vi PREFACE 
The key to genera includes all the genera found grow- 
ing wild or in common cultivation in the United States. 
More complete descriptions of the more important genera 
are added under each tribe. For use as a reference work 
it would have been desirable to give full descriptions of 
each genus. But again the author’s course was modified 
by didactic requirements. It is unnecessary for the 
student to acquire information on the unimportant genera. 
The nomenclature followed is that of the American 
Code. Synonyms are introduced whenever a species or 
genus has been commonly known under another name. 
After careful consideration, the English system is 
used for all measurements except the small fractions of 
an inch. But for the smaller measurements the milli- 
meter is adopted as the unit. The English system is as 
yet more familiar than the metric for the larger measure- 
ments. The small fractions of an inch however are incon- 
venient. The line might be used but is unfamiliar and is 
too large a unit. The millimeter meets the requirements 
as to convenience and size of unit, and is sufficiently 
familiar to botanical students. | 
The habit drawings have been made by Mrs. Mary 
Wright Gill, the detailed drawings of the spikelet by Mrs. 
Agnes Chase. 
A..S. HITCHCOCK. 
WASHINGTON, D. C. 
May 12, 1914. 
