HOW TO KNOW THE FRESH-WATER ALGAE 



189a (35) Plant a microscopic unbranched filament, attached or free- 

 floating: or a macroscopic thallus in the form of an expanded 

 sheet, a tube, or an arbuscular (tree-like) gelatinous and beaded 

 growth 190 



189b Plant a branched filament, a coenocytic tube (without cross walls) 

 or an attached cushion or disc in which the branching habit is 

 obscure because of closely oppressed branches 241 



190a Cells constricted in the midregion, with 2 large chloroplasts, one 



in either 'semicell' 191 



(In some species of filamentous desmids the constriction is very 

 slight, being only a slight invagination or concavity of the lateral 

 walls. See illustrations of Hyalotheca, Fig. 137, and of Gymnozy- 

 ga. Fig. 136 before proceding in the key.) 



190b Cells not constricted in the midregion 198 



191a Cells adjoined by the interlocking of short, straight, horn-like or 

 hooked processes at the poles 192 



191b Cells adjoined by their end walls, either along the entire apical 

 surface, or at the ends of arms which project from the apex. . 194 



192a Interlocking polar processes simple, slender and horn -like. Fig. 

 133 ONYCHONEMA 



Fig. 133. Onychonema laeve var. 

 latum West <£ West. 



There are 2 rather common spe- 

 cies of this filamentous desmid 

 genus. O. tiliioime (Ehr.) Roy <& 

 Biss. has the lateral angles ot 



Figure 1 33 . 



the semicells furnished with a 

 spine and the polar processes are 

 relatively long. O. laeve Nordst. has cells without lateral spines and 

 appears like a small Cosmaiium (Fig. 113) in a filament, the cells ad- 

 joined by short (sometimes scarcely evident) polar processes. 



88 



