HOW TO KNOW THE SEAWEEDS 



80b Thallus attached by rhizoids on the under side; cells not especially 

 small; tetrasporangia cruciate, borne in a superficial layer. Fig. 

 115 Peyssonelia 



Fig. 115. Peyssonelia sp. 



A vertical section through a tetraspor- 

 angia! thallus showing the rhizoids from 

 the basal cell layer and the position of 

 a tetrasporangium in a special, nemathe- 

 cial, superficial layer (nemathecium). Spe- 

 cies of this genus may be encountered en- 

 crusting rocks at low intertidal levels, or 

 infratidally, on either of our coasts. 



ooooooooooooo 

 mOOQOOOOOOOO 



Figure 1 1 5 



81a Crusts relatively thin, firm, somewhat brittle or woody. Fig. 116. 

 Ralisia 



Figure 1 16 



Fig. 116. Ralisia sp. 



An example of a loosely attached form 

 showing concentric lines of growth, X 1. 

 A number of species occur on rocks, some 

 of them very firmly attached and so flat, 

 thin, and dark that they may be mistaken 

 by the layman for patches of tar. Some 

 times they completely cover rock surfaces 

 to give the intertidal area a dark brown 

 color. They cne widespread along the whole 

 Pacific Coast and in the cooler northern 

 waters along the Atlantic. They are usually 

 at rather high intertidal levels and are re- 

 sistant to considerable desiccation. 



89 



