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).
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.
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.