158 A TEXT-BOOK OF GRASSES 
genera and allied genera into higher groups without com- 
mitting ourselves as to how the various groups came to 
be what they are. The modern tendency is toward a 
grouping of species in all large genera. Some of these 
groups are recognized under the formal titles of sub- 
genera, sections and subsections. But it is often con- 
venient to form smaller groups centering around well- 
known or widespread species. 
Ascherson and Grabner bring together closely allied species 
under the heading Gesammtart (Syn. Mit.-Eur. Fl.). In our recent 
revision of the North American Species of Panicum (Contr. U. §. 
Nat. Herb. 15) these minor groups or species were indicated by the 
plural of the leading species, e.g., the allies of Panicum dichotomum 
were grouped under Dichotoma. 
