CHAPTER IV. 



THE THALLUS OF THE HIGHER ALG^. 



30. The larger algae. — From the thread-form algae, 

 whose body is a single row of cells, it is but a step to those 

 forms whose body consists of a single sheet of cells. One 

 common form has a leaf-like body, which grows attached to 

 stones or other algae. The broader forms are sometimes 

 20—25 cm. wide. 



The body of the sea-lettuce is somewhat similar in structure, 

 but consists of two layers of cells, 

 and, as fig. 23 shows, is very clearly 

 distinguishable into an organ of 

 attachment, the rhizoid, and the leaf- 

 like part for which the name thallus 

 may be kept. 



So, from the thread-like bodies we 

 pass through sheet-like to massive 

 bodies of a broadly extended form. 

 Likewise there may be found all in- 

 termediate forms between the thread- 

 like algae and those whose bodies 

 are slender, but are more than one 

 row of cells thick. 



Fig. 23. — A small pla 

 iactuca, the sea^ettuce^ show- 

 ing thallus, and rhizoid for 

 attaching it to rocks. Natural 

 size. — From Bessey. 



In other marine algae a still higher specialization of members is reached. 

 One of the red seaweeds may be used to show the gradual advance in 

 complexity. 



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