LITMIR.BIZ

      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.

      The Siphoneæ are algæ occurring both in fresh and salt water, and are distinguished from other algæ by having the form of a tube, undivided by partition walls, except when reproduction occurs. The only common representatives of the order in fresh water are those belonging to the genus Vaucheria, but these are to be had almost everywhere. They usually occur in shallow ditches and ponds, growing on the bottom, or not infrequently becoming free, and floating where the water is deeper. They form large, dark green, felted masses, and are sometimes known as “green felts.” Some species grow also on the wet ground about springs. An examination of one of the masses shows it to be made up of closely matted, hair-like threads, each of which is an individual plant.

      In transferring the plants to the slide for microscopic examination, they must be handled very carefully, as they are very easily injured. Each thread is a long tube, branching sometimes, but not divided into cells as in Spirogyra or Cladophora. If we follow it to the tip, the contents here will be found to be denser, this being the growing point. By careful focusing it is easy to show that the protoplasm is confined to a thin layer lining the wall, the central cavity of the tube being filled with cell sap. In the protoplasm are numerous elongated chloroplasts (cl.). and a larger or smaller number of small, shining, globular bodies (ol.). These latter are drops of oil, and, when the filaments are injured, sometimes run together, and form drops of large size. No nucleus can be seen in the living plant, but by treatment with chromic acid and staining, numerous very small nuclei may be demonstrated.

      Fig. 21.—A, C, successive stages in the development of the sexual organs of a green felt (Vaucheria). an. antheridium. og. oögonium. D, a ripe oögonium. E, the same after it has opened. o, the egg cell. F, a ripe spore. G, a species in which the sexual organs are borne separately on the main filament. A, F, × 150. G, × 50. cl. chloroplasts. ol. oil.

      When the filaments are growing upon the ground, or at the bottom of shallow water, the lower end is colorless, and forms a more or less branching root-like structure, fastening it to the earth. These rootlets, like the rest of the filament, are undivided by walls.

      One of the commonest and at the same time most characteristic species is Vaucheria racemosa (Fig. 21, A, F). The plant multiplies non-sexually by branches pinched off by a constriction at the point where they join the main filament, or by the filament itself becoming constricted and separating into several parts, each one constituting a new individual.

      The sexual organs are formed on special branches, and their arrangement is such as to make the species instantly recognizable.

      The first sign of their development is the formation of a short branch (Fig. 21, A) growing out at right angles to the main filament. This branch becomes club-shaped, and the end somewhat pointed and more slender, and curves over. This slender, curved portion is almost colorless, and is soon shut off from the rest of the branch. It is called an “antheridium,” and within are produced, by internal division, numerous excessively small spermatozoids.

      As the branch grows, its contents become very dense, the oil drops especially increasing in number and size. About the time that the antheridium becomes shut off, a circle of buds appears about its base (Fig. 21, B, og.). These are the young oögonia, which rapidly increase in size, assuming an oval form, and become separated by walls from the main branch (C). Unlike the antheridium, the oögonia contain a great deal of chlorophyll, appearing deep green.

      When ripe, the antheridium opens at the end and discharges the spermatozoids, which are, however, so very small as scarcely to be visible except with the strongest lenses. They are little oval bodies with two cilia, which may sometimes be rendered visible by staining with iodine.

      Fig. 22.—A, non-sexual reproduction in Vaucheria sessilis. B, non-sexual spore of V. geminata, × 50.

      The oögonia, which at first are uniformly colored, just before maturity show a colorless space at the top, from which the chloroplasts and oil drops have disappeared (D), and at the same time this portion pushes out in the form of a short beak. Soon after the wall is absorbed at this point, and a portion of the contents is forced out, leaving an opening, and at the same time the remaining contents contract to form a round mass, the germ or egg cell (Fig. 21, E, o). Almost as soon as the oögonium opens, the spermatozoids collect about it and enter; but, on account of their minuteness, it is almost impossible to follow them into the egg cell, or to determine whether several or only one enter. The fertilized egg cell becomes almost at once surrounded by a wall, which rapidly thickens, and forms a resting spore. As the spore ripens, it loses its green color, becoming colorless, with a few reddish brown specks scattered through it (F).

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