Elements of Structural and Systematic Botany. Douglas Houghton Campbell
href="#ulink_ab38cf38-7e06-54d6-b6a6-34c900159d08">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.
Fig. 17.—A, plant of Coleochæte, × 50. B, a few cells from the margin, with one of the hairs.
Much like Œdogonium, but differing in being branched, is the genus Bulbochæte, characterized also by hairs swollen at the base, and prolonged into a delicate filament (Fig. 16, G).
The highest members of the Confervaceæ are those of the genus Coleochæte (Fig. 17), of which there are several species found in the United States. These show some striking resemblances to the red seaweeds, and possibly form a transition from the green algæ to the red. The commonest species form bright-green discs, adhering firmly to the stems and floating leaves of water lilies and other aquatics. In aquaria they sometimes attach themselves in large numbers to the glass sides of the vessel.
Growing from the upper surface are numerous hairs, consisting of a short, sheath-like base, including a very long and delicate filament (Fig. 17, B). In their methods of reproduction they resemble Œdogonium, but the reproductive organs are more specialized.
CHAPTER V.
Green Algæ—Continued.
Order III.—Pond Scums (Conjugatæ).
The Conjugatæ, while in some respects approaching the Confervaceæ in structure, yet differ from them to such an extent in some respects that their close relationship is doubtful. They are very common and familiar plants, some of them forming great floating masses upon the surface of every stagnant pond and ditch, being commonly known as “pond scum.” The commonest of these pond scums belong to the genus Spirogyra, and one of these will illustrate the characteristics of the order. When in active growth these masses are of a vivid green, and owing to the presence of a gelatinous coating feel slimy, slipping through the hands when one attempts to lift them from the water. Spread out in water, the masses are seen to be composed of slender threads, often many centimetres in length, and showing no sign of branching.
Fig. 18.—A, a filament of a common pond scum (Spirogyra) separating into two parts. B, a cell undergoing division. The cell is seen in optical section, and the chlorophyll bands are omitted, n, nʹ, the two nuclei. C, a complete cell. n, nucleus. py. pyrenoid. D, E, successive stages in the process of conjugation. G, a ripe spore. H, a form in which conjugation takes place between the cells of the same filament. All × 150.
For microscopical examination the larger species are preferable. When one of these is magnified (Fig. 18, A, C), the unbranched filament is shown to be made up of perfectly cylindrical cells, with rather delicate walls. The protoplasm is confined to a thin layer lining the walls, except for numerous fine filaments that radiate from the centrally placed nucleus (n), which thus appears suspended in the middle of the cell. The nucleus is large and distinct in the larger species, and has a noticeably large and conspicuous nucleolus. The most noticeable thing about the cell is the green spiral bands running around it. These are the chloroplasts, which in all the Conjugatæ are of very peculiar forms. The number of these bands varies much in different species of Spirogyra, but is commonly two or three. These chloroplasts, like those of other plants, are not noticeably different in structure from the ordinary protoplasm, as is shown by extracting the chlorophyll, which may be done by placing the plants in alcohol for a short time. This extracts the chlorophyll, but a microscopic examination of the decolored cells shows that the bands remain unchanged, except for the absence of color. These bands are flattened, with irregularly scalloped margins, and at intervals have rounded bodies (pyrenoids) imbedded in them (Fig. 18, C, py.). The pyrenoids, especially when the plant has been exposed to the light for some time, are surrounded by a circle of small granules, which become bluish when iodine is applied, showing them to be starch. (To show the effect of iodine on starch on a large scale, mix a little flour, which is nearly all starch, with water, and add a little iodine. The starch will immediately become colored blue, varying in intensity with the amount of iodine.) The cells divide much as in Cladophora, but the nucleus here takes part in the process. The division naturally occurs only at night, but by reducing the temperature at night to near the freezing point (4° C., or a little lower), the process may be checked. The experiment is most conveniently made when the temperature out of doors approaches the freezing point. Then it is only necessary to keep the plants in a warm room until about 10 p.m., when they may be put out of doors for the night. On bringing them in in the morning, the division will begin almost at once, and may be easily studied. The nucleus divides into two parts, which remain for a time connected by delicate threads (Fig. 18, B), that finally disappear. At first no nucleoli are present in the daughter nuclei, but they appear before the division is complete.
New filaments are formed by the breaking up of the old ones, this sometimes being very rapid. As the cells break apart, the free ends bulge strongly, showing the pressure exerted upon the cell wall by the contents (Fig. 18, A).
Spores like those of Œdogonium are formed, but the process is somewhat different. It occurs in most species late in the spring, but may sometimes be met with at other times. The masses of fruiting plants usually appear brownish colored. If spores have been formed they can, in the larger species at least, be seen with a hand lens, appearing as rows of dark-colored specks.
Two filaments lying side by side send out protuberances of the cell wall that grow toward each other until they touch (Fig. 18, D). At the point of contact, the wall is absorbed, forming a continuous channel from one cell to the other. This process usually takes place in all the cells of the two filaments, so that the two filaments, connected by tubes at regular intervals, have the form of a ladder.
In some species adjoining cells of the same filament become connected, the tubes being formed at the end of the cells (Fig. 18, H), and the cell in which the spore is formed enlarges.
Soon after the channel is completed, the contents of one cell flow slowly through it into the neighboring cell, and the protoplasm of the two fuses into one mass. (The union of the nuclei has also been observed.) The young spore thus formed contracts somewhat, becoming oval in form, and soon secretes a thick wall, colorless at first, but afterwards becoming brown and more or less opaque. The chlorophyll bands, although much crowded, are at first distinguishable, but later lose the chlorophyll, and become unrecognizable. Like the resting spores of Œdogonium these require a long period of rest before germinating.
Fig. 19.—Forms of Zygnemaceæ. A, Zygnema. B, C, D, Mesocarpus. All × 150.
There are various genera of the pond