26 



FIRST BOOK OF GRASSES 



or be along the margins only. These differences 

 distinguish the species. A genus is composed of one 

 to many species having few to several important 

 characters in common, and presumably descended 

 from a common ancestor. Groups of species having 

 less important characters in common form related 

 genera. Festuca (Fig. 12) is related to Bromus, 



having few to several-flowered 

 spikelets, disarticulating above 

 the glumes and between the 

 florets and with several-nerved 

 lemmas; but the lemmas are 

 awned from the tip, or pointed 

 only, and not toothed, and 

 the palea is not grown to the 

 grain. 



A large nimiber of grasses 



have laterally compressed 



spikelets of this general type, 



,„ . ., , . r E. disarticulating above the 



Fig. 12. A, spikelet of Fes- ^ n 



tuca ovina; B, lemma de- glumes and between the few 

 *^'^"^- to many florets. They are 



sorted into genera and the genera separated from 

 each other chiefly according to the modifications 

 of the lemma. In Panicularia (Fig. 13) the lemmas 

 are broad and obtuse with strong parallel nerves. 

 In Poa (Fig. 14) the lemmas are keeled on the back, 

 and have five nerves converging toward the acute 

 but never awned apex. The species figured (Ken- 

 tucky blue-grass, Poa pratensis) and many others 



