THE NORTH AMERICAN SPECIES OF STEREOCAULON 
LINCOLN WARE RIDDLE 
(WITH NINE FIGURES) 
The American species of Stereocaulon fall naturally into two 
groups. One is a typically boreal group, with S. paschale the 
central species, and a general distribution throughout British 
North America and the northern United States, as far south as 
North Carolina in the Alleghany Mountains, and in the Rocky 
Mountains extending into Colorado, and in the case of some of the 
species even into the mountains of Mexico and the South American 
Andes. The other group is a tropical one, represented by S. 
ramulosum and its allies, and characteristic of tropical America, 
from which it extends rather widely into the southern hemisphere. 
No American herbarium contains sufficient material for a satis- 
factory study of these tropical species, and for that reason the 
present paper is confined to a consideration of the boreal species. 
Anyone who has attempted to determine material of Stereo- 
caulon, or has studied the material in our herbaria, must have 
realized the confusion which exists in regard to the distinctive 
characters of the species. This confusion arises in part from the 
great variability of the species, and their tendency, in some cases, 
to intergrade. But another cause of the confusion comes from the 
lack of literature available to the general student and containing 
comparative notes on the species. This lack of comparative notes 
makes TUCKERMAN’S Synopsis of the North American Lichens, which 
has been the guide for most of the study of the American material, 
a difficult treatment to use. The classic work on the genus Siereo- 
caulon is to be found in the papers of Tu. M. Fries, his De Stereo- 
caulis et Pilophoris commentatio being published in 1857, and his 
Monographia Stereocaulorum et Pilophororum in 1858. These must 
orm the basis for any careful study, but unfortunately they are 
tather inaccessible to American students. 
The present paper is an attempt to clear up some of the existing 
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ans [Botanical Gazette, vol. 50 
