GENUS 2. 



DUCKWEED FAMILY. 



447 



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



Thallus disc-shaped, usually provided with a central nerve and with or without two or 

 four lateral nerves. Each thallus produces a single rootlet, which is devoid of vascular tis- 

 sue and is commonly provided with a thin blunt or pointed rootcap. The ovary contains 

 from one to six orthotropous, amphitropous or anatropous ovules. Fruit ovoid, more or less 

 ribbed. Endosperm in one or three layers. [Greek, in allusion to the growth of these 

 small plants in swamps.] 



About 8 species, in temperate and tropical regions. Besides the following, Lemna angolensis is 

 a native of Lower Guinea. Type species: Lemna trisulca L. 



Thalli long-stipitate. i. L. trisulca. 



Thalli short-stipitate or sessile. 

 Spathe open. 



Thalli i -nerved or nerveless. 



Thalli thin, without papules ; rootcap strongly curved, tapering. 2. L. cyclostasa. 



Thalli thick, with a row of papules along the nerves ; rootcap little curved, cyiindric. 



3. L. minima. 



Thalli 3-nerved ; rootcap cylindric. 4. L. perpusilla. 



Spathe sac-like. 



Thalli green or purplish beneath ; fruit not winged. 5. L. minor. 



Thalli pale beneath, usually strongly gibbous ; fruit winged. 6. L. gibba. 



i. Lemna trisulca L. Ivy-leaved Duckweed. Star 

 Duckweed. Fig. 1121. 



Lemna trisulca L. Sp. PI. 970. 1753. 



Thallus lanceolate, submerged and devoid of stomata in 

 the primary aquatic form, ovate to oblong-lanceolate, 6"-9" 

 long, floating and provided with stomata in the later flower- 

 ing stage. The later and more common form is narrowed at 

 the base to a slender 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 ditches, springs, ponds, shallow lakes and sloughs. Nova Scotia 

 to British Columbia, south to New Jersey, New Mexico and Cali- 

 fornia. Also in Europe, Asia, Africa and Australia. July-Aug. 



2. Lemna cyclostasa (Ell.) Chev. Valdivia Duck- 

 weed. Fig. 1 122. 



L. minor cyclostasa Ell. Bot. S. C. and Ga. 2: 518. 1824. 

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

 Lemna valdiviana Philippi, Linnaea 33: 239. 1864. 



Thallus oblong-elliptical, \"-\\" long, thin, subfalcate and 

 shortly stalked at the base, provided with numerous stomata, 

 except on the borders, nerveless; rootcap short-tapering, 

 curved; spathe reniform; fruit ovoid-oblong, unsymmetrical ; 

 seed prominently i2-2p-ribbed. 



In pools and rivers, Massachusetts to Florida, west to Illinois, 

 Wyoming and California. Also in Jamaica and in South America. 

 June-July. 



3. Lemna minima Philippi. Least Duckweed. Fig. 



1123. 



Lemna minima Philippi, Linnaea 33: 239. 1844. 



Thallus oblong to elliptic, i"-2" 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 i6-ribbed, with many transverse striations. 



Georgia and Florida to Kansas, Wyoming and California. 



