STRUCTURE, MORPHOLOGY, AND PHYSIOLOGY. 33 



are probably about 3500 well-defined species. As in 

 most isolated families, the subdivision of the grasses is 

 very difficult and nowhere consists in single characters, 

 but in a combination of them. No single tribe, no large 

 genus numbering over 50 species, can be definitely 

 characterized. The keys of analysis in Part II, espe- 

 cially the first, are therefore subject to many exceptions. 



