8 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



The geographical range of a form is to be taken into consideration 

 along with the morphological characters, in determining the taxonomic 

 standing of a given group. A group distinguished by small differ- 

 ences in morphological characters may be assigned subspecific or 

 even specific rank when the differences are supported by a distinct 

 or at least a different geographical range. Groups that are not 

 sufficiently distinct to be assigned subspecific rank are mentioned 

 after the general description of the species and specimens are cited as 

 illustrations. In a few cases these citations are given in a formal 

 manner to emphasize the distinctness of the group, in which cases the 

 same specimens do not occur among those cited in the general dis- 

 tribution. Usually the specimens mentioned as illustrating a par- 

 ticular departure from the typical form are cited by collector and 

 number or date only, and are repeated under "Distribution." The 

 locality is given in such cases only when the geographical range is of 

 significance. 



GEOGRAPHICAL DISTRIBUTION. 



The species of the genus Panicum, numbering probably about five 

 hundred., are found in the Tropics and warm temperate regions of both 

 hemispheres. In North America the genus extends throughout the 

 West Indies, Central America, Mexico, and the United States and into 

 the southern part of the Dominion of Canada. The subgenus Dichan- 

 thelium, which includes over one-half the species of the entire genus 

 as represented in North America, is confined to the Western Hemi- 

 sphere and almost to North America. The center of distribution of 

 this group is the Atlantic Coastal Plain of the Southeastern States, 

 whence it extends north to the eastern Canadian Provinces, west to 

 the Pacific coast and British Columbia and south through Mexico and 

 the West Indies to northern South America. A few species of true 

 Panicum, such as P. virgatum and species of Capillaria, also extend 

 over a wide range. The latter group is represented from Maine to 

 British Columbia and southward into South America. The genus is 

 poorly represented in the mountainous regions of the United States 

 and not at all at high altitudes. Panicum thermale is found around 

 hot springs in the Rocky Mountains at an altitude of 2,000 meters, but 

 this is an exceptional case. The genus is poorly represented on the 

 Great Plains and especially in the arid regions of the Great Basin. 

 As to habitat, the species are found growing under a variety of con- 

 ditions, but rarely in shady, hard- wood forests. 



CITATION OF SPECIMENS. 



The general range of all species is given, so far as indicated by 

 specimens, even when this range extends beyond the limits of 

 North America. The detailed citation of specimens is given by 



