276 



Ulvales 



In Enteromorpha the adult thallus is tubular, having arisen by the 

 splitting of a two-layered membranous expansion. The cells are very 

 compact, with polygonal outlines (fig. 177 B), and in some species they are 

 arranged in longitudinal rows. Ilea greatly resembles Enteromorpha, but is 

 unbranched and the cells are arranged in regular groups of four. 



Fig. 177. A and B, Enteromorpha gracillima G. S. West. A, slightly less than natural size; 

 B, surface view of small portion of branch showing outlines of cells, x 440. C E, 

 Monostroma membranacea W. & G. S. West. C, entire thallus rather less than natural size; 

 D, section of thallus, x 495 ; py, pyrenoid ; E, conjugation of gametes (</) to form zygote (z) ; 

 gametes, x 495 ; ' zygozoospores ' and zygote, x 690. F, section of thallus of Ulva Lactuca 

 Linn, x 495; /, disc filaments. G, longitudinal section through the attaching disc of Ulva, 

 somewhat simplified and diagrammatic ; in this case the Ulva is attached to another seaweed 

 which is acting as host (h), x 87; /, disc filaments. F, after Thuret; G, after Delf. 



The cells are mostly disposed with their long axes at right angles to the 

 plane of the thallus (fig. 177 D and F). Each cell contains a single nucleus 

 and one large parietal chloroplast, often with lobed or deeply incised margins, 

 and with one pyrenoid. 



