GRAMINEAE. 79 



[Old Latin name for some grass, probably the cultivated Sorghum, referring to its 

 panicle, taken from Pliny.] About 300 species, in temperate and tropical regions. 

 The geographic distribution of many of our species is not well ascertained. The 

 old English name Panic or Panic-grass is often applied to any of the species. 



Palet of the fourth scale much enlarged at maturity, forcing the spikelet open and mak- 

 ing it gape. i. P. hians. 

 Palet of the fourth scale not enlarged, smaller than the scale. 



Spikelets roughened with numerous tubercles, glabrous. 2. P. verrucosum, 



Spikelets not tuberculate, or if so, the tubercles bearing hairs. 



(A) Basal and culm leaves similar, usually elongated ; spikelets lanceolate to 



ovate, commonly the former, acute to acuminate. 

 Lower sheaths round or but little flattened, not keeled. 



Culms branched, at least at maturity ; no scaly rootstocks nor stolons. 

 Sheaths very pubescent. 



Spikelets lanceolate, 3 mm. long or less. 



Annuals; pedicels not usually more than 2-3 times as long 



as the spikelets. 



Culms stout ; panicle usually included at the base, its 

 many branches repeatedly divided and bearing nu- 

 merous spikelets. 3. P. capillare. 

 Culms slender ; panicle exserted, its few branches 

 but little divided, and bearing comparatively few 

 spikelets. 

 Spikelets 2 mm. long, acute. 



4. P. Philadelphicum. 

 Spikelets 3 mm. long, acuminate. 



5. P. flexile. 

 Perennial ; pedicels commonly many times as long as 



the spikelets. 6. P. cognatum. 



Spikelets ovate, 5 mm. long. 7. P. miliaceum. 



Sheaths glabrous. 8. P. proliferum. 



Culms simple, from stout, often scaly, rootstocks. 



Culms tufted ; rootstocks scaly. 9. P. virgatum. 



Culms not tufted ; rootstocks not scaly ; leaves glaucous and 



very thick. 10. P. amarum. 



Lower sheaths much compressed, broad, keeled, often equitant. 

 Fourth scale of the spikelet sessile. 



Spikelets 3-3.5 mm. long, the apex curved. 13. P. rostratum. 

 Spikelets less than 3 mm. long, the apex not curved, 

 Ligule naked ; culms finally much branched. 



11. P. agrostoides. 

 Ligule ciliate ; culms simple or nearly so. 



12. P. longifolium. 

 Fourth scale of the spikelet distinctly stalked. 14. P. stipitatum. 



(B) Basal and culm leaves dissimilar, the former generally much shorter and 



broader than the latter ; rosulate tufts of leaves often present in the 

 fall ; spikelets oval to obovate or globose, obtuse, or rarely acutish (acute 

 in No. 15). 



(A) Middle leaves of the main culm less than 1.5 cm. wide, usually less than 

 i cm. in width, the base rounded or truncate, rarely subcordate, 

 sometimes narrowed toward the base. 

 (a) Leaves elongated, erect, narrow, distinctly narrowed toward the 



base, especially the lower ones. 

 Secondary panicles borne on short basal branches, which are nearly 



concealed in the dense tufts. 

 Spikelets less than 3 mm. long, obtuse or acutish, pubescent. 



16. P. linearifolium. 

 Spikelets 3-4 mm. long, acute, glabrous. 



15. P. depauperatum. 

 No concealed basal panicles. 



Sheaths glabrous, or merely ciliate on the margins. 

 Leaves linear, less than 5 mm. wide. 



17. P. Wcrneri. 

 Leaves linear-lanceolate, 6-10 mm. wide. 



1 8. P. Bicknellii. 



Sheaths densely pubescent. 19. P. laxiflorum. 



() Leaves not elongated, lanceolate, not manifestly narrowed toward 

 the base, 



