SEAWEEDS AND THE WATER-FLORA. l8l 



red colouring matter. All, however, possess chloro- 

 phyll, as may easily be proved by soaking them in fresh 

 water ; when both the brown and red colours diffuse 

 away, leaving the ordinary green chlorophyll. The 

 particular species will vary very greatly according to the 

 character of the shore. Hard rocks, which do not 

 weather easily, often show the richest variety. Mud is 

 generally only occupied by the rich deep-green carpets 

 of Vaucheria, or by webs of yellow-brown Ectocarpus. 

 Shingle and sand, or continually shifting mudbanks, 

 afford no hold to the larger Algae, but when they are 

 allowed to remain at rest they soon become covered. 

 They are probably first attacked by crusts of diatoms ; 

 then the Ectocarpi or Vancherias on mud, and the 

 Enteromorphas on sand, begin to cover them. Stones 

 which are not disturbed, soon become a dark purple-red, 

 or rich olive-brown colour. No one would suppose this 

 stain to be the result of vegetable life, but by carefully 

 scraping the stone and examining it microscopically, it is 

 easy to see that there is a crust of algal cells covering 

 it. There are, in fact, some i6 genera and 20-30 

 species of Algae, which are specialised to cover rocks 

 and stones. In some, such as Hildenbrandtia and 

 Petrocelis, the cells are arranged in parallel vertical 

 rows, and are closely held together by mucilage. In 

 others, they are arranged in radiating lines, and are 

 easily visible, as reddish or olive-black gelatinous excre- 

 tions on the stones. 



The tenacity with which all Algae are able to 

 maintain their position is very remarkable ; a Vaucheria 

 carpet is very common on the estuarine mud of the 

 Clyde, and it has been found that a depth of nearly an 

 inch of mud is entangled in the Algae ; a foot square 

 with the mud attached can be supported by one corner 



