ALGA, a natural order of plants, belonging to the class cryptogamia of Linnteus, and to the acotyledones of the natural system. It contains a great number of species, about 2000 being known and described, and among these there is a great variety of forms. They grow for the most part in water, some in fresh and some in salt water, but some also on moist rocks or ground; whilst others are frequently found covering the glass and pots of hot-houses. Some species occur even upon diseased animal tissue, as achlya prolifera from the gills of fish, whilst sarcinula ventriculi (q.v.) appears to be formed in the human stomach. They are most numerous in still or stagnant water and in warm climates. Their structure is very various; they are found of all grades, from the little microscopic vesicle to great sea-weeds, which ramify like trees. The diversity in size is as great as in form; some species being visible only through the microscope, and resembling mold or rust; some a few inches, others several feet in length; whilst the laminarke, which float in the South American seas, measure more than 100 ft.; and macrocystis pyrzfera of the Pacific ocean reaches the length of 1500 ft. Yet they are seldom to be found as thick as the finger, or as broad as the hand, although some far exceed these dimensions, the trunk of lessonta fuscescins attaining the thickness of a man's thigh. Some species are firmly fixed at the bottom of the water, some adhere to rocks and stones left dry by the retiring tide; some frequently break loose, and float about upon and beneath the sur face. They have in no case proper roots, but merely processes for their attachment to the surfaces on which they are fixed; they seem to derive their nourishment by all parts of their surface from the water or moist air in which they grow. The gulfweed (sargassum) floats in long pieces in the Atlantic ocean and all the great seas; a large por tion of the sea between the West Indies and the Canary islands is specially called the .3fer de Sargasse. The weed is carried in such quantities by the current into the gulf of Mexico, that it covers the sea in tracts of many miles in breadth, and gives it the appearance of a meadow. Many fabulous stories were related of this gulfweed by the mariners of the 15th century. Ships were said to have been stopped in their course, and the crews obliged to cut their way through with hatchets. The discoveries of Columbus put an end to these exaggerated reports.
A. are entirely cellular in their structure, however elongated may be their fronds, having no proper vessels, but consisting of an irregular tissue of utricular cells. The fronds of many are articulated. Some of the simplest or lowest organization are propagated by spontaneous separation ; in others, the reproductive organs consist of spores (see AtOT}tEDDNot8 PLANTS) inclosed in perispores, and variously disposed in receptacles of different kinds ; sometimes in the interior of the cells. Antheridia (q.v.)
also occur in some ; and zoospores, or spores with moving cilia, which exhibit phenomena of motion resembling those of animal life. The diatomaceo2, in which the ordinary mode of reproduction is by spontaneous separation, have by some been referred to the animal kingdom. They are entirely microscopic, resemble the animalcules called infusoria, and are generally found in still waters and moist places, but occur in prodigious numbers in some parts of the Antarctic ocean, where they give a color to the water.
A differ from fungi (q.v.) in deriving their nourishment exclusively, as it would seem, from the medium by which they are surrounded, and not from the substance upon which they grow. The substance of which they are composed is also very different. Yet it has been felt not a little difficult to determine to which order some of the lowest forms of vegetable life should be referred.
As to their substance, A. consist chiefly of vegetable gelatine, which dissolves in water when they are boiled in it. The harder parts of their fronds are sometimes coriaceons, or horny, or cartilaginous, but never really ligneous. Their color is not always green, but mostly brown or yellow, sometimes purple or violet, or rose-color ; and many of them present a very beautiful appearance when examined through a microscope. Many contain an abundance of iodine. Different species of WRACK (fueus) (q.v.), which are cast on shore in vast confused masses by the waves, are gathered and burned in the Orkney islands, in Normandy, and other parts of the world, the ashes forming an article of commerce under the name of KELP (q.v.), and containing much of the iodide of sodium. Sea-weeds of all kinds are an excellent manure. None of the species are poisonous, and some.of them are used for food, a CARRAGEEN (q.v.) or Irish-moss, DOLSE (q.v.), LAVER (q.v.), etc. The edible swallows nest of the Indian archipelago are composed of a species of sea-weed. Several kinds are eaten as articles of luxury by the Chinese. Roca ria tena,r, one of the species so used, furnishes them also with an admirable glue, of which great quantities are prepared and brought to the market. Plocarki helminthocorton, Corsican moss, a native of the Mediterranean, and found principally around the shores of Corsica, is used as a vermifuge. See PLOCARIA.
This natural order is divided into 5 sub-orders, regarded by some as distinct orders— namely, CILUZACErE (q.v.), FUCACEX (q.v.), CERA311ACEX (q.v.), CONFERVACE‘E (see satvA), and DLkTOALACEX (q.v.). The Characem are sometimes separated as a dis tinct order of higher organization, whilst the rest are united under the name algte. See Efitzing's Phycologia Generalis (Leip. 1843), and his Species Algarum (Leip. 1849) ; Greville's A. Britannieo3 (Load. 1830) ; British Sea-weeds, nature-printed (London : Bradbury and Agnew).