8 4 



THE BOOK OF NATURE STUDY 



The brown weeds are often large, and, being frequently exposed 

 to the air at low tide, often produce a large amount of mucus 

 which prevents them drying up, but renders them undesirable 

 inhabitants for the aquarium. The aquarium - keeper is thus 

 wise to confine his attention to the green weeds, which are also 

 the best oxygen producers. In the excursions to the shore to 

 find specimens, however, one would call attention to the bladders 

 of the common bladder- wrack, and their use in enabling the 

 plants to float up near the surface and so get the light, and also 

 to the long stalks of oarweed (Laminaria), which have a similar 

 function, in that they bring the long starch-forming fronds up 

 to the surface. The red colour of the red weeds is an adaptation 

 to enable these plants to catch the last rays of light in the misty 

 depths in which they dwell. 



The next point is to realise that, with rare exceptions, the 

 seaweeds are inhabitants of the shore-zone, where the ebb and 

 flow of the tide is strong therefore while the algae of stagnant 

 freshwater ponds are often without fixing organs, and float at 



the surface, the seaweeds are fixed to 

 rocks or stones. Free, floating frag- 

 ments have been torn off, and are 

 dying, or will shortly die. This is an 

 important point to remember, for algae 

 decay rapidly, and are then apt to 

 poison the water. Do not, therefore, 

 put seaweed into your tank unless it 

 is attached to a stone or fragment of 

 rock. 



As to kinds, the most useful is the 

 common Green Laver (Ulva latissima), 

 also called sea-lettuce. This is bright 

 green in colour, and forms a thin, 

 waved, or curled membrane, from 6 

 inches to 14 inches long, and attached 

 to the rock by a minute disc. Some- 

 what similar is the Purple Laver (Por- 

 phyra), which is of a purplish colour. Very useful also are the 

 species of Enter omorpha, known as sea-grass, or mermaid's hair, 



FIG. 39. A marine green alga 

 (Bryopsis plumosd). 



