of a single stamen; pistillate flowers usually 1 to a spathe and having a single 

 pistil; ovary 1- to 3-ovuled; seeds usually early-ribbed and containing a distinct 

 operculum. 



About 15 species, mostly world-wide in distribution. 



Identification of the species can be more certain when live, fresh plants are 

 available. Although the highest power of a dissecting microscope can be used 

 in studying the flowers and fruits, best results can be obtained by using a some- 

 what higher magnification. 



1. Thallus elliptic to lanceolate, commonly dilated below middle, usually sub- 

 mersed, long-stipitate, frequently with many remaining attached to 



form long chains, commonly denticulate near and at apex 



1. L. trisulca. 



1. Thallus obovate to oblong or oblong-elliptic, usually floating, short-stipitate or 



sessile, characteristically 2 to 5 attached, entire (2) 



2(1). Thallus typically oblong to oblong-elliptic; dorsal surface flat, smooth, 

 with no prominent protuberances, nerveless or very obscurely 1- 

 nerved (3) 



2. Thallus typically obovate to suborbicular; dorsal surface with more or less 



prominent protuberances, indistinctly to prominently 3-nerved (4) 



3(2). Thalluses often 8 to 10 attached, obliquely oblong, thin and flat, without 

 papules, the surface texture uniform throughout 2. L. valdiviana. 



3. Thalluses seldom more than 2 remaining attached, oval, symmetrical, thick, 



with a low median ridge bearing 2 or more papules, usually with 

 a thin margin 3. L. minima. 



4(2). Root sheath with definite wings or appendages; thallus thin or somewhat 

 thickened and light green, typically without pigmentation (5) 



4. Root sheath without wings or appendages; thallus thickish, yellow- or dark- 



green and typically with some pigmentation (6) 



5(4). Thallus usually not prominently nerved, generally biconvex, the apical 

 papilla prominent 4. L. perpusilla. 



5. Thallus distinctly 3-nerved, flat, thin 5. L. trinervis. 



6(4). Thallus usually much less than 3 mm. long and 2 mm. wide, symmetric at 

 apex 6. L. obscura. 



6. Thallus usually more than 3.5 mm. long and 2 mm. wide (7) 



7(6). Thallus symmetric at apex; ventral surface of thallus flat or slightly 

 convex but not inflated; dorsal surface of thallus dark-green, the 

 air spaces not prominent; fruits broad but not winged at the 

 shoulders; seed 1 7. L. minor. 



1. Thallus asymmetric at apex; ventral surface of thallus noticeably convex, 

 usually inflated; dorsal surface of thallus mottled yellow-green, the 

 air spaces prominent; both surfaces of thallus showing red-purple 

 coloring; fruits winged at the shoulder; seeds usually 2.. ..8. L. gibba. 



1. Lemna trisulca L. Ivy duckweed. Fig. 290. 



Often forming dense masses, usually floating just beneath the surface except 

 when flowering, mother- and daughter-thalluses often remaining attached for 

 several generations by long attenuate stipes; thallus 5-15 mm. long, 2.5-5 mm. 

 wide, lanceolate-elliptic to oblanceolate, flat on both surfaces, translucent, attenu- 

 ate below into a slender stipe, entire or mostly denticulate at the apex, each with 

 a solitary root or in some thalluses the root lacking; rootcap acutely pointed. 



In ponds and slow-moving streams, reported from Tex. and in N. M. (Catron, 

 Grant and Socorro cos.) and Ariz. (Apache Co.); in temp, zones of the N. Hemis. 



567 



