Howe: American Species of Alectoria 



109 



the limits of variation in this genus, and I take it that to make 

 possible a scientific comprehension of species is one aim of syste- 

 matic botany. 



Key to the Species 



A. Thallus dark (brown) throughout. 



a. Prostrate. 



Rigid. 



Unicolored. [boreal 

 Stout, apices furcate, esoraliate diver gens] 



Unicolored or partially bicolored. [austral to subboreal 

 Stout, apices simple, soraliate chalybeiformis] 



Subrigid. 



Bicolored. [subboreal to boreal 

 Slender, apices simple, esoraliate bicolor] 



b. Pendulous. 



Lax. 



Unicolored. 



Slender throughout. [transitional to subboreal 



White soraliate implexa] 



Stout, extremities capillaceous. 

 White soraliate. 



Apothecial margins entire. [transitional 



Disk brown jubata] 



Apothecial margins ciliate. [alpine 



Disk brown oregana] 



Yellow soraliate. 



Apothecial margins entire. [transitional to subboreal 

 Disk yellow Fremontii'] 



B. Thallus light (virescent) or partially so. 



c. Erect. 



Rigid. 



Unicolored. [alpine 

 Slender, apices minute osteina] 



Bicolored, darker above. [boreal 



Slender, apices livid nigricans'! 



[boreal 



Stout, apices black ochroleuca] 



b. Prostrate. 



Rigid. 



Unicolored, occasionally blackening throughout. 



Stout, cavernous. [boreal 

 Extremities noncapillaceous Cincinnati] 



c. Pendulous. 



Subrigid. 



Unicolored. 



Slender, esulciform. 



Extremities subcapillaceous. [transitional 



