112 Alexander W. Evans, 



k. Surface-cells of axis about half as wide as the interior cells 

 or less ; branches often tapering gradually to blunt points 



9. R. corralensis (p. 143). 



k. Surface-cells of axis more than half as wide as the interior 

 cells ; branches often narrow but rounded at the tips /. 



/. Plants flaccid ; axis mostly four to six cells thick in the median 

 portion; dorsal surface-cells of. axis usually 60-80 [x wide, 

 the interior cells narrower 14. R. conimitra (p. 156). 



/. Plants rigid; axis mostly eight to twelve cells thick in the 

 median portion ; interior cells not narrower than the dorsal 

 surface-cells m. 



m. Dorsal surface-cells of axis mostly 30-40 ix wide n. 



m. Dorsal surface-cells of axis mostly 12-20 /x wide 0. 



n. Plants pale to bright green; main axis mostly 0.8-1 mm. wide 

 and 0.3 mm. thick 10. R. chilensis (p. 145). 



n. Plants pale to dark brown ; main axis mostly 0.3-0.4 mm. wide 

 and 0.2 mm. thick 12. R. fuscobrunnea (p. 152). 



0. Thallus smooth throughout ii. R. alcicornis (p. 148). 



o. Thallus roughened by crowded tooth-like or spine-like out- 

 growths 13. R. spinulif era (p. 1^4). 



p. Thallus vaguely differentiated into axis, photosynthetic 

 branches, and stolons ; interior cells of axis averaging 

 about 30 /x in width 15. R. autoica (p. 159). 



p. Thallus undifferentiated ; interior cells of axis averaging 

 about 50 fx in width 16. R. tenerrima (p. 164). 



q. Plants prostrate throughout the greater part of their extent 

 or at least with a prostrate axis r. 



q. Plants soon becoming ascending or suberect w. 



r. Thallus copiously branched s. 



r. Thallus sparingly branched u. 



s. Branches often bounded by a unistratose wing two cells wide 



17. R. diversiflora (p. 167), 



s. Branches rounded to subacute at the sides in cross section but 

 never winged t. 



t. Thallus apparently prostrate throughout ; axis and its branches 

 with a median ventral band of narrow cells containing 

 fungus hyphae 19. R. mycophora (p. lys)- 



