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.
But even on sandy shores extensive submarine meadows are frequently formed by the Grass Wrack (Zostera marina), whose creeping stems, rooting at the joints and extending to a considerable depth in the sand, are admirably adapted for seeming a firm position on the loose ground. Its long riband-like leaves, of a brilliant and glossy green, wave freely in the water, and afford shelter and nourishment to numerous marine animals and plants. In the tropical seas it forms the submarine meadows on which the turtles graze, and in the North of Europe it is used for the manufacture of cheap bedding. It also furnishes an excellent material for packing brittle ware.
Sea-weeds are usually classed in three great groups, green, olive-coloured, and red; and these again are subdivided into numerous families, genera, and species.
On the British coasts alone about 400 different species are found, and hence we may form some idea of the riches of the submarine flora. Thousands of algæ are known and classified, but no doubt as many more at least still wait for their botanical names, and have never yet been seen by human eye.
The Green sea-weeds, or Chlorospermeæ, generally occur near high-water mark, and love to lead an amphibious life, half in the air and half in salt-water. The delicate Enteromorphæ, similar to threads of fine silk, and the broad brilliant Ulvæ, which frequently cover the smooth boulders with a glossy vesture of lively green, belong to this class. Many of them are remarkable for their wide geographical distribution. Thus the Ulva latissima and the Erderomorpha compressa of our shores thrive also in the cold waters of the Arctic Sea, fringe the shores of the tropical ocean, and project into the southern hemisphere as far as the desolate head-lands of Tierra del Fuego. But few animals or plants possess so pliable a nature, and such adaptability to the most various climates.
The Olive-coloured group of sea-weeds, or Melanospermeæ, plays a much more considerable part in the economy of the ocean. The common fuci, which on the ebbing of the tide impart to the shore cliffs their peculiar dingy colour, belong to this class; as well as the mighty Laminariæ, which about the level of ordinary low water, and one or two fathoms below that limit, fringe the rocky shore with a broad belt of luxuriant vegetation.
The first olive-coloured sea-weed we meet with on the receding of the flood is the small and slender Fucus canaliculatus, easily known by its narrow grooved stems and branches, and the absence of air-vessels. Then follows Fucus nodosus, a large species, with tough thong-like stems, expanding at intervals into knob-like air-vessels, and covered in winter and spring with bright yellow berries. Along with it we find the gregarious Fucus vesiculosus, with its forked leaf traversed by a midrib, and covered with numerous air-vessels situated in pairs at each side of the rib. Finally, about the level of half-tide, a fourth species of fucus appears, Fucus serratus, distinguished from all the rest by its toothed margin and the absence of air-vessels.
These four species generally occupy the littoral zone of our sea-girt isle, being found in greatest abundance on flat rocky shores, particularly on the western coasts of Scotland and Ireland, where they used formerly to be burnt in large quantities for the manufacture of kelp or carbonate of soda, which is now obtained by a less expensive process. In Orcadia alone more than 20,000 persons were employed during the whole summer in the collection and incineration of sea-weeds, a valuable resource for the poverty-stricken islanders, of which they have been deprived by the progress of chemical science.