[Illustration: FIG. 19.--Forms of _Zygnemaceæ_. _A_, _Zygnema_. _B_, _C_, _D_, _Mesocarpus_. All × 150.] There are various genera of the pond scums, differing in the form of the chloroplasts and also in the position of the spores. Of these may be mentioned _Zygnema_ (Fig. 19, _A_), with two star-shaped chloroplasts in each cell, and _Mesocarpus_ (Fig. 19, _B_, _D_), in which the single chloroplast has the form of a thin median plate. (B shows the appearance from in front, _C_ from the side, showing the thickness of the plate.) _Mesocarpus_ and the allied genera have the spore formed between the filaments, the contents of both the uniting cells leaving them. [Illustration: FIG. 20.--Forms of Desmids. _A_, _B_, _Closterium_. _C_, _D_, _Dʹ_, _Cosmarium_. _D_, and _Dʹ_ show the process of division. _E_, _F_, _Staurastrum_; _E_ seen from the side, _F_ from the end.] Evidently related to the pond scums, but differing in being for the most part strictly unicellular, are the desmids (Fig. 20). They are confined to fresh water, and seldom occur in masses of sufficient size to be seen with the naked eye, usually being found associated with pond scums or other filamentous forms. Many of the most beautiful forms may be obtained by examining the matter adhering to the leaves and stems of many floating water plants, especially the bladder weed (_Utricularia_) and other fine-leaved aquatics. The desmids include the most beautiful examples of unicellular plants to be met with, the cells having extremely elegant outlines. The cell shows a division into two parts, and is often constricted in the middle, each division having a single large chloroplast of peculiar form. The central part of the cell in which the nucleus lies is colorless. Among the commonest forms, often growing with _Spirogyra_, are various species of _Closterium_ (Fig. 20, _A_, _B_), recognizable at once by their crescent shape. The cell appears bright green, except at the ends and in the middle. The large chloroplast in each half is composed of six longitudinal plates, united at the axis of the cell. Several large pyrenoids are always found, often forming a regular line through the central axis. At each end of the cell is a vacuole containing small granules that show an active dancing movement. The desmids often have the power of movement, swimming or creeping slowly over the slide as we examine them, but the mechanism of these movements is still doubtful. In their reproduction they closely resemble the pond scums. ORDER IV.--_Siphoneæ_.