230 LEMNACEAE 
Plant-body elliptic or oblong: utricle elongated. 8. L. minima. 
Plant-body unsymmetrical. 
Plant-body thick, obliquely obovate, papillose on the median line. 4. L. perpusilla. 
Plant-body thin, oblong, smooth 
Plant-body manifestly 3-nerved: root-sheath with lateral appendages. 5. L. trinervis. 
Plant-body obscurely 1-nerved : root-sheath without appendages. 6. L. cyclostasa. 
l. Lemna trisülca L. Plants prolific, often forming dense colonies of several gen- 
erations. Root-sheath acute: plant-bodies both submerged and aérial, oblong to oblong- 
lanceolate, often faleate, 5-10 mm. long, acute, smooth, attenuate at the base or hastate 
when young: aérial plants smaller than the submerged and short-stalked, cavernous ; sub- 
merged plants with twisted stipes: seeds prominently 12-15-ribbed. 
Mostly in springs and flowing water, Nova Scotia to British Columbia, North Carolina, Alabama, 
New Mexico and California. Alsoin the eastern hemisphere. Summer. 
2. Lemna minor L. Plants solitary or few in colonies, prolific. Root-sheath short, 
obtuse: plant-bodies thickish, elliptic, obovate or suborbicular, 2-4 mm. long, obscurely 
mostly 3-nerved, hollow, commonly keeled on the back and papillose on the midrib; 
apical papillae prominent : spathes cleft, finally ruptured: seeds uneven, 12-15-ribbed. 
In ponds, lakes and stagnant water, Ontario to Vancouver Island, Florida and California. Summer. 
3. Lemna mínima Philippi. Plants solitary or 2, or sometimes 4, in colonies Root- 
sheath short, curved: plant-body oblong or elliptic, 1.5-3.9 mm. long, rounded at the 
apex, sometimes slightly oblique, nearly nerveless, convex above, narrowly margined, 
cavernous in the central portion : seeds oblong, acute, 1.5 mm. long, commonly 16-ribbed. 
Instill water, Wyoming to Georgia. Florida and California. Summer. 
4. Lemna perpusílla Torr. Plants solitary or more commonly 2-6 in colonies. 
Root-sheath acute, 0.8-2 mm. long: plant-body thick, obovate, usually obliquely so, un- 
symmetrical, 2-3.5 mm. long, obtuse, hollow, contracted at the base, 3-nerved, often papil- 
lose along the midrib and with a larger apical papilla: utricle ovoid or oblong, tipped 
with the eccentric style : seeds oblique in the utricle 12-16-ribbed. 
In ponds and streams, Massachusetts to Minnesota, Nebraska and Florida. Spring to fall. 
5. Lemna trinérvis (Austin) Small. Plants solitary or in colonies of 2 or3 or 
rarely more. Root-sheath with lateral appendages : plant-body thin, oblong or nearly so, 
slightly unsymmetrical, little convex above, flat beneath, strongly 3-nerved, abruptly nar- 
rowed to the blunt apex, smooth : utricle tipped with the terminal style: seeds erect or 
nearly so in the utricle. [L. perpusi/la var. trinervis Austin. ] 
In still water, New Jersey to Missouri, Louisiana and the Indian Territory. Spring to fall. 
6. Lemna cyclostasa (Ell) Chev. Plants solitary or usually 2-8 in colonies. 
Root-sheath rather elongated, without appendages: plant-body oblong or nearly so, 2.3- 
4.5 mm. long, obtuse, hollow in the middle, often somewhat falcate, unsymmetrical at the 
‘base, obscurely 1-nerved : utricle tipped with the style: seeds 12-29-ribbed. 
In ponds and rivers, Massachusetts to California, Florida, Mexico and South America. Summer. 
4. SPIRODELA Schleid. 
Plant-body disk-like, conspicuously several-nerved, with the stipe peltately attached 
back of and under the basal margin. Roots several slender, each with a sheath. Propa- 
gative pouches 2, triangular, opening by slits in either margin of the basal portion of the 
plant-body. Spathe sacdike. Spadix of 1 pistillate and 2 staminate flowers arising from 
a propagative pouche. Anthers 2-celled : sacs opening lengthwise. Ovary with 1 am- 
phitropous ovule or with 2 anatropous ovules. Utricle lenticular, with winged margins. 
1. Spirodela polyrrhiza (L.) Schleid. Plants solitary or 2-5 in colonies, mostly 
sterile and perennial. Roots 4-16 ; sheaths 1.3-1.5 mm. long, acute : plant-body obovate 
or suborbicular, 2.5-8 mm. long, 5-15-nerved : spathe sac-like, opening at the upper end : 
pollen-grains globose to ellipsoidal, 0.18-0.23 mm. in diameter, spinulose : utricle slightly 
winged : seeds somewhat flattened, smooth. [Lemna polyrrhiza L.] 
In ponds or pools, Nova Scotia to British Columbia, Florida, Texas and California. Also in tropi- 
cal America and the Old World. DUCKWEED. 
Order8. XYRIDALES. 
Terrestrial, commonly bog-inhabiting herbs, or epiphytes (BROMELIACEAE). 
Leaves often basal: blades prevailingly narrow. Inflorescence usually not a 
spadix, but often with a spathe-like bract or a spathe. Flowers mostly com- 
plete. Corolla regular or nearly so (except in some genera of COMMELINACEAE, 
