the thallus that becomes hyaline near the base, the air chambers in 1 layer. 



In ponds, lakes, canals and lagoons in s.e. Okla. (McCurtain Co.), general 

 but mostly in the w. half of Tex., N.M. (Grant, Sandoval and Socorro cos.) and 

 Ariz. (Maricopa and Santa Cruz cos.); in w. U.S. and S.A. 



Vegetatively, what one might consider to be the largest thallus of this species 

 could easily be taken for the smallest thallus of L. valdiviana. An arbitrary 

 separation of species based on size differences is most unsatisfactory, but in this 

 case this condition exists. 



4. Lenina perpusilla Torr. Fig. 293. 



Thallus solitary or in small clusters, obovate to orbicular-obovate, oblique, 

 1-2.5 mm. long, 0.7-2 mm. wide, obscurely 1- to 3-nerved, rather thick, usually 

 light-green, not pigmented, with large air spaces within that are in 1 layer, the 

 apical papilla usually prominent, sometimes with a row of papules along the 

 mid-nerve. L. aequinoctiales Welwitch. 



In ponds and lakes in e. Okla. (Johnston, McCurtain and Washington cos.) e., 

 cen. and s. Tex. and Ariz. (Pima and Santa Clara cos.); distributed more or less 

 throughout the world. 



5. Lemna trinervis (Aust.) Small. 



Thallus solitary or several attached, 2.5-5 mm. long, 1.5-3 mm. wide, obovate 

 to elliptic-obovate, with an obtuse to rounded apex and subacute base, very 

 nearly symmetrical, thin and membranous, flat on both surfaces, light to medium 

 green, typically not pigmented but occasionally with some pigment, papillae lack- 

 ing or at least not prominent, with 3 distinct and prominent nerves; root sheath 

 winged, the rootcap acute. L. perpusilla var. trinervis Aust. 



In lakes, streams, ditches and canals in Okla. (Adair, Alfalfa, Blaine, Bryan, 

 Comanche, Harper and Ottawa cos.), throughout Tex., in N. M. (Catron, Mora, 

 Rio Arriba, San Juan and Taos cos.) and Ariz. (Maricopa, Navajo and Yavapai 

 cos.); mostly in the W. Hemis. 



6. Lenina obscura (Austin) Daubs. 



Thallus solitary or only 2 or 3 attached, 1.5-3.5 mm. long, 1-2.5 mm. wide, 

 usually toward the smaller range, obovate to suborbicular, slightly asymmetrical 

 at the obtuse to rounded apex, nerveless or indistinctly nerved, the margin entire, 

 thin and frequently curling when dried; dorsal surface medium green, slightly 

 laterally convex; ventral surface strongly red-purple, rounded-convex slightly 

 inflated; root sheath short, the young root usually lying in a furrow similarly 

 to L. minor. 



In sluggish streams, sloughs, quiet lakes and ponds in s.e. Okla. (McCurtain 

 Co.), e., cen. and s. Tex., N. M. (Grant and Valencia cos.) and Ariz. (Apache 

 and Navajo cos.) ; e. and s. U.S., w. to Calif, and Mex. 



Those plants whose thallus is less than 3 mm. long and is somewhat ventrally 

 convex and inflated are arbitrarily referred here. There is little doubt that it is 

 part of a complex involving L. gihha and L. minor. It is more probable that 

 these plants should be considered as merely small forms of L. minor. 



7. Lemna minor L. Water lentil. Fig. 293. 



Thallus solitary or clustered, suborbicular to elliptic-obovate, 2-4 mm. long, 

 1.5-3 mm. wide, opaque, with 2 layers of air spaces within that are not inflated, 

 nearly flat on both sides, obscurely 1 -nerved above, the dark-green surface often 

 suft'used with red or purple, a low median ridge often terminated by a conspicuous 

 papilla or sometimes with a row of papules along the median ridge. 



In quiet waters of sloughs, lakes, canals and ponds in Okla. (Beaver, McCurtain 

 and Texas cos.), general but mostly in the w. half of Tex., N.M. (Catron, Grant, 



571 



