‘The fruit of these three great Laminariæ is imbedded here and there in the surface of the frond, thickening it and forming cloudy patches.’[[45]] It consists of thick club-shaped perpendicular cells in which the endochrome is ultimately divided into four parts. This is certainly the case in the Laminaria bulbosa, and also in the Alaria Pylaii, a species of which latter genus, the Alaria esculenta of our own coasts, is a much esteemed British dulse.
Abundance of colossal Algæ are found in the North Pacific, about the Kurile and Aleutian Islands, and along the deeply indented and channel-furrowed northwestern coast of America. The Nereocystis Lutkeana forms dense forests in Norfolk Bay, and all about Sitka. Its stem resembles whipcord, and is sometimes 300 feet long. It is exceedingly slender at the top, where it terminates in an enormous air-bladder six or seven feet long, and about four feet and a half in diameter at its widest part, the lower extremity passing into the stem. This huge air-vessel, which is the usual seat of the sea otter, is crowned with a tuft of twin leaves mostly rising on five stalks. These leaves, which are membranous and lanceolate when young, and from one to two feet long and two inches broad at the centre, are only marked with a few faint nerves, but they ultimately split lengthwise, cover a large space, and attain a length of twenty-seven or thirty feet, or even more. The growth of the Nereocystis must be enormously rapid, since it is an annual, and must therefore develop its whole gigantic proportions in one summer.[[46]] Boats cannot pass through the floating masses of this plant, whose stem is used for fishing lines, and whose cylindrical air-vessel serves as a siphon for pumping water out of boats.
The Thalassiophyllum Clathrus is also an inhabitant of the Russian coast of North America. It is about six feet high, very bushy and branched, each branch bearing a broad leaf at its extremity which unfolds spirally, and by this gradual development produces the stem with its branches and lateral divisions. A spiral border wound round the stem indicates the growth of the frond, which presents a large convex bent lamina without nerves, or a leaf of which one-half is wanting. Numerous long narrow perforations, arranged in a radiating form, give it the appearance of a cut fan.
The Macrocystis pyrifera and the Laminaria radiata are the most remarkable of marine plants, for their gigantic size and the extent of their range. They are met with on the antarctic coasts two degrees nearer the pole than any other vegetable, except the Diatomaceæ. The stem of the Macrocystis is slender, smooth, round, and slimy, rising from a fibrous root, like other Laminariæ, and bearing at its tip a lanceolate or oblong lanceolate frond. This frond divides at the base; the fissures extend upwards so as to form two petioles, each of which swells into an oblong or pyriform air-vessel. Another fissure is formed in a similar way a little above, and so on, till a single frond may at the same time have eight or ten fissures, each of which will ultimately gain the common apex. The margins of the fissures are at first perfectly smooth, but they soon become ciliated like the outer edge. The continuity with the fibrous base is at last broken, and the divisions of the leaves going on indefinitely, the whole reaches the length of some hundred feet, forming enormous floating masses which are wafted by the waves hundreds of miles from their origin. Fructification only takes place in young plants; consequently in such as are still attached to their native rocks. Even in that youthful state, Mr. Darwin mentions that such is the buoyancy of this powerful weed, that there is scarcely a loose block of stone on the coasts of Cape Horn that is not buoyed up by it.[[47]] The Macrocystis is native on the shores of the Atlantic, from Cape Horn to 43° S. latitude; but on the Pacific coast, according to Dr. Hooker, it extends to the river San Francisco in California, and perhaps to Kamschatka. The plant is reproduced by pyriform cells, full of endochrome, in nearly parallel rows imbedded in the fronds.
The rocky coasts of the Falkland Islands are covered with a vast growth of the gigantic Macrocystis mixed with forests of the arborescent Lessonia, which forms large dichotomous trees with a stem from eight to ten feet high and a foot in diameter. The leaves are two or three feet long, drooping from the forked branches like weeping willows. In the Lessonia nigrescens the quadripartite endochrome, ultimately resolved into spores, is contained in thickened club-shaped cells springing vertically between the surfaces of the frond.
A transverse section of the stem of many of the larger sea weeds presents zones, formed period by period, corresponding with the development of the laminæ, roots, and branches. The stem of the Lessonia bears a strong analogy to that of dicotyledons in having rings of growth, though there is a great difference. As increase in Lessonia takes place by the constant division of a flat leaf, the basilar portion of which becomes the petiole and ultimately swells into a branch, the stems have always a more or less elliptical form, and their section exhibits an elliptical core. This form of the core is not however peculiar, but exists in other Algæ. It is probable that the Lessoniæ, although attaining so large a size, are really of rapid growth.[[48]]
The Ecklonia is essentially a southern genus, though one species ascends to Spain and the Canaries. The frond is pinnatifid, the segments arising from the evolution of marginal teeth. The stem of the Ecklonia buccinalis, which is three or four inches thick and strongly inflated above, exhibits rings of growth with an orbicular central pith.
The group of the Fucaceæ exhibits the highest structure of all the olive-green Algæ, and forms a large portion of the sea weeds on our coasts, but they abound more in individuals than in the number of genera and species. A few have cylindrical stems and branches swelling out at intervals into large oblong inflated air-vessels, which gives them buoyancy in the water. The rest have a flat, ribbon-like stem, and for the most part dichotomous branches with a decided midrib, but no air-vessels, because they chiefly grow at half-tide level, and are exposed twice every twenty-four hours. The most common of our fuci, the Fucus vesiculosus, or bladder-wrack, has a midrib with air-vessels, generally in pairs on each side of it, formed by the inflation of the frond; these vessels, however, are frequently wanting, for it is the most variable in form and most widely spread of the Fuci. The fructification of this group is contained in large clavate receptacles or expansions of an orange or greenish yellow colour situated at the extremities or borders of the branches.
MM. Thuret and Decaisne discovered, by microscopic investigation, that the fuci have a truly sexual fructification, consisting of male and female cells inclosed in these receptacles. In the common Fucus vesiculosus it was found that the male and female cells are either in different individuals, or in different conceptacles on the same individual; whilst in the Fucus platycarpus, both the male and female cells were found to be contained in a globular cavity enclosed in the flattened receptacles which grow at the extremities of the branches. The cavity is lined with jointed hair-like filaments formed of cells, some of which are so long as to project through a pore on the surface of the receptacle in a spreading brush (see [fig. 31], where the whole is highly magnified). Towards maturity, the cells of some of these filaments assume an ovoid form; the white viscous, granular matter in their interior acquires an orange hue, and is divided into a multitude of hyaline particles, each having an orange spot and two cilia of unequal lengths, which enable these spermatozoids to swim with great vivacity in the water as soon as they are set free by the rupture of the cell in which they are inclosed. Besides these, dark olive-green female cells, of a large pyriform shape, are fixed to the walls of the same cavity by very short stems; their contents spontaneously divide into eight spore cells, never more; each contains a colourless viscous liquid, which is mixed with protein and yellow-green matter, and is inclosed in a double coat. ‘The coats are united at the base, and when the spores are ready for dispersion, the inner coat bursts through the apex of the outer one, dragging with it a portion of the latter in the form of a little peduncle. The immediate covering of the spores at length bursts, and they are set free.’[[49]] In Fucus serratus, vesiculatus, and nodosus, swarms of spermatozoids are produced, but M. Thuret has proved by experiment that they never come to anything of themselves, and the unfertilized spores perish.
