r 



MORPHOLOGY OF THE FLORAL ORGANS 127 



Morphologically the awn is thought to represent the 

 blade, and the lemma the sheath of a primary leaf. If the 

 awn is dorsal the free portion of the lemma above the 

 insertion of the awn probably represents the ligule. (See 

 Domin, Ann. Jard. Bot. Buitenzorg 24:200. 1910.) 



157. Twisted awns. — Not infrequently the awns are 

 spirally twisted. This torsion is well shown in the large 

 awns of certain species of Stipa, such as the porcupine- 

 grass of the prairies {S. spartea Trin.). The awns are 

 several inches long, stout at base but tapering to a fine 

 point (Fig. 36). The awn at first is straight and untwisted, 

 but at maturity it bends at 2 points and becomes closely 

 spirally twisted up to the second bend. The torsion is 

 very sensitive to atmospheric moisture so that the awn 

 becomes less twisted or almost straight in moist air and 

 twists tightly again in dry air. Twisted awns are found 

 especially in Andropogonese (Fig. 16), Avenese (Fig. 44), 

 and Stipa. 



158. The palea is the bract standing between the 

 flower and the rachilla. It is usually 2-nerved or 2- 

 keeled with the space between the nerves concave and 

 with the margins bent forward about the flower. It is 

 homologous with the prophyllum which it resembles in 

 structure. The palea is usually embraced by the lemma at 

 the margins, or sometimes entirely inclosed as in Stipa 

 and Aristida, although it may project more or less at the 

 apex. Though the palea is usually 2-nerved, it is appar- 

 ently 1-nerved in a few genera because the 2 nerves are 

 so close together (e.g., Cinna). So-called 1-nerved paleas 

 occur only in 1-flowered spikelets. The apex of the palea 

 is usually rounded or notched but may be toothed, the 

 teeth being rarely awned. The keels are usually smooth 

 or scabrous but may be ciliate (Eragrostis), winged (Pleu- 



