LEMNACEAE. 233 



slendei stipe, thin, denticulate, with or without rootlets, and slightly 3-nerved; 

 several individuals often remain connected, so as to form a chain-like series; root- 

 cap pointed; fruit symmetrical; seed deeply 12-15 -ribbed. In water, N. S. to 

 X. J., N. Mex., Br. Col. and Cal. Also in Europe, Asia, Africa and Australia. 

 July-August. 



2. Lemna cyclostasa (Ell.) Chev. Valdivia Duckweed. (I. F. f. 886.) 

 Thallus oblong-elliptic, 2-4.5 mm - l° n g? thin, subfalcate and shortly stalked 

 at the base, provided with numerous stomata, except on the borders, nerveless; 

 rootcap blunt; spathe reniform; fruit ovoid-oblong, unsymmetrical; seed prom- 

 inently 12-29-ribbed. In pools and rivers, Mass. to Fla., 111., Wyo. and Cal. 

 Also in South America. June-July. (L. Valdiviana Phil.) 



3. Lemna minima Philippi. Least Duckweed. (I. F. f. 886a.) Thallus 

 oblong to elliptic, 2-4 mm. long, obscurely I -nerved, or nerveless, with a row of 

 papules along the nerve, the lower surface flat, or slightly convex; the apex 

 rounded, rootcap usually short, a little curved, rarely perfectly straight, cylindric, 

 blunt; spathe open; pistil short-clavate; stigma concave; ovule solitary, obliquely 

 orthotropous ; seed oblong, pointed, about 16-ribbed, with many transverse stri- 

 atums. Ga. and Fla. to Kans., Wyo. and Cal. 



4. Lemna perpusilla Torr. Minute Duckweed. (I. F. f. 887.) Thallus 

 small, 2-3.5 mm - l° n g> obovate, often purplish tinged beneath, unsymmetrical and 

 abruptly narrowed to a very short stalk, provided throughout with numerous 

 stomata, obscurely 3-nerved; rootcap straight, pointed; fruit ovoid; seed 12-20- 

 ribbed. In ponds, rivers, springs and lakes, Mass. to N. Y., Fla., Minn., Neb. 

 and Kan. June-July. 



Lemna perpusilla trinervis Aust. Thallus oblong to oblong-obvate, less unsym- 

 metrical, thin, prominently 3-nerved. N. J. to Mo., La. and the Ind. Terr. 



5. Lemna minor L. Lesser Duckweed. (I. F. f. 888.) Thallus obovate 

 or subcircular, 1.5-5 mm. long, thickish, rarely reddish or purplish tinged, short- 

 stalked when young, provided throughout with stomata, nearly symmetrical, 

 obscurely 3-nerved, very rarely 4-5 -nerved. Rootcap obtuse or subtruncate; fruit 

 symmetrical, subturbinate; seed with a prominent protruding hilum, deeply and 

 unequally 12-15-ribbed; ovule I. In ponds, lakes and stagnant waters, through- 

 out N. Am., except the extreme north. Also in Europe, Asia, Africa and 

 Australia. Summer. 



6. Lemna gibba L. Gibbous Duckweed. (I. F. f. 889.) Thallus un- 

 symmetrical, obovate or short-obovate, 2-6 mm. long, thickish or more or less 

 strongly gibbous beneath, short-stalked when young, soon separating, provided 

 with stomata which are sparse beneath, obscurely 3-5 -nerved; rootcap mostly 

 short-pointed, cylindric. rarely long-pointed or obtuse; fruit symmetrical; seed 

 thick, deeply and unequally ribbed. In ponds and rivers, Neb. to Tex., Wyo., 

 Ariz, and Cal. Also in Mexico, Europe, Asia, Africa and Australia. June-July. 



3. WOLFFIA Horkel. 



Thallus small, globose, ovoid-oblong, subcylindric or irregular, rootless, nerve 

 less and leafless. The vegetative growth is from a cleft near one end of the plant. 

 the branch being mostly sessile and soon detached. The ovary contains one or- 

 thotropous ovule. Fruit spherical or short ovate, smooth. Endosperm in a single 

 layer. [Name in honor of Nath. Matth. v. Wolff, 1 724-1784, Polish physician 

 and naturalist.] A genus of about 12 species, mostly in tropical and subtropical 

 regions. 



Thallus globose to ellipsoid, not punctate. 1. W. Columbiana. 

 Thallus flattened above, at least on the margin, brown-punctate. 



Apex of thallus acute. 2. W. punctata. 



A large conic papule on the upper surface. 3. W. papulifera. 



I. Wolffia Columbiana Karst. Columbia Wolffia. (I. F. f. 890.) 

 Thallus spherical or subellipsoidal, 0.5-1 mm. long, with a limited number of sto- 

 mata (1 to 6), loosely cellular and clear green throughout, not punctate nor 

 gibbous. Floating as minute alga-like grains just beneath the surface of the 

 water in stagnant ponds, pools and shallow lakes, Mass. to Ont., N. J., S. Car., 

 Minn., Mo. and La. Also in Mexico and South America. June-July. 



