HOW TO KNOW THE SEAWEEDS 



151b Surface of blades smooth. 



152 



152a Thallus dark brown or yellowish-brown (as opposed to reddish, 

 purplish or greenish) 153 



152b Thallus reddish, purplish, or sometimes greenish, but not brown 

 or yellowish-brown 154 



153a Thallus several layers of cells thick, the medullary cells very dif- 

 ferent, especially in size, from the surface cells. Fig. 191 



Ilea (Petalonia) fascia 







Figure 191 



Fig. 191. Ilea fascia (MUller) Fries 



Part of a transection of a blade showing the difference in size be- 

 tween the medullary cells and the surface cells. The vertical rows of 

 small cells represent gametangia, X 150. 



This is a common species along practically the entire Pacific and 

 Atlantic coasts. Its gross appearance is essentially like that shown for 

 Punctaiia in Fig. 193, but its structure is quite distinct from that genus. 

 In southern California it may be confused with Endarachne binghamiae 

 J. Agardh which is almost identical in external appearance. Again, 

 however, a transection readily distinguishes the two, for Endarachne 

 has a medulla of intertwined filaments. 



148 



