466 THE BRITISH NATURE BOOK 



used in special forms in medicine, while it is from the ashes of the RocKweeds 

 that the chief supply of this important element is obtained. 



All seaweeds, except the Grass-wrack (Zoster a maritima), belong to the 

 flowerless division of plant life, being made up of cells, without woody or 

 other tissues. They have no roots, that portion in the larger species which 

 looks like a root being really a sucker, by means of which the plant adheres 

 firmly to the rock. 



Turning to the Chlorospermece those with green spores it should be 

 borne in mind that whilst usually light green, the plant is sometimes olive or 

 purple. 



Every one who visits the seaside knows the thin, flat, transparent fronds 

 of the Sea-lettuce or GREEN LAYER (Ulva latissima), with its crisped, folded 

 margins a valuable plant for the marine aquarium, as its leaves give off a 

 plentiful supply of oxygen. A companion is the Purple Laver, known in Ire- 

 land as " SLOKE," and often boiled down into a jelly for food, as is also the case 

 with the Green. 



Conferva masses of green hairy threads may be found in any tidal 

 pool, and correspond to those found in fresh water; while another common 

 green weed is Sea-grass (Enteromorpha compressa), which forms a narrow tube 

 of rugged shape, occasionally found inflated with oxygen gas. 



The Zostera, or Grass-wrack, is not really a seaweed, but a flowering plant 

 which somehow has taken to living in the sea. It may be found thrown up 

 on the shore after a storm, or growing in the deeper pools just above low-water 

 mark. Its leaves are less than | inch wide, and may be 3 or 4 feet long, and 

 it has real flowers which grow in a sheath, springing from one of the shorter 

 leaves. Below low-water mark it abounds in large stretches, the abode of 

 all sorts of sea animals. 



Bryopsis plumosa is another green weed, fairly common, found in rock 

 pools, and with fronds like fine feathers. 



Cladophora rupestris and C. arcta are two other common green seaweeds, 

 the former coarse and horsehair-like, growing in thick tufts, 4 or 5 inches 

 long, the latter a brighter green, and growing in a radiating manner from 

 a broad disc. 



The Brown Seaweeds (Melanospermece), always of an olive colour, include 

 our gigantic oarweeds and tangles, and afford a great variety of forms. The 

 most striking on account of their size and abundance are the Fucacece. 



The Channelled Wrack (Fucus canaliculatus) is a small leathery plant, 

 growing abundantly on rocks. It looks so dry and shrivelled that most people 

 consider it to be dead, but it softens when the water reaches it. It is much 

 branched, and the edges of the fronds turn in so much as to give the appear- 

 ance of being channelled or furrowed. The receptacles containing the spores 

 are pod-like organs, dark orange in colour, found at the end of some of the 

 fronds. 



The Knotted Wrack (F. nodosus) may be as much as 6 feet long, and has 



