THE GRASS WRACK. 391 



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 contri- 

 bute ; 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 con- 

 stantly retained, are thus the favourite abode of most algse, 

 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 fre- 

 quently formed by the Grass Wrack {Zostera marina), whose 

 creeping stems, rooting at the joints and extending to a consider- 

 able 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 algae 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 Chlorospermese, generally occur near 

 hioh-water mark, and love to lead an amphibious life, half in the 

 air and half in salt-water. The delicate Enteromorphse, similar 

 to threads of fine silk, and the broad brilliant iflvse, which fre- 

 quently cover the smooth boulders with a glossy vesture of lively 

 green, belong to this class. Many of them are remarkable for 



