230 LEMNACEAE 



Plant-body elliptic or oblong: utricle elongated. 3. L. minima. 

 Plant-body uusymmetrical. 



Plant-body thick, obliquely obovate, papillose on the median line. 4. L. perpnsilla. 

 Plant-body thin, oblong, smooth. 



Plant-body manifestly 3-nerved : root-sheath with lateral appendages. 5. L. trinen-is. 



Plant-body obscurely 1-nerved : root-sheath without appendages. 6. L. cydostasa. 



1. Lemna trisiilca L. Plants prolific, often forming dens*' colonies of several gen- 

 erations. Root-sheath acute : plant-bodies both submerged and aerial, oblong to oblong- 

 lanceolate, often falcate, 5-10 mm. long, acute, smooth, attenuate at the base or hastate 

 when young : aerial 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. Also in 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 minima 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. 



In still water, Wyoming to Georgia. Florida and California. Summer. 



4. Lemna perpusilla 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 trin6rvis (Austin) Small. Plants solitary or in colonies of 2 or 3 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. perpusilla 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 sac-Jike. 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. 



Order 8. 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. Flow r ers mostly com- 

 plete. Corolla regular or nearly so (except in some genera of COMMELINACEAE, 



