12 STRUCTURE 
the genera and species; more especially is this so with the flower- 
ing glume. The glumes are always nerved, ze. with unbranched 
veins, the central or dorsal nerve usually stronger than the lateral 
ones. The glumes may either be rounded on the back, or com- 
pressed and keeled, the keel being formed by the prominence of 
the dorsal nerve on the outer surface. The nerves vary in num- 
ber—one, three, five, etc., always an odd number—and either run 
parallel, or converge as they approach the apex of the glume; some- 
times they slightly project beyond the apex (excurrent), (fig. 21), or 
they may vanish below it. The glumes may be hairy or downy on 
the outer surface, or scabrous along the nerves. The two empty 
glumes at the base of the spikelet 
often differ from each other, as well 
as from the flowering glume, both 
in size and in the number of nerves 
(fig. 27). 
The fine hair or bristle which is 
often present on the flowering glume 
is called the awn. It maybe simply 
a prolongation of the dorsal nerve 
(figs. 16, 18), or it may be inserted 
at some point along that nerve (figs. 
8, 36), or it may arise from the base 
of the glume, and is denominated 
terminal, dorsal, or basal, according- 
ly. It varies in length in different 
species, and may be smooth or, as 
is often the case, scabrid. When 
the tip of the glume is split (bifid) 
or notched, the awn arises from the 
bottom of the notch or sinus, and is 
styled subterminal. Sometimes the 
awn is sharply bent (geniculate or 
é Hac Beatie she a ue kneed) as in the Oat, and the part 
feo, the sate ait sinptye plumes below the knee spirally twisted ; the 
removed to show dorsal awn of upper awn is then hygroscopic, the torsion 
flowering glume. increasing or diminishing with the 
varying humidity of the atmos- 
phere. This hygroscopic movement can readily be observed by 
placing a few glumes so awned into water, or upon a wet surface. 
In one exotic genus, Avés/¢da, the awn is compound, having three 
forks or branches ; another exotic genus, Pappophorum, is remark- 
able in having the flowering glume armed with a dozen or more 
awns. Generally speaking, the lateral nerves of the flowering 
glume are not sufficiently excurrent to form awns. In many cases 
the dorsal nerve too is only slightly excurrent, forming a short 
point or mucro (fig. 31). The empty glumes are, as a rule, either 
mucronate or awnless; Hordeum and Polypogon are familiar ex- 
ceptions, having long-awned empty glumes. The Panzcacee, one 
of the primary series into which grasses are classified, are in- 
