Fig. 156.
Thallus of Coleochæte scutata.
323. The shield-shaped coleochæte.—This plant (C. scutata) is in the form of a flattened, circular, green plate, as shown in [fig. 156]. It is attached near the center on one side to rushes and other plants, and has been found quite abundantly for several years in the waters of Cayuga Lake at its southern extremity. As will be seen it consists of a single layer of green cells which radiate from the center in branched rows to the outside, the cells lying so close together as to form a continuous plate. The plant started its growth from a single cell at the central point, and grew at the margin in all directions. Sometimes they are quite irregular in outline, when they lie quite closely side by side and interfere with one another by pressure. If the surface is examined carefully there will be found long hairs, the base of which is enclosed in a narrow sheath. It is from this character that the genus takes its name of coleochæte (sheathed hair).
Fig. 157.
Portion of thallus of Coleochæte scutata,
showing empty cells from which zoogonidia
have escaped, one from each cell;
zoogonidia at the left. (After Pringsheim.)
Fig. 158.
Portion of thallus of Coleochæte
scutata, showing four antheridia
formed from one thallus cell; a
single spermatozoid at the right.
(After Pringsheim.)
324. Fruiting stage of coleochæte.—It is possible at some seasons of the year to find rounded masses of cells situated near the margin of this green disk. These have developed from a fertilized egg which remained attached to the plant, and probably by this time the parent plant has lost its color.
325. Zoospore stage.—This mass of tissue does not develop directly into the circular green disk, but each of the cells forms a zoospore. Here then, as in œdogonium, we have another stage of the plant interpolated between the fertilized egg and that stage of the plant which bears the gametes. But in coleochæte we have a distinct advance in this stage upon what is present in œdogonium, for in coleochæte the fertilized egg develops first into a several-celled mass of tissue before the zoospores are formed, while in œdogonium only four zoospores are formed directly from the egg.
326. Asexual reproduction.—In asexual reproduction any of the green cells on the plant may form zoogonida. The contents of a cell round off and form a single zoogonidium which has two cilia at the smaller end of the oval body, [fig. 157]. After swimming around for a time they come to rest, germinate, and produce another plant.
327. Sexual reproduction.—Oogonium.—The oogonium is formed by the enlargement of a cell at the end of one of the threads, and then the end of the cell elongates into a slender tube which opens at the end to form a channel through which the spermatozoid may pass down to the egg. The egg is formed of the contents of the cell ([fig. 159]). Several oogonia are formed on one plant, and in such a plant as C. scutata they are formed in a ring near the margin of the disk.