Fig. 12. Fucus serratus, showing a transverse section of the Conceptacle, and Antheridium with Antherozoids escaping.
They are deep sea plants, or at least grow about low water mark. The largest of the group is Desmarestia ligulata, which, with the other British species, D. aculeata, is often cast ashore. The latter species, at an early period of its existence, is clothed with tufts of slender hairs, springing from the margin of the frond. Desmarestia viridis is the most delicate and also the rarest of the three. Nothing like fruit has been discovered on any of them. Arthocladia villosa and Sporochnus pedunculatus are branched sea-weeds, covered also with tufts of closely set hairs. Carpomitra Cabreræ, a rare species, bears, in common with the two preceding species, its spores in a special receptacle. In the first the receptacle is pod-like; in the second knotted; and in the last mitriform.
The concluding group of Algæ is the Fucaceæ, including the universally known sea wrack (Fucus). The frond in all of them is jointless. They are reproduced by means of antheridia and oogonia developed in conceptacles, clustered together at the apex of the branches. Both from their bulk and their decided sexual distinctions, they deserve to rank at the head of the order. Of all sea-weeds they are also perhaps of the greatest use to man. One of the most interesting among them is the Gulfweed (Sargassum bacciferum), occupying a tract of the Atlantic extending over many degrees of latitude. Pieces of it, and of its congener, S. vulgare, are occasionally drifted to our shores, and they consequently find a place in works on British Algæ, although they have no claim to be considered native plants. On rocky coasts the various species of Fucus occupy the greater part of the space between tide-marks, the most plentiful being Fucus vesiculosus. F. serratus ([Fig. 12]) is the handsomest of the genus, the other species being F. nodosus, said to be the most useful for making kelp, and F. canaliculatus. Halidrys siliquosa is remarkable for its spore receptacles, which have quite the appearance of the seed vessel of a flowering plant. The species of Cystoseira, chiefly confined to the southern coasts, are also very interesting. Their submerged fronds are beautifully iridescent, and the stems, of the largest species at least, are generally covered with a great variety of parasites, animal and vegetable, the former consisting of Hydrozoa and Polyzoa, and other curious forms. Himanthalia lorea is another remarkable plant. It has conspicuous forked fruit-bearing receptacles; but the real plants are the small cones at the base of these, and from which they are shed when ripe.
As to conditions of site and geographical distribution, Algæ do not differ from land plants. Latitude, depth of water, and currents influence them in the same way as latitude, elevation, and station operate on the latter; and the analogy is maintained in the almost cosmopolitan range of some, and the restricted habitat of others. Not many extra-European species of Desmids are known, but those of Diatoms are far more widely diffused, and extend beyond the limits of all other vegetation, existing wherever there is water sufficient to allow of their production; and they are found not only in water, but also on the moist surface of the ground and on other plants, in hot springs and amid polar ice. They are said to occur in such countless myriads in the South Polar Sea as to stain the berg and pack ice wherever these are washed by the surge. A deposit of mud, chiefly consisting of the shells of Diatoms, 400 miles long, 120 miles broad, and of unknown thickness, was found at a depth of between 200 and 400 feet on the flanks of Victoria Land in 70° south latitude. Such is their abundance in some rivers and estuaries that Professor Ehrenberg goes the length of affirming that they have exercised an important influence in blocking up harbours and diminishing the depth of channels. The trade and other winds distribute large quantities over the earth, which may account for the universality of their specific distribution; for Sir Joseph Hooker found the Himalayan species to closely resemble our own. Common British species also occur in Ceylon, Italy, Virginia, and Peru. The typical species of the Confervaceæ are also distributed over the whole surface of the globe. They inhabit both fresh and salt water, and are found alike in the polar seas and in the boiling springs of Iceland, in mineral waters and in chemical solutions. Some of the tropical ones are exceedingly large and dense. Batrachospermum vagum, in the next tribe, a native of England, is also found in New Zealand. An edible species of Nostochineæ, produced on the boggy slopes bordering the Arctic Ocean, is blown about by the winds sometimes ten miles from land, where it is found lying in small depressions in the snow upon the ice. The common Nostoc of moist ground in England occurs also in Kerguelen's Land, high in the southern hemisphere. Floating masses of Monormia are often the cause of the green hue assumed by the water of ponds and lakes. Certain species of Oscillatoria of a deep red colour live in hot springs in India, and the Red Sea is supposed to have derived its name from a species of this tribe, which covers it with a scum for many miles, according to the direction of the wind. The lake of Glaslough in County Monaghan, Ireland, owes its colour and its name to Oscillatoria ærugescens, and large masses of water in Scotland and Switzerland are tinted green or purple by a similar agency. A few species of Siphoneæ have a very wide range, two British species of Codium occurring in New Zealand. The Ulvaceæ abound principally in the colder latitudes. Enteromorpha intestinalis, a common British species, is as frequent in Japan, where it is used, when dried, in soup. The Rhodosperms are found in every sea, although the geographical boundaries of genera are often well-marked. Gloiosiphonia, one of our rarest and most beautiful Algæ, is widely diffused. Of Melanosperms the Laminariæ affect the higher northern latitudes, Sargassa abound in the warmer seas, while Durvillæa, Lessonia, and Macrocystis characterize the marine flora of the Southern Ocean. The Fucaceæ are most abundant towards the poles, where they attain their greatest size. The marine meadows of Sargassum, conceived by some naturalists to mark the site of the lost Atlantis, and which give its name to the Sargasso Sea, extending between 20° and 25° north latitude, in 40° west longitude, occupy now the same position as when the early navigators, with considerable trepidation, forced through their masses on the way to the New World. Sargassum is drifted into this tract of ocean by currents, the plants being all detached; and they do not produce fruit in that state, being propagated by buds, which originate new branches and leaves. ([Fig. 13.])
Fig. 13. The Gulf-weed (Sargassum bacciforum).
Owing to their soft, cellular structure, Algæ are not likely to be preserved in a fossil state; but what have been considered such have been found as low down as the Silurian formation, although their identity has been disputed, and several of them, it is more than probable, belong to other orders, and some even to the animal kingdom. Freshwater forms, all of existing genera and species, are believed to have been detected in the carboniferous rocks of Britain and France; others also of the green-coloured division are said to occur from the Silurian to the Eocene, and the Florideæ to be represented from the Lias to the Miocene. The indestructible nature of the shells of the Diatomaceæ has enabled them to survive where the less protected species may have perished. Tripoli stone, a Tertiary rock, is entirely composed of the remains of microscopic plants of this tribe. It is from their silicious shells that mineral acquires its use in the arts, as powder for polishing stones and metals. Ehrenberg estimates that in every cubic inch of the tripoli of Bilin, in Bohemia, there are 41,000,000 of Gaillonella distans. Districts recovered from the sea frequently contain myriads of Diatoms, forming strata of considerable thickness; and similar deposits occur in the ancient sites of lakes in this and other countries.