The form of the Rhodosperms, as well as the limits of the species, like those of other Algæ, are affected by many circumstances known and unknown, such as the depth, temperature, saltness, and currents in the water. The Gelidium corneum varies to such an extent that its forms may not only be considered as distinct species, but even as belonging to different genera. The Delesseria alata is sometimes destitute of its margin, and then its midribs alone being left, it has the form of the Delesseria angustissima. Several species of the florid Algæ, which in their natural state have the tips of their fronds even and straight, occasionally produce hooked and clasping tips.
Fig. 28. Polyzonia cuneifolia.
Brackish water is often a cause of change. The Irish moss, Chondrus crispus, when exposed to the fresh water of an estuary acquires great breadth and thickness, while at low water mark it is thin and has narrow forked branches, and there are many intermediate forms. The fruit rarely varies with these changes; its disposition and intimate structure, as well as that of the frond, are the points of prime importance for the determination of genera and species in the Algæ.
The Melanospermeæ, or Melanosperms, are olive-green Algæ, sometimes inclining to brown. They have fewer species than the Rhodosperms, but the individuals exceed in abundance and in magnitude all the other Algæ.
These large Melanospermous Algæ, which form marine forests in both hemispheres, are excessively strong and tough on the exterior but of a looser texture within, so that the cells of their tissue are of different sizes and forms, according to the degree of pressure. The stems and branches are more dense than the leaves. This highest order, however, has small and delicate Algæ united to the largest by many intermediate forms. The Melanosperms are either monœcious or diœcious, and bear their olive-green spores in cases, that is cysts, variously disposed on the plants. Many have two kinds of zoospores differing in nothing but size; they are produced in different organs; in some species both are fertile, in others only one, and, in these cases, the other is therefore supposed to be a fertilizing body, but however that may be, there are certainly antherozoids in this group of Algæ, especially in the order Fucaceæ.
The Ectocarpeæ have many representatives on our coasts, all of which are tufts of articulated threads from one to eighteen inches long, branched or simple. They are generally soft, some so flaccid that they cling together, but sometimes they are firm and stiff. The cysts which are attached to these threads have various forms; they are spherical, siliquose (that is, like long pods), or of other shapes, according to the species; but whatever form they may assume, they are filled with a dense endochrome. Besides these they have active granules contained in other distinct organs. M. Thuret has decided beyond a doubt that the latter are small zoospores, and it is presumed that the endochrome in the cysts is resolved into zoospores, but of a different order, as in the Ulvas. These two organs are for the most part situated on different individuals; in Ectocarpus pusillus ([fig. 29] b) they are on the same. The different forms of fruit carpels are represented magnified in [fig. 29].
Fig. 29. Fruit of Ectocarpus:—a, E. sphærosporus; b, E. pusillus; c, E. fenestratus; d, E. fasciculatus.
The Ectocarpeæ contain little or no gelatine, whereas the genera of the group Chordariæ have soft gelatinous fronds of many forms, either incrustations, convex lumps, or tubers, like the Leathesia so common on our coasts; small plants as the Mesogloias, which have soft slippery filiform stems beset with myriads of moniliferous worm-like branches; or lastly the Chorda filum, a simple unbranched slimy cylindrical cord, varying from a quarter of an inch to the thickness of a pencil, and from one to twenty or even forty feet in length in deep water. The cord is tubular, divided into chambers by transverse partitions, formed of interlaced vertical and horizontal articulated threads. It tapers at each extremity, and the exterior, which is brown, is clothed with pellucid hairs. Vertical spores are immersed throughout the whole surface of the cord, and Dr. Harvey says that, mixed with these, there are numerous narrow, elliptical, transversely striated cells, which according to M. Thuret produce zoospores. Each plant rises solitary from its own little disc, but as the Chorda filum is a social plant, vast assemblies of it cover extensive areas of sand and mud, and form dense thickets in our northern seas. There are bands of it in the North Sea 15 to 20 miles long, and more than 600 feet wide; there is a submarine forest of it in Skapta Bay, Orkney; and in passing through the sounds of the western islands, as between Kerrera and the mainland, there are others. The long cords always lean in the direction of the tide, and must oscillate between two zones of rest, one at the turn of the flood, and another at the turn of the ebb. When dried the people use them for fishing lines. In the Chordaria divaricata both kinds of spore cysts are external, and give rise to zoospores.