The genus Hormotrichum, which forms tufts several inches long, of bright grass green, differs from the Confervas in being soft and gelatinous, and even more by its mode of increase, which, however, is still by zoospores. The H. collabens may be taken as the type of this genus. It forms a long and large tuft of soft gelatinous and slippery filaments of glossy green. The joints of the filaments are once, or once and a half, longer than they are broad, and the green granular matter within them is collected into a round sac or sporidium in the centre of each, and after being converted into zoospores, the sac comes through a rupture in the joint into the water, opens, and sets the zoospores free.
The genus Cladophora, which has twenty-five species in the British seas alone, forms tufts of jointed filaments from four to eight, ten, or even twenty inches high. In some species the filaments are rigid, bristly, and wiry; in others they are soft and silky; but they are always richly, variously, and sometimes densely, branched and rebranched. In some the branches and branchlets are forked, in others tripartite; the Cladophora pellucida, which is a rigid, wiry plant, combines both these forms.
The genus Bangia consists of purple filamentous jointed and unbranched Algæ, which are distinguished from all others by the microscopic arrangement of their endochrome, which is enclosed in little cells placed according to a definite plan within the transparent and tubular joints of the filaments. In the Bangia fuscopurpurea, whose blackish purple tufts, several inches long, cling closely to the rocks near high-water mark, the tubular joints contain rows of minute colour cells radiating from a centre. In the narrow filaments there is but one colour cell in a joint, but in the broader filaments there are from three to five, forming a tesselated line across it. In this plant one spore is produced in each joint. The Bangia ciliaris forms a scarcely perceptible rosy pink fringe of hair-like jointed filaments on the Zostera marina, and also on other Algæ. The filaments are not more than the tenth or fifth of an inch long, consequently their joints are most minute, yet the microscope shows that they contain from two to three colour cells set as if radiating from a centre, and that the granular endochrome in each cell is converted into two zoospores. The Bangia ceramicola, which forms purplish pink tufts on small Algæ in rock pools, differs from both of the preceding. The joints of its filaments are once or twice as long as they are broad, and contain colour cells like long upright lines. By aided vision zoospores are seen to be formed within the linear colour cells, then the cells run together into a globular mass, which bursts through the cell wall, leaving the joint empty. The whole genus is soft and sometimes gelatinous.
The Enteromorpha genus is characterized by a cylindrical and tubular stem and branches. These plants form two groups, one whose filaments and branches swell from a narrow base upwards and terminate in a blunt extremity, while in the other group the tips of the branches are pointed. The Enteromorpha intestinalis, which is an inhabitant of many seas, has a thin membranous, tubular, cylindrical, and unbranched stem, inflated upwards into a broad round head, being more or less wrinkled and curled throughout. Downwards it tapers to a fine thread, and although attached at first, at last it becomes floating. Several of these plants rise from the same root, sometimes to the height of two feet, at others not more than an inch, and they are of every width, from the tenth of an inch to three inches, their colour being grass green. The typical form of the other group is much branched, and all the branchlets are finely pointed.
The three genera Codium, Bryopsis, and the marine Vaucherias are all soft plants characterized by their filaments being tubular, however much they may be branched. They agree also in being reproduced by zoospores developed from the green matter within little sacs attached to the exterior of their filaments. The species of the genus Codium differ much, although formed of similar elements. In the C. tomentosum, which is from three to twelve inches long, the dark green stem is thicker than a crow’s quill and much branched; while Codium Bursa, on the contrary, is a dark green round spongy lump of tubular filaments, densely interwoven and matted together. These masses, which are from one to eight inches in diameter, become hollow when old, and different sizes and ages grow together in a group.
The Bryopsis is a yellowish green tubular plant, from two to four inches high, plumed like a feather, and sometimes replumed. It is a rare plant in England, and grows on the larger Algæ in deep water.
The Vaucheria marina forms soft limp tufts of hair-like filament filled with bright green matter, which often runs partially out. It is from one to three inches high, and has a few long upright branches, to which are attached small stalked pear-shaped sacs containing zoospores. Both this plant and the Vaucheria velutina, grow on muddy shores.
Fig. 22. Ulva latissima: a, portion of ordinary frond; b, cells in which the endochrome is beginning to break up; c, cells from the boundary between the coloured and colourless portion, some containing zoospores; d, ciliated zoospores; e, development of zoospores.
The Ulvas, which are the grass green layers seen on all our coasts, originate in the simple vegetable cell, whatever form their foliaceous fronds may ultimately assume. When the cell is divided in one direction only, a confervoid filament is the result; and if the filament should increase in breadth as well as length, according to a determinate law, a ribbon-shaped frond may be produced; but when the original cell is divided into four cells, and each of these four and all their successors undergo similar division, the increase being as the series 1, 4, 16, 64, &c., a membranous expansion is formed, in which all the cells are firmly attached to one another, and every portion is the exact counterpart of another. The cells of the Ulvas frequently exhibit an imperfect separation of the granular endochrome into four parts preparatory to multiplication by double division, and the entire frond or leaf shows the groups of cells arranged in clusters containing some multiple form of four, as in [fig. 3], page 171.