HITCHCOCK-——MEXICAN GRASSES. 
Spikelets glabrous or minutely pubescent only; rachis green. 
Racemes 30 or more, approximate, the rachis scabrous; 
spikelets pointed; aquatic plants, the sheaths of 
the floating stems inflated, bladder-like. 
Spikelets in 2 rows, more or less pubescent, about 
0.8 mm. wide........- pee eee eee ee eeeeeeee 
Spikelets in a single row, glabrous, about 1.2 mm. 
wide, the tip of the rachis elongated... -.-.-- 
Racemes 4 to 12, scattered, the rachis glabrous; plants 
terrestrial. 
Spikelets pointed, 3.5 to 4 mm. long, imbricated, 
Spikelets blunt, 2 to 2.2 mm. long, alternate, not 
imbricated, white..........---..----+------ 
Rachis without broad membranaceous wings, or if somewhat 
winged, the spikelets not long-silky. 
Fertile lemma with five longitudinal ridges; both glumes 
wanting (§ ANACHYRIS)........--------++-+-0+----- 
Fertile lemma not ridged (§ OrisTHIon). 
Racemes 2, conjugate at the summit of the culm, rarely 
a third below; spikelets solitary (see also P. cla- 
vuliferum). 
Spikelets elliptical or ovate; first glume occasion- 
ally present; plants with extensively creep- 
ing rootstocks. 
Second glume not wrinkled, the sterile lemma 
pubescent..........-------0+- eee eee 
Second glume wrinkled in the internerves; 
spikelets glabrous...........------------ 
Spikelets nearly orbicular to obovate. 
Racemes slender, elongated; spikelets concavo- 
convex, light yellow, sparsely long-silky 
around the margin; plants stoloniferous. 
Racemes rather stout, not elongated; spikelets 
plano-convex, green, glabrous; plants 
with short stout rootstocks but not stolo- 
niferous. 
Spikelets 2 to 2.5 mm. long......-------- 
‘Spikelets 3 to 3.8 mm. long......--.------ 
Racemes 1 to many, racemose on the axis, not conjugate 
at the summit of the culm, or if occasionally so the 
spikelets in pairs. 
Fruit dark brown, polished; second glume and 
sterile lemma thin, fragile. 
Plants perennial, erect; second glume trans- 
versely wrinkled inside the margin.-...- 
Plants annual, more or less spreading or pros- 
trate; second glume not wrinkled ....-- 
Fruit not dark brown (brown but not polished in 
P. virgatum). 
Spikelets clothed with long silky hairs; lower 
sheaths densely appressed-villous...-.-..-.- 
26. 
25 
bo 
3. 
.&P. 
.P. 
. =P, 
. P. 
.P. 
* phyllum. 
hy hy 
2, 
227 
repens. 
longicuspe. 
acuminatum. 
prostratum., 
matlaco- 
. distichum. 
. vaginatum. 
. conjugatum. 
. minus. 
. notatum. 
. plicatulum. 
cOnveELUM. 
. erianthum. 
