The Diatomaceæ, closely allied to the preceding group in structure and reproduction, are however distinguished from them by their flinty shells, which are often beautifully sculptured. Their endochrome is a golden brown, instead of green as in the Desmideaceæ. The latter, also, are confined to fresh water, while the Diatomaceæ are found, though not exclusively, in the sea, where their shells sometimes, microscopically minute as they are individually, form banks extending several hundred miles. It is stated that in the collection made by Sir Joseph Hooker in the Himalayas the species closely resemble our own.
In the next group, Confervaceæ, we are introduced to forms more like the general notion of what a plant should be. The individuals of which it consists are composed of threads, jointed, either simple or branched, mostly of a grass-green colour, and propagating either by minute zoospores or by metamorphosed joints. They are found both in fresh and salt water, and in damp situations. The number of species is very great. A considerable number consist of unbranched threads; the branched forms grow sometimes so densely as to assume the form of solid balls. After floods, when the water stands for several days, they sometimes increase to such an extent, as to form on its subsidence a uniform paper-like stratum, which while decomposing is extremely disagreeable. The name Conferva has been almost discontinued as a generic title, the majority of British species being now ranged under Clado- and Chæto-phora. The latter are branched, and require great care and attention in order to distinguish them, on account of their general resemblance to each other. Good characters are however to be found in their mode of branching and the form and comparative size of the terminal joints.
The Batrachospermeæ constitute a small but very beautiful group, consisting of gelatinous threads variously woven into a branched cylindrical frond. The branches are sometimes arranged, as in the British species, so that the plants appear like necklaces. In colour they pass from green, through intermediate shades of olive and purple, to black. In common with some of the higher Algæ, the threads of the superficial branches send joints down the stem, changing it from simple to compound. The native species are all fluviatile.
The Hydrodicteæ are among the most remarkable of Algæ. Hydrodictyon utriculatum, the solitary British species, is found in the large pond at Hampton Court, and in similar situations in various parts of the country, but not very generally. It resembles a green purse or net, from four to six inches in length, with delicate and regular meshes, the reticulations being about four lines long. Its method of reproduction is no less remarkable than its form. Each of the cells forms within itself an enormous mass of small elliptic grains. These become attached by the extremities so as to form a network inside the cell, and, its walls being dissolved, a new plant is set free to grow to the size of the parent Hydrodictyon.
The Nostochineæ grow in fresh water, or attached to moist soil. They consist of slender, beaded threads surrounded by a firm jelly, and often spreading into large, wavy fronds. The larger beads on the inclosed threads are reproductive spores. ([Fig. 4, A.])
Fig. 4. A, Fragment of a Filament of Nostoc. B, End of a Filament of Oscillatoria.
The Oscillatoreæ are another remarkable group, on account of the peculiar animal-like motions they exhibit. They occur both in salt and fresh water, and on almost every kind of site in which there is sufficient moisture. The threads of which they are composed are jointed, and generally unbranched; they are of various tints of blue, red, and green, and, where their fructification has been ascertained, are propagated by cell division. The most curious point about them is, however, the movements of their fronds. According to Dr. Harvey, these are of three kinds—a pendulum-like movement from side to side, performed by one end, whilst the other remains fixed, so as to form a pivot; a movement of flexure of the filament itself, the oscillating extremity bending over from one side to the other, like the head of a worm or caterpillar seeking something on its line of march; and lastly, a simple onward movement of progression, the whole phenomenon being, Dr. Harvey thinks, resolvable into a spiral onward movement of the filament. Whatever is the cause of this motion, it is not, as used to be supposed, of an animal nature; for the individuals of this group are undoubted plants. ([Fig. 4, B.]) Several species of Rivularia, belonging to the Oscillatoreæ, are found both in the sea and in fresh water. They are gelatinous, and have something of the appearance of Nostoc, in aspect as well as in minute structure.
The Conjugatæ are freshwater articulated Algæ, which reproduce themselves by the union of two endochromes. They are very interesting objects under the microscope, owing to the spiral or zigzag arrangement of the endochrome of many of them, and the delicacy of all.