Their reproduction is effected by division and conjugation. In the process of division the cells gradually separate at the suture, and a new half-cell is formed upon each old half, which grows until it attains the size and form of the original half of the parent-cell. The conjugation is effected by two cells approximating so that their sutures are near together, the cells then open at the sutures, and the effused contents become united to form a spore or sporange, from which one or more individuals are formed. These spores are often elegantly spinous on the surface.
Among the species selected for illustration is Clostérium acerósum ([Pl. V.] fig. 9), in which the cells are single, elongate, very slightly curved or lunate; the endochrome forms long bands, often containing numerous globules or transparent vesicles. At each end of the cells is a round transparent vesicle, containing exceedingly minute granules, which exhibit a trembling kind of motion. Between the cell-wall and the cell-contents very fine currents may also be detected, forming a circulation resembling that in the hairs of Tradescantia.
In Micrastérias rotáta ([Pl. V.] fig. 16) the cells, which are single, are deeply cleft into two segments at the suture, the segments being again regularly cut into five lobes, which are toothed or dentate.
In Hyalothéca dissil´iens ([Pl. V.] fig. 21) the cells are united into a cylindrical filament, and are surrounded by a very delicate gelatinous sheath. In Ankistrodes´mus falcátus ([Pl. V.] fig. 19) the cells resemble those of Closterium in shape, but are aggregated into faggot-like bundles, and are very much smaller. In the beautiful little Pedias´trum boryánum ([Pl. V.] fig. 20) the cells are aggregated into a disk, the marginal cells being bidentate or having each two points, so that the whole resembles a star. The species of Pediastrum are reproduced by the contents of each cell subdividing into numerous ciliated segments or zoospores, which subsequently escape in a mass from the cell, ultimately losing their cilia, and reuniting to form a new individual.
In Scenedes´mus quadricau´da ([Pl. V.] fig. 25) the oblong cells are united, side by side, the outermost cells being furnished with a bristle at each end. The division of these cells takes place obliquely, so that in the divided groups the cells are situated in two alternate rows.
The spores of many of the Desmidiaceæ are spinous, and they are often found fossil in flint ([Pl. V.] fig. 15). To detect them in this substance, thin slips of flint may be examined under a half-inch power; or the chips of flint may be cemented to a slide with balsam, and ground down on a hone.
The Desmidiaceæ must be mounted in the moist state: the smaller ones will keep well in chloride of calcium; but the larger ones are injured both by that liquid and by glycerine. The remarks made upon mounting, at page 15, are especially applicable to these delicate organisms.
Diatoma´ceæ, or Siliceous Algæ.—The members of this family are singly very minute; but when existing in large numbers, as they are often found at the bottom of ditches and ponds, on the submerged stems of water-plants, or upon damp ground, they form yellowish-brown evident masses or strata. They occur both in sea-and in fresh water. They usually consist, like the Desmidiaceæ, of single cells, which are called frustules. But they are especially characterized by the cell-walls being imbued with silica or flint, so that if the frustules be heated to redness upon the point of a knife or a slip of platinum-foil, which destroys the organic part of the cells, the coat of silica remains, exhibiting the perfect form of the original cells or frustules. The form of the frustules is very different in the various genera and species, as represented in [Pl. V.] figs. 22, 23, 27, 30, 31, and [Pl. VI.] figs. 16, 17, 23; and it will be noticed that, in the figures, two views are given of each frustule, f indicating the front view, and s the side view. In all the front views, as in [Pl. V.] fig. 22, one or more lines will be observed running longitudinally down the middle of the frustules, and corresponding to the indications of division existing in the cells of the Desmidiaceæ. Each half of a frustule is called a valve, and the line at which these valves meet is called the suture. That side or aspect of the frustule in which the suture lies (fig. 22 f) is the front view; and the other aspect of the frustule (fig. 22 s) is the side view. The frustules are mostly four-sided—the main breadths of the two opposite valves forming two sides, and the bent margins of the valves, with the back and front of the hoop, forming the two other sides; so that the view presented by the side of a frustule is the same as that of a single valve. The suture is the line at which the division of the frustules takes place in the formation of new individuals. In this process the cell-contents divide into two parts, as in ordinary endogenous cell-formation,—the two new surfaces thus produced becoming coated with a new portion of cell-wall or valve, so that two frustules now occupy the place of the original one. At the same time a siliceous band, encircling the frustules at the line of suture, is formed to fill up the interval between the edges of the parent valves; this is the hoop ([Pl. V.] fig. 22 f; [Pl. VI.] fig. 10 f), and beneath it lie the two newly formed valves. In many cases I believe that each half-frustule becomes coated with a new entire cell-wall, with its siliceous valves.
The frustules of the Diatomaceæ are constantly undergoing division when in vigorous growth. After the frustules have divided, the new ones either separate entirely, as is perhaps most commonly the case; or they remain united, sometimes completely, so as to constitute a filament (Pl. V. fig. 23), while at others the frustules cohere only at the angles (Pl VI. fig. 23), so as to form a zigzag chain.
In some species, the frustules are attached to foreign bodies by means of a gelatinous cushion ([Pl. V.] fig. 5; [Pl. VI.] fig. 7); while in others they are situated upon a simple or branched gelatinous stalk ([Pl. V.] fig. 17) or stipes (stipes, a stem).