Conferva´ceæ.—On removing some of the soft green matter found adhering to the stems of water-plants in any pool or pond, one of the species of Conferva, C. flocculosa, is almost sure to be met with. On close inspection with the naked eye, the green filaments of which it consists are just visible, as extremely fine, soft, silky threads; and, under a high power of the microscope, the filaments are seen to be unbranched, and composed of a single row of cells ([Pl. V.] fig. 1), or joints, as they are called in technical works; these are 2 or 3 times as long as broad. In some specimens the joints are swollen, so as to present a rounded outline. In another common species, C. bombyc´ina, the filaments are somewhat more slender, and the joints are from 3 to 5 times as long as broad.
Cladoph´ora crispáta.—This Confervoid forms large, entangled, dull-green masses, composed of branched, tufted, somewhat rigid and coarse filaments. It is often a troublesome overrunner of the fresh-water vivarium. The filaments are composed of thick-walled cells ([Pl. V.] fig. 6), from 4 to 6 times as long as broad, and often containing minute starch-granules.
The Confervaceæ have two modes of reproduction. The first of these consists in the division of the endochrome of the joints into a number of distinct segments, each of which becomes furnished at one end with two very slender cilia ([Pl. V.] fig. 6 a). After a time, these ciliated bodies, which are called zo´ospores (ξῶον, animal, σπορἀ, seed) or gonid´ia (λονἠ, seed, εἶδος, resemblance), escape from the cells either by their rupture or through a papillary orifice, and swim about in the water, ultimately losing their cilia and growing into cells resembling those of the parent plant. In the second method, which occurs, for instance, in Conferva bombycina, certain of the joints enlarge so as to become rounded or inflated; their endochrome then becomes coated with a new cell-wall, and so forms a spore, which subsequently escapes from the cell and germinates.
Chætophora´ceæ.—Draparnal´dia glomeráta forms small green jelly-like masses, adhering to sticks and stones in water. These consist of branched filaments ([Pl. V.] fig. 10), prolonged at the ends into colourless hair-like points, and composed of single rows of cells, the green endochrome forming a band across the middle of each cell, the ends being colourless.
In Coleochæ´te scutáta ([Pl. V.] fig. 14) the cells are closely united, so as to form a minute flat green disk. In the natural state, this beautiful little object adheres to the submerged leaves and stems of water-plants, and is therefore difficult to be found. But if a few healthy water-plants be kept for some time in a glass jar, the little Coleochæte, which is about as large as a pin’s head, will often be found adhering to the side of the glass.
Bat´rachosper´meæ.—The members of this family resemble to the naked eye the little masses of Draparnaldia, and they are found in the same localities. They are of various colours, being green, brown, purple, or red. They consist, as in Bat´rachosper´mum monilifor´me ([Pl. V.] fig. 8), of branched filaments, which have a knotty appearance under a low power. The larger filaments are composed of cells arranged end to end, the knots consisting of numerous smaller whorled filaments, i. e. filaments arising from around them at the same level (fig. 7). The cells composing the whorled filaments are beaded or moniliform, and are prolonged into colourless hair-like points. The globules seen among the branches (fig. 7) consist of groups of spores.
Zygnema´ceæ.—The members of this family resemble the Confervaceæ in consisting of simple cellular filaments ([Pl. V.] figs. 11, 13), but differ from them in the elegant arrangement of the endochrome: this forms beautiful spiral bands, as in Spirog´yra quini´na (fig. 11), or star-shaped masses, as in Zygne´ma crucia´ta (fig. 13). A remarkably curious phenomenon met with in them is the manner in which the spores are formed, and which is known as conjugation. In this process the opposite cells of two distinct filaments, lying near together, push out protrusions of the cell-walls, which meet and open into each other, forming cross tubes, as in Spirog´yra nit´ida ([Pl. V.] fig. 12). The contents of the opposite cells of the filaments then unite, forming large spores, which remain either in the cells of one of the filaments or in the cross tubes.
The three species figured are common in clear pools.
Desmidia´ceæ.—The Desmidiaceæ are truly microscopic, few of them being even perceptible to the naked eye without the very closest examination. They are very beautiful, on account of their bright green colour and often elegant forms. Many of them are very common, existing in every pond or ditch; but they abound most in clear open boggy pools on heaths. On placing some water containing them in a glass jar and exposing it to the light, they will often be found adhering to the glass, or forming a layer on the surface of the muddy sediment.
The Desmidiaceæ consist mostly of single cells ([Pl. V.] figs. 9, 16); and these consist of two equal halves or segments, as indicated either by a paleness of the endochrome or a deep constriction at the line of junction, which is called the suture. The cells are often elegantly lobed and cut, or spiny; and in many the surface exhibits minute markings, consisting of little protrusions of the cell-wall outwards, or inflations, as they are called.