LITMIR.BIZ

Скачать книгу

sometimes a metre or more in length. By floating out a little of it in a saucer, it is easy to see that it is made up of branching filaments.

      The microscope shows (Fig. 13, A) that these filaments are rows of cylindrical cells with thick walls showing evident stratification. At intervals branches are given off, which may in turn branch, giving rise to a complicated branching system. These branches begin as little protuberances of the cell wall at the top of the cell. They increase rapidly in length, and becoming slightly contracted at the base, a wall is formed across at this point, shutting it off from the mother cell.

      The protoplasm lines the wall of the cell, and extends in the form of thin plates across the cavity of the cell, dividing it up into a number of irregular chambers. Imbedded in the protoplasm are numerous flattened chloroplasts, which are so close together as to make the protoplasm appear almost uniformly green. Within the chloroplasts are globular, glistening bodies, called “pyrenoids.” The cell has several nuclei, but they are scarcely evident in the living cell. By placing the cells for a few hours in a one per cent watery solution of chromic acid, then washing thoroughly and staining with borax carmine, the nuclei will be made very evident (Fig. 13, B). Such preparations may be kept permanently in dilute glycerine.

      Fig. 13.—Cladophora. A, a fragment of a plant, × 50. B, a single cell treated with chromic acid, and stained with alum cochineal. n, nucleus. py. pyrenoid, × 150. C, three stages in the division of a cell. i, 1.45 p.m.; ii, 2.55 p.m.; iii, 4.15 p.m., × 150. D, a zoöspore × 350.

      If a mass of actively growing filaments is examined, some of the cells will probably be found in process of fission. The process is very simple, and may be easily followed (Fig. 13, C). A ridge of cellulose is formed around the cell wall, projecting inward, and pushing in the protoplasm as it grows. The process is continued until the ring closes in the middle, cutting the protoplasmic body completely in two, and forms a firm membrane across the middle of the cell. The protoplasm at this stage (C iii.) is somewhat contracted, but soon becomes closely applied to the new wall. The whole process lasts, at ordinary temperatures (20°-25° C.), from three to four hours.

      At certain times, but unfortunately not often to be met with, the contents of some of the cells form, by internal division, a large number of small, naked cells (zoöspores) (Fig. 13, D), which escape and swim about actively for a time, and afterwards become invested with a cell wall, and grow into a new filament. These cells are called zoöspores, from their animal-like movements. They are provided with two cilia, closely resembling the motile cells of the Protococcaceæ and Volvocineæ.

      There are very many examples of these simple Confervaceæ, some like Conferva being simple rows of cells, others like Stigeoclonium (Fig. 14, A), Chætophora and Draparnaldia (Fig. 14, B, C), very much branched. The two latter forms are surrounded by masses of transparent jelly, which sometimes reach a length of several centimetres.

      Fig. 14.—Confervaceæ. A, Stigeoclonium. B, Draparnaldia, × 50. C, a piece of Draparnaldia, × 2. D, part of a filament of Conferva, × 300.

      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.

      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

Скачать книгу