Ulvacece 95 



Genus Trentepohlia Martins, 1817. [Chroolepus Ag., 1824.] 

 The thallus is filamentous, simple or ramified. The branches are 

 alternate, of the same diameter as the principal filaments, and do 

 not terminate in a point or hair. The chloroplasts are numerous, 

 discoidal, and without pyrenoids, but are generally masked by 

 the presence of a red or orange-red oil which colours blue with 

 iodine. 



The most abundant species in the British Islands is T. aurea Mart., which 

 occurs principally in hilly and mountainous districts, forming broad expanded 

 sheets of a bright red or orange-red colour. It is chiefly found attached to 

 rocks, particularly carboniferous limestone or Silurian rocks, and generally on 

 the windward side. The filaments are 10 20 p. in thickness (fig. 34 A C). 

 T. odorata (Ag.) Wittr [=T. umbrina (Kiitz.) Bornet] and T. calamicola 

 (Zeller) De Toni (thickness of filaments 7'5 10 p; fig. 34 D F) are much 

 smaller British species of the genus. 



Order III. UL VALES. 



This order is mainly distinguished from the Cha3tophorales by 

 the expanded, parenchymatous thallus, which is attached when 

 young to a substratum by ' rhizoids.' The cells are uninucleate 

 and they contain a single parietal chloroplast, often of considerable 

 bulk and containing one pyrenoid. 



Asexual reproduction takes place by zoogonidia and also by 

 gemmation. Sexual reproduction is by isogamous planogametes 

 with two cilia. 



There is only one family which has few freshwater repre- 

 sentatives. 



Family 1. ULVACEJE. 



The Algae belonging to this family are more often marine or 

 brackish in habit than freshwater. They consist of flat, ribbon- 

 shaped or expanded plates, or more rarely they exhibit a vesicular 

 or intestiniform structure. These flat or tubular structures consist 

 of one (Monostroma) or two (Ulva) layers of cells which may be 

 somewhat scattered and rounded in form, in which case they are 

 frequently arranged in groups of four, or they may be closely 

 compact with polygonal outlines. The two genera Monostroma 

 and Enteromorpha have freshwater representatives, and in each 

 case the thallus consists of a single layer of cells, division only 

 taking place in one plane. The cells are usually compact and 



