348 



LEMNACEAE 



4. Lemna minor L. 



Lesser Duckweed. 



Lemna minor L. Sp. PL 9~0. 1753. 



Fig. 850. 



Fronds rounded to obovate-oblong, 2-4 mm. long, symmetrical, 

 thickish, convex and green or purplish on both surfaces, upper 

 surface sometimes keeled and with a row of papulae along the 

 midnerve, obscurely 3-nerved; fruit not winged; seeds solitary, 

 with a prominent protruding hilum, 12-15-ribbed. 



In ponds, lakes and stagnant waters, widely distributed throughout 

 North America, except the extreme north; also in Europe, Asia, Africa 

 and Australia. Type locality : Europe. 



5. Lemna gibba L. 



Gibbous Duckweed. 



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



Fig. 85 L 



Fronds rounded to obovate, 2-5 mm. long, unsym- 

 metrical, thick, convex, slightly keeled and green on the 

 upper surface, pale beneath and more or less gibbous, 

 usually 3-5-nerved ; fruit winged with rounded lobes on 

 either side of the stigma ; seeds thick, deeply and unequally 

 ribbed. 



In ponds and slow streams; Nebraska and Texas to Cali- 

 fornia and Mexico; also in Europe and Asia. In California this 

 is a common species, especially in the valleys and foothills. Type 

 locality : Europe. 



3. WOLFFIELLA Hegelm. in Engler Bot. Jahrl). 21 : 303. 1895. 



Fronds rootless, thin, unsymmetrical, curved in the form of a segment of a band, the 

 two ends submerged, abundantly punctuate on both surfaces with brown epidermal pigment 

 cells. Reproductive pouch triangular, opening as a cleft in the basal margin of the frond. 

 Flowers and fruit unknown. [Name a diminutive of Wolffia, a closely related genus.] 



Seven species, mostly of tropical distribution. Besides the following, IV. floridana (J. D. Smith) 

 Thompson occurs in the southern states. Type species, Woljficlla oblonga (Philippi) Hegelm. 



Fronds saber-shaped; stipe insertion at the lower angle of the two walls of the pouch. 



1. W. oblonga. 

 Fronds tongue-shaped; stipe insertion on the margin of the lower wall of the pouch. 2. W. lingulata. 



I. Wolffiella oblonga (Philippi) Hegelm. 

 Saber-shaped Wolffiella. Fig. 852. 



Lemna oblonga Philippi, Linnaea 29: 45. 1857. 

 Wolffia oblonga Hegelm. Monogr. Lemnac. 131. 1868. 

 Wolffiella oblonga Hegelm. in Egler Bot. Jahrb. 21: 303. 1895. 



Fronds solitary or in pairs, oblong or commonly tapering 

 from the obliquely rcrtmded base to the slightly narrower 

 bluntly rounded apex; 0.5-1 mm. wide, 1.7-4.6 mm. long; 

 stipe insertion at the lower angle of the two walls of the 

 pouch. 



A Mexican and South American species, which was collected in 

 1881 with Spirodela polyrhiza near San Bernardino, southern Cali- 

 fornia, but has not been rediscovered, and is otherwise unknown in 

 the United States. Type locality : Santiago, Chile. 



