DUCKWEED FAMILY 



347 



2. LEMNA L. Sp, PI. 970. 1753. 



Thallus disc-shaped, stipitate or sessile, usually with a central nerve and with or without 

 2-4 lateral nerves. Rootlet solitary on each thallus, devoid of vascular tissue, the rootcap 

 thin, blunt or pointed. Reproductive pouches 2, triangular, opening as clefts in either margin 

 of the basal portion of the frond. Sp^dix of 1 pistillate and 2 staminate flowers. Spathe 

 sac-like or open. Stamens with tilaments curving upwards from the margin of the frond ; 

 anthers 2-celled, transversely dehiscent. Ovule 1-6, orthotropus, amphitropus or anatropus. 

 Utricle ovoid more or less ribbed. Endosperm in 1 or 3 layers. [Greek, in allusion to the 

 growth of these plants in swamps.] 



A genus of about eight, mostly widely distributed species. Type species, L. tiisuUa L. 



Fronds long-stipitate. 1- ^- tnsulca. 



Fronds short-stipitate, or sessile. 



Spathe open; thallus 1-nerved or nerveless. 



Frond thin, without papules: rootcap strongly curved, tapering. 2. L. cyclostasa. 



Frond thick, with a row of papules along the nerve; rootcap little curved, cyhndric. 



3. L. ini)ni)ia. 



Spathe sac-like; thallus more or less faintly 3-,S-nerved. 

 Frond green or purplish beneath; fruit not winged. 

 Frond pale beneath, usually strongly gibbous; fruit winged. 



4. L. 



5. L. 



iiitnor. 

 gibba. 



1. Lemna trisulca L. 

 Ivy-leaved Duckweed. Fig. 847. 



Lcmna trisulca L. Sp. PI. 970. i;53. 



Fronds remaining connected, and forming chain-like 

 colonies oblong to oblong-lanceolate, 6-10 mm. long, narrowed 

 at base to a slender stipe, denticulate toward the apex, ob- 

 scurely 3-nerved. with or without rootlets ; spathe sac-like ; 

 seeds ovate, amphitropus, 12-15-ribbed. 



Growing in pools and slow streams, widely distributed over North 

 America and the Old World. On the Pacific Coast ranging from British 

 Columbia to the San Bernardino Mountains, southern California. Type 

 locality: Europe. 



2. Lemna cyclostasa (Ell.) Chev. 



Valdivia Duckweed. 



Fig. 848. 



Lcmna minor cyclostasa Ell. Bot. S. C. and Ga. 2: 518. 1S24. 

 Lcmna cyclostasa Chev. Fl. Paris 2: 256. 1827. 

 Lemna valdiviana Philippi, Linnaea 33: 239. 1864. 



Fronds elliptic-oblong, 2.5-4 mm. long, rather thick, sub- 

 falcate and shortly stalked at the base, obscurely 1-nerved; 

 rootcap curved, short-tapering; spathe broadly reniform ; 

 fruit ovoid-oblong, pointed by the long style, unsymmetrical ; 

 seed oblong, 12-29-ribbed, amphitropus. 



Widely distributed over North and South America, but on the Pacific 

 Coast apparently not found north of Lake County, California, and com- 

 mon only in southern California. Type locality: southeastern United 

 States. 



3. Lemna minima Philippi. 



Least Duckweed. 



Fig. 



849. 



Lemna minima Philippi, Linnaea 33: 239. 1844. 



Frond oblong to elliptic. 1.5-4 nun. long, 1-nerved or com- 

 monly nerveless, with a row of papules along the nerve on the 

 upper surface, the lower surface flat or slightly convex ; root- 

 cap slightly curved or straight, cylindric and blunt ; spathe 

 open; ovary short, clavate ; ovule 1, obliquely orthotropus; 

 seed oblong, pointed, about 16-ribbed. 



In pools and streams, Georgia to Florida and Kansas, also in 

 Wyoming and California, w'here it ranges from the San Francisco^ Bay 

 region, and the San Joaquin Valley to southern California. Type 

 locality : Santiago, Chile. 



