44 S . LEMNACEAE. Vol. I. 



4. Lemna perpusilla Torr. Minute Duckweed. Fig. 11 24. 



Lemna perpusilla Torr. Fl. N. Y. 2 : 245. 1843. 



L. paucicostata Hegelm. in A. Gray, Man, Ed. 5, 681. 1868. 



L. perpusilla trinervis Austin, in A. Gray, Man. Ed. 5, 479. 1867. 



Thallus small, i"-ii" long, oblong to obovate, often purplish- 

 tinged beneath, unsymmetrical and abruptly narrowed to a very 

 short stalk, provided throughout with numerous stomata, more or 

 less 3-nerved; rootcap pointed; fruit ovoid, unsymmetrical; seed 

 12-16-ribbed, and transversely striated. 



In ponds, rivers, springs and lakes, New York and New Jersey to 

 Minnesota, Nebraska and Missouri. June-July. 



5. Lemna minor L. Lesser Duckweed. Fig. 1125. 



Lemna minor L. Sp. PL 970. 1753. 



Thallus obovate or subcircular, i"-3" long, thickish, rarely 

 reddish or purplish-tinged, short-stalked when young, provided 

 throughout with stomata, obscurely 3-nerved, very rarely 4-5- 

 nerved. Rootcap obtuse or subtruncate; fruit symmetrical, sub- 

 turbinate, wingless; seed with a prominent protruding hilum, 

 deeply and unequally 12-15- ribbed. 



In ponds, lakes and stagnant waters, throughout continental North 

 America except the extreme north, and in the Bahamas. Also in 

 Europe, Asia, Africa and Australia. Duck's-meat. Toad-spit. Water 

 lentils. Mardling. Summer. 



Lemna gibba L. Gibbous Duckweed. Fig. 1126. 



Lemna gibba L. Sp. PI. 970. 1753. 



Thallus slightly unsymmetrical, obovate or short-obovate, 

 1 4"— 3" long, thickish or more or less strongly gibbous be- 

 neath, short-stalked when young, soon separating, provided 

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

 rootcap mostly short-pointed, rarely long-pointed or obtuse; 

 fruit symmetrical, winged ; seed thick, deeply and unequally 

 ribbed. 



In ponds and rivers, Nebraska, Texas, Arizona and California. 

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



3. WOLFFIA Horkel ; Schleid. Linnaea, 13 : 389. 1839. 



Thallus small, globose, ovoid-oblong, subcylindric or irregular, rootless, nerveless 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 orthotropous ovule. Fruit spher- 

 ical or short-ovate, smooth. Endosperm in a single layer. [Name in honor of Nath. Matth. 

 v. Wolff, 1724-1784, Polish physician and naturalist.] 



A genus of few species, mostly in tropical and subtropical regions. Type species : Lemna 

 hyalina Delile. 



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. 



