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 Andropogonee (Fig. 16), Avenes (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- 
