114 BULLETIN 772, U. S. DEPARTMENT OF AGRICULTURE. 



49. AIRA L. 

 (Descliampsia Beauv.) 



Spikelets 2-flowered, disarticulating above the glumes, the hairy 

 rachilla prolonged behind the upper floret as a stipe, this sometimes 

 bearing a reduced floret; glumes about equal, acute 

 or acutish, membranaceous ; lemmas thin, truncate 

 and 2 to 4 toothed at the summit, bearing a slender 

 awn from or below the middle, the awn straight, bent, 

 or twisted. 



Low or moderately tall 

 annual or usually peren- 

 nial grasses, with shining 

 pale or purplish spikelets 

 in narrow or open pani- 

 cles. Species about 35, in 

 the temperate and cool 

 regions of both hemispheres, 6 of these 

 being in the United States. 



Type species : Aira caespitosa L. 



Aira L., Sp. PI. 63, 1753 ; Gen. PI., eel. 5, 31. 

 1754. Fourteen species are described. The 

 name was first used for a genus by Linnaeus 

 in his Flora Lapponica in 1737, where he 

 describes four species. These four species 

 are named in the Species Plantarum : 7. A. 

 spicata, 8. A. caespitosa, 9. A. flexuosa, 10. 

 A. montana. The first of these, A..spicata, 

 is referred to Trisetum ; the other three be- 

 long to Deschampsla, as recognized in most 

 American botanies. The genus Aira, as 

 accepted by Bentham and Hooker in the 

 Genera Plantarum and by Hackel in the 

 Natiirlichen Pflanzenfamilien, is based upon 

 the last two of the original Linnseari 



FIG. 59. Tall oat-grass, Arrhenathcrum elatius. Plant, X i ; spikelet and fertile floret, 



X 5. 



