HOW TO KNOW THE SEAWEEDS 



32a Thalli with typical, externally visible pericentral cells. Fig. 54 . . . 

 49 



B 



Figure 54 



Fig. 54. Appearances of pericentral cells in two different plants. 



A. The apex of a Polysiphonia species showing short pericentral 

 cells, scar-cells, trichoblasts and apical cells. X 100. B. A short piece 

 of a Lophosiphonia species with a large number of long pericentral 

 cells surrounding developing tetrasporangia, X 150. Lophosiphonia 

 is a small, inconspicuous plant not treated in the key. 



32b Thalli without typical pericentral cells, or if present, these ob- 

 scured by cortication 33 



53 



