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.