THE GRASS WRACK, 291 
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 algz, 
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 securing 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 algze 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 Chlorospermez, generally occur near 
high-water mark, and love to lead an amphibious life, half in the 
air and half in salt-water. The delicate Enteromorphe, similar 
_ to threads of fine silk, and the broad brilliant Ulvze, which fre- 
quently cover the smooth boulders with a glossy vesture of lively 
green, belong to this class, Many of them are remarkable for 
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