The lowest forms of algæ are microscopic in size, each individual being a single cell; but in the ascending scale they attain curious and beautiful shapes, some growing to a gigantic size and in forms that resemble shrubs and trees. The green surface commonly seen on the shady side of trees, on stone steps, and in other damp places is one of the species of algæ which consist of a single cell. This plant or cell divides, and the separate divisions divide and subdivide again and again, and in time the aggregate number is great enough to spread over a comparatively large surface, and thus become visible to the naked eye. This plant, the /Pleurococcus vulgaris/, is a fresh-water alga. The /Protococcus nivalis/, or red snow, described on page 33, is a closely allied species. The green and blue-green scums and slimes on [pg026] brackish ditches and on the stones and woodwork of wharves are also species of the lowest orders of algæ and increase by cell-division. Many of them are in colonies incased in gelatinous matter. These, together with plants of a little higher order, though still of low organization, the /Confervaceæ/, form a large part of the green vegetation between tide-marks. The vegetative body of a thallophyte is a /thallus/, and corresponds to stem and leaf. It is also called a /frond/. What corresponds to the root of flowering plants is in algæ a disk or conical expansion of the base of the plant. It is simply a holdfast by which the frond attaches itself to any submerged material. The algæ which grow on sandy shores and on corals have holdfasts which branch like fibrous roots and penetrate porous substances in all directions; but this is only for greater stability, and is an adaptation to the habitat. Holdfasts do nothing for algæ other than the name implies, whereas real roots absorb the nourishment upon which plants live. Algæ are nourished by the substances held in solution by the water which surrounds them. Algæ are the lowest and simplest in organization of all plants, because they are composed of but one class of cells, such as in flowering plants are called the /parenchyma/, or soft cells, these being the ones which compose the pulp of the leaf. In the lowest orders of algæ single cells constitute individual plants, as in /Pleurococcus/; but in the higher forms, such as /Sargassum/, they arrange themselves in such a variety of combinations as to resemble plants which have leaf and stem. The botanical distinction is that in leaf and stem there would exist the woody and the vascular cells as well as the parenchyma cells. Beginning with plants composed of a single cell, the next development is into filamentous plants, which are single thread-like rows of cells, as in /Cladophora/. In /Ulva/ is seen the earliest type of an expanded leaf. The cells are here arranged in a horizontal surface of plate-like or ribbon-like shape.