HOW TO KNOW THE SEAWEEDS 



5b Thallus of various form and structure, but consisting neither of 

 one or more branched or imbranched uniseriate filaments, nor of 

 a branched, free, essentially cylindrical filament without cellular 

 septations. (Note that some young stages, such as in Bangia. may 

 be uniseriate; some Caulerpa species are coenocytic but not cylin- 

 drical; Codium is coenocytic but consists of interwined filaments 

 whose only free parts are the external utricles) 6 



6a Thallus membranous, only 1-2 cells thick, at least at the margins. 

 Fig. 16 52 



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Fig. 16. A cross section of a distromatic 

 thallus (2 cells thick) as occurs in Ulva. 



Figure 16 



6b Thallus cylindrical or flattened; if membranous, with more than 

 two layers of cells at the margins 7 



7a Vegetative portions of thallus dominontly compressed, flattened, or 

 complanate. Fig. 17 125 



C 



D 



B 



Fig. 17, Cross sectional forms of (A.) 

 compressed, (B.) flattened, and (C.) 

 complanate thalli. 



Figure 17 



7b Vegetative portions of thallus dominantly of cylindrical form, or, if 

 compressed, only slightly so or only in restricted areas such as the 

 points of branching 8 



30 



