The whole of the plants which have been described in the preceding pages belong to the group of green Algæ, although many are inhabitants of fresh water. The structure of the marine Algæ is entirely cellular. Deprived of vascular tubes, they can have no circulation of sap, consequently they derive their nourishment by absorption throughout their whole surface from the medium in which they live, for their root, or rather fulcrum, only serves to fix them to the rocks and stones to prevent them from being buffeted by the waves. Since solar light and heat decrease rapidly with the depth, each family of Algæ has a zone peculiar to itself. The first zone extends from high to low water mark, and is inhabited by plants periodically exposed to the atmosphere, to the direct light and heat of the sun, and occasionally to rain. Some of the Algæ that are long left dry are believed to derive some nourishment from the substances to which they are fixed. The second zone, which extends from low water mark to a depth of fifteen fathoms, is the region of the great marine forests which encircle the globe in both hemispheres. Other two zones follow at greater and greater depths, but all are divided into various minor regions, below the last of which the Algæ decrease as the depth increases, till, as far as we know, vegetation ceases altogether; that depth, however, must be very great, as diatoms are sometimes found, and in great quantities, three hundred fathoms deep. The marine Confervas, like those growing in fresh water, are slender-jointed filaments formed of one series of cells joined end to end. The cells become more or less flattened on the surface of contact, while the side walls retain their natural curvature, which may be cylindrical or oval. The filament may, therefore, be cylindrical or beaded. The cells are almost always longer than they are broad, and for the most part equal and similar in the same plant, although there are exceptions to uniformity of size. The cells contain a transparent liquid through which minute solid particles of various shades of green are pretty evenly scattered. The conversion of these particles into zoospores has already been described. Since these Algæ have no roots, and the cell wall no opening, each cell of a Conferva elaborates independently the nutriment it absorbs from the water. Some species form a fleecy layer over rocks, and on the bottoms of salt-water pools and estuaries, others extend in bundles in salt-water ditches, and some are found on rocks, between tide marks, rising in long, straight, stiff, and wiry tufts, from three to eight or twelve inches high.