The dry land develops the most exuberant vegetation on the lowest grounds, the plains and deep valleys, and the size and multiplicity of plants gradually diminish as we ascend the higher mountain regions, until at last merely naked or snow-covered rocks raise their barren pinnacles to the skies: but the contrary takes place in the realms of ocean; for here the greater depths are completely denuded of vegetation, and it is only within 600 or 800 feet from the surface that the calcareous nullipores begin to cover the sea-bottom, as mosses and lichens clothe the lofty mountain-tops. Gradually corallines and a few algæ associate with them, until finally about 80 or 100 feet from the surface begins the rich vegetable zone which encircles the margin of the sea. The plants of which it is composed do not indeed attain the same high degree of development as those of the dry land, being deprived of the beauties of flower and fruit: but as the earth at different heights and latitudes constantly changes her verdant robe, and raises our highest admiration by the endless diversity of her ornaments, thus also the forms of the sea-plants change, whether we descend from the brink of ocean to a greater depth, or wander along the coast from one sea to another; and their delicate fronds are as remarkable for beauty of colour and elegance of outline, as the leaves of terrestrial vegetation. The difference of the mediums in which land- and sea-plants exist naturally requires a different mode of nourishment, the former principally using their roots to extract from a varying soil the substances necessary for their perfect growth, while the latter absorb nourishment through their entire surface from the surrounding waters, and use their roots chiefly as holdfasts. The constituent parts of the soil are of the greatest importance to land-plants, to whose organisation they are made to contribute; while to the sea-plant it is generally indifferent whether the ground to which it is attached be granite, chalk, slate, or sandstone, provided only its roots find a safe anchorage against the unruly waters. Flat rocky coasts, not too much exposed to the swell of the waves, and interspersed with deep pools in which the water is constantly retained, are thus the favourite abode of most algæ, while a loose sandy sea-bottom is generally as poor in vegetation as the Arabian desert.