The sexual reproduction takes place as follows: Certain cells of a filament become distinguished by their denser contents and by an increase in size, becoming oval or nearly globular in form (Fig. 16, _A_, _B_). When fully grown, the contents contract and form a naked cell, which sometimes shows a clear area at one point on the surface. This globular mass of protoplasm is the egg cell, or female cell, and the cell containing it is called the "oögonium." When the egg cell is ripe, the oögonium opens by means of a little pore at one side (Fig. 16, _A_). In other cells, either of the same filament or else of the small male plants already mentioned, small motile cells, called spermatozoids, are formed. These are much smaller than the egg cell, and resemble the zoöspores in form, but are much smaller, and without chlorophyll. When ripe they are discharged from the cells in which they were formed, and enter the oögonium. By careful observation the student may possibly be able to follow the spermatozoid into the oögonium, where it enters the egg cell at the clear spot on its surface. As a result of the entrance of the spermatozoid (fertilization), the egg cell becomes surrounded by a thick brown wall, and becomes a resting spore. The spore loses its green color, and the wall becomes dark colored and differentiated into several layers, the outer one often provided with spines (Fig. 16, _F_). As these spores do not germinate for a long time, the process is only known in a comparatively small number of species, and can hardly be followed by the ordinary student. [Illustration: FIG. 17.--_A_, plant of _Coleochæte_, × 50. _B_, a few cells from the margin, with one of the hairs.] Much like _Œdogonium_, but differing in being branched, is the genus _Bulbochæte_, characterized also by hairs swollen at the base, and prolonged into a delicate filament (Fig. 16, _G_). The highest members of the _Confervaceæ_ are those of the genus _Coleochæte_ (Fig. 17), of which there are several species found in the United States. These show some striking resemblances to the red seaweeds, and possibly form a transition from the green algæ to the red. The commonest species form bright-green discs, adhering firmly to the stems and floating leaves of water lilies and other aquatics. In aquaria they sometimes attach themselves in large numbers to the glass sides of the vessel. Growing from the upper surface are numerous hairs, consisting of a short, sheath-like base, including a very long and delicate filament (Fig. 17, _B_). In their methods of reproduction they resemble _Œdogonium_, but the reproductive organs are more specialized. CHAPTER V. GREEN ALGÆ--_Continued_. ORDER III.--POND SCUMS (_Conjugatæ_).