But the longest of all known plants is the alga /Macrocystis/. Its thin naked stem, the diameter of which seldom exceeds one quarter [pg036] of an inch, is reported by one author to be seven hundred feet in length, by another fifteen hundred feet. It is terminated by a lamina fifty feet long, resembling a pinnatifid leaf, each leaflet of which, at its point of division on the stem, expands into an air-vessel as large as an egg. These air-vessels sustain the immense frond which floats on the surface of the water, its leaflets depending in a vertical position from the stem. /M. pyrifera/, the only species, is found in the Southern oceans and on the Pacific coast of North America. /Lessonia/, another genus, resembles a palm-tree. It grows erect to a great height and has a stem like the bole of a tree. It branches in a forking manner and has depending from its branches laminæ two or three feet long. The large stems from which the laminæ have been torn by the storms, and which have been cast ashore on the Falkland Islands, as described by Sir Joseph Hooker, resemble driftwood, as they lie in piles three or four feet high and extending for many miles. /Agarum/ and /Thalassiophyllum/ are arctic genera, but they are found within our limits, the former in the North Atlantic. It has a simple but enormous leaf-like frond. The latter, which is found on the North Pacific coast, has a compound frond. Both are characterized by their fronds being perforated throughout with holes, giving them the name of sea-colander. [pg037] X USES OF ALGÆ Water covers two thirds of the surface of the earth, and algæ, with a very few exceptions, constitute the whole vegetation which exists in that enormous area. They have, therefore, an important part to perform in the economy of nature. Algæ do not, like land plants, derive their nourishment from the soil to which they are attached, but from substances held in solution by water. In their growth they effect changes in the water analogous to those effected by land plants in the air; that is, they change so-called impurities in the water into materials essential to animal life. The function of plants is that of transforming or manufacturing inorganic matter, which they assimilate, into organic matter (such as starch, albumen, sugar), which forms their own structure and which is the food essential to animals. In this process, plants inhale carbonic acid gas which animals breathe out, and exhale oxygen which animals breathe in. Plants feed on mineral substances and furnish vegetable food, thus keeping up the balance of life.