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A TEXTBOOK OF BOTANY 



[Ch. X 



of moderate size, from near microscopic to a yard in length, 

 perhaps oftenest about the size of the hand. They live 

 almost wholly below low water mark, and extend to greater 

 depths than any other Algae, — down indeed as far as light 

 can reach, viz. some 200 feet in clear waters. A few occur 



in fresh water; but 

 such kinds are small, 

 and tend towards a 

 greenish color. Some 

 3000 species are known, 

 of which a few have 

 limited economic uses. 

 The plant body con- 

 sists typically of a flat, 

 thin, flexible thallus, 

 cut to divisions which 

 are often filamentous 

 (Fig. 310), such char- 

 acters being usual in 

 wholly submerged 

 water plants (page 562) . 

 In this group, as in the 

 Brown Algae, the thallus 

 is sometimes differenti- 

 ated into parts strongly 

 simulative of the leaf, 

 stem, and root of the 

 higher plants, though of quite different morphology. The cells 

 of the fronds usually he in definite lines, suggesting an origin 

 from amalgamated filaments ; and the continuity of the proto- 

 plasm from cell to cell is often evident, even more clearly 

 than in the higher plants (page 40). The walls form much 

 gelatinous material, often in quantity and quality sufficient 

 to give these plants value as food, as in case of the Irish Moss, 

 from which a blanc-mange is made, the Dulse, a favorite raw 

 comestible of children in seaport towns, and the Agar- Agar, 



Fig. 310. — Rhodomela subfusca, var. gra- 

 cilis, a typical Red Alga ; X §• (From 

 Hervey's Sea Mosses.) 



