PREFACE. 
The accompanying paper by A. 5. Hitchcock, Systematic Agros- 
tologist of the United States Department of Agriculture, and Agnes 
Chase, Assistant Agrostologist, is supplemental to a revision of the 
genus Panicum published by the same authors in volume 15 of the 
Contributions under the title of ‘‘ The North American Species of 
Panicum.” As stated in the preface to that paper, it was impossible 
at that time to offer a satisfactory treatment of the tropical species. 
Since then much additional material, including collections made by 
the authors, has been accumulated. The present paper includes the 
results of the study of these collections and presents a fairly detailed 
account of the genus in tropical North America. 
There are included 116 species and 3 subspecies. Of these, 9 species 
are described as new; 5 are South American species which were un- 
known within the limits of the area covered; 1 is a species recently 
described ; and 4 are species that were included in the list of synonyms 
or of dubious species but are now considered valid. In all 19 species 
are added, making the total for the genus Panicum in North America 
216. Each species is accompanied by an outline map graphically 
representing the geographical distribution within the Tropics of North 
America. 
FREDERICK V. COVILLE, 
Curator of the United States National Herbarium. 
Ill 
