Fig. 284.—Confervaceæ.
1. Volvox globator; 2. A section of volvox, showing the flagellate margin of the cell; 3. A portion more highly magnified, to show the young volvocina, with their nuclei and thread-like attachments; 4. Spirogyra, near which are spores in different stages of development; 5. Conferva floccosa; 6. Stigeoclonium protensum, jointed filaments and single zoospores; 7. Staurocarpus gracilis, conjugating filaments and spores.
Confervaceæ are a genus of algals. The species consist of unbranched filaments composed of cylindrical or moniliform cells, with starch granules. Many are vesicular, and all multiply by zoospores generated in the interior of the plant at the expense of the granular matter. They are, for the most part, found in fresh water attached or floating, some in salt water, and a few in both, in colour usually green, but occasionally olive, violet, and red. The Confervaceæ proper are often divided into four families: 1. Hydrodictidæ; 2. Zygnemidæ; 3. Confervidæ; 4. Chætophoridæ. To the microscopist all the plants of this genera are extremely interesting as subjects for the study of cell multiplication. The process usually takes place in the terminal cell, the first step in which is the division of the endochrome, and then follows a sort of hour-glass contraction across the cavity of the parent cell, whereby it is divided into two equal parts. This is better seen in some of the desmids than in [Fig. 284], Nos. 4, 5, and 6. Some species are characterised by a different mode of reproduction; these possess a number of nuclei, and multiply by zoospores of two kinds, the largest of which have either two or four cilia, which germinate directly the smaller are biciliated; conjugation has been seen to take place in a few instances.
Allied to the Confervaceæ is an interesting plant, Sphæroplea annulina, which has received careful attention from Cohn. The oospores of this plant are the product of a process partaking of a sexual nature, and when mature are filled with reddish fat vesicles which divide by segmentation.
The Ædogoniaceæ also closely resemble Confervaceæ in habits of life, but differ in some particulars, especially so in the mode of reproduction (only a single large zoospore being set free from each cell) and by the almost complete fission of the cell-wall or one of the rings which serve as a hinge. The zoospores are the largest known among algals, and each is described as having a red eye-spot. The Chætophoraceæ form an interesting group of confervoid plants, and are usually found in running streams, as they prefer pure water. One of the characteristics of the group is that the extremities of the branches are prolonged into an acute-shaped termination, as represented in [Fig. 284], No. 6. A very pretty object under the microscope is Draparnaldia glomerata, belonging to this species. It consists of an axis composed of a row of cells, and at regular intervals whorls of slender prolongations, containing chlorophyll or endochrome of a deeper green; these attain to an extraordinary length.
The Batrachospermæ bear a strong resemblance to frog-spawn, from which they derive their name, and are chiefly a marine group of algals allied to the Rhodespermeæ or red seaweeds. The late Dr. A. Hassall first described them; they have since received more careful attention from M. Sirodot. They are reddish-green, extremely flexible, and nothing can surpass the grace of their movements in water; but when removed from their element they lose all form, and resemble a jelly-like substance without a trace of organisation; but if allowed to remain quiet they regain their original shape.
The presence of the cell-membrane will be best demonstrated by breaking up the filaments, either by moving the thin glass cover, or by cutting through a mass of them in all directions with a fine dissecting knife. On now examining the slide, in most instances many detached empty pieces of the cell-membrane, with its striæ, will be seen, as well as filaments partly deprived of protoplasm. On the application of iodine all these appearances become more distinguishable in consequence of the filament turning red or brown, while the empty cells remain either unaffected, or present a slight yellowish tint, as is frequently the case with cellulose when old.
Fig. 285.—Mesoglia vermicularis.
With regard to the contents of the cell, the endochrome is coloured in the Oscillatoriæ, and is distinguishable by circular bands or rings around the axis of the cylindrical filament. Iodine stains them brown or red, and syrup and dilute sulphuric acid produce a beautiful rose colour. As to their mode of propagation, nothing positive is known. If kept for some time they gradually lose their green colour; a portion of the mass, becoming brown, sinks to the bottom of the vessel, and presents a granular layer.