HOW TO KNOW THE SEAWEEDS 



171a Medulla of middle parts of blades composed of a single, uniform 

 layer of large cells. Fig. 214 172 



171b Medulla composed throughout of more than one layer of large 

 cells, or at least the medullary cells not uniform 173 



172a Blades with both medulla and cortex monostromatic throughout. 

 Fig, 214 Dictyota 



Fig. 214. Dictyota sp. 



Transection of the margin of a 

 blade showing the monostromatic 

 cortex and monostromatic medul- 

 Figure 214 la. In the genus Pachydictyon 



(Fig. 215) the medulla is distro- 

 matic at the margins and the cortex of two or more layers. 



Dictyota is a warm water genus represented by one common spe- 

 cies, D. flabellata (Collins) Setchell & Gardner, in southern California, 

 and by several species in Florida. D. dichotoma (Hudson) Lamouroux, 

 which extends from North CaroHna southward, may be considered 

 the commonest Atlantic species. Although some of the species are 

 quite broad and resemble the figure of Pachydictyon (Fig. 215), others 

 are narrow and only 1 to 2 mm. broad. Altogether, about nine species 

 are known from our coasts. 



172b Blades with distromatic medulla at the margins and di-polystro- 

 matic cortex at the margins. Fig. 215 Pachydictyon coriaceum 



Fig. 215. Pachydictyon coriace- 

 um (Holmes) Okamura 



The upper portion of a plant 

 to show the branching habit, 

 X 0.56. This is the only mem- 

 ber of this genus in our terri- 

 tory. It is common in southern 

 Cahfornia and is readily dis- 

 tinguished externally from the 

 similar Dictyota flabellata by 

 its coarser texture and usually 

 darker color. 



Figure 215 



162 



