bands remain unchanged, except for the absence of color. These bands are flattened, with irregularly scalloped margins, and at intervals have rounded bodies (pyrenoids) imbedded in them (Fig. 18, _C_, _py._). The pyrenoids, especially when the plant has been exposed to the light for some time, are surrounded by a circle of small granules, which become bluish when iodine is applied, showing them to be starch. (To show the effect of iodine on starch on a large scale,