Among the marine forms related to these may be mentioned the sea lettuce (_Ulva_), shown in Figure 15. The thin, bright-green, leaf-like fronds of this plant are familiar to every seaside student. [Illustration: FIG. 15.--A plant of sea lettuce (_Ulva_). One-half natural size.] Somewhat higher than _Cladophora_ and its allies, especially in the differentiation of the reproductive parts, are the various species of _Œdogonium_ and its relatives. There are numerous species of _Œdogonium_ not uncommon in stagnant water growing in company with other algæ, but seldom forming masses by themselves of sufficient size to be recognizable to the naked eye. The plant is in structure much like _Cladophora_, except that it is unbranched, and the cells have but a single nucleus (Fig. 16, _E_). Even when not fruiting the filaments may usually be recognized by peculiar cap-shaped structures at the top of some of the cells. These arise as the result of certain peculiarities in the process of cell division, which are too complicated to be explained here. There are two forms of reproduction, non-sexual and sexual. In the first the contents of certain cells escape in the form of large zoöspores (Fig. 16, _C_), of oval form, having the smaller end colorless and surrounded by a crown of cilia. After a short period of active motion, the zoöspore comes to rest, secretes a cell wall about itself, and the transparent end becomes flattened out into a disc (_E_, _d_), by which it fastens itself to some object in the water. The upper part now rapidly elongates, and dividing repeatedly by cross walls, develops into a filament like the original one. In many species special zoöspores are formed, smaller than the ordinary ones, that attach themselves to the filaments bearing the female reproductive organ (oögonium), and grow into small plants bearing the male organ (antheridium), (Fig. 16, _B_). [Illustration: FIG. 16.--_A_, portion of a filament of _Œdogonium_, with two oögonia (_og._). The lower one shows the opening. _B_, a similar filament, to which is attached a small male plant with an antheridium (_an._). _C_, a zoöspore of _Œdogonium_. _D_, a similar spore germinating. _E_, base of a filament showing the disc (_d_) by which it is attached. _F_, another species of _Œdogonium_ with a ripe spore (_sp._). _G_, part of a plant of _Bulbochæte_. _C_, _D_, × 300; the others × 150.]