A REVISION OF THE AMERICAN LEMNACEAE. i 
Spirodela Schleiden, Linnaea. 18: 391. 1839. 
Stipe attached (peltately ) to the frond back of and under 
the basal margin. Reproductive pouches two, triangular, 
opening as clefts in either margin of the basal portion of 
the frond. Roots more than one, fascicled. Spadix of 
one pistillate and two staminate flowers from the vegeta- 
tive reproduction pouches; spathe sac-like; filaments curv- 
ing upward from the margin of the frond; anthers 
bilocellate, dehiscing longitudinally. Fruit rounded len- 
ticular, with wing margins. 
SprropELA poLyrRuiza (L.) Schleiden, Linnaea. 13: 392. 
1839. Lemna polyrhiza Linn. Sp. Plant. 970. 1753. 
Lemna thermalis Beauv. Mémoire sur les Lemna. 13. 
11 Sept. 1815. Lemna umbonata A. Br. ex Hglm. 
Monogr. Lemnac. 156. May 1868. 
Fronds solitary or united in colonies of 2-5, roundish 
obovate, slightly unsymmetrical, flat on both sides, sessile 
or short stipitate, stipe deciduous; apex obtuse or acute ; 
5-15-nerved, 2.5-4.5 mm. wide by 2.5-8 mm. long (usu- 
ally 3-6 mm. long). Roots 4-16; rootcap large, straight, 
176-.235 mm. in diameter by 1.3-1.5 mm. long, sharp 
pointed. Usually sterile and wintering over by specialized 
resting fronds. Fertile fronds extremely rare. Spathe a 
complete sac, opening at the upper end. Anthers .33 mm. 
in transverse diameter; pollen grains spinulose, globose to 
ellipsoidal, .018-.023 mm. in diameter. Pistil flask shaped ; 
ovules 2, anatropous, or frequently 1 and then amphitro- 
pous. Fruit somewhat winged along the margin; seed 
slightly compressed, smooth; spongy testa very thick; 
operculum distinct; endosperm dark brown, thin; albu- 
men considerable; embryo cylindrical, nearly as long as 
the seed.— Abundant in ponds, rarely occurring in running 
water, thriving for a considerable time on very wet mud. 
A cosmopolitan species occurring abundantly throughout 
our range — Plate 1 A. 
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