Rio Arriba, Sandoval, San Juan and Valencia cos.) and Ariz. (Apache, Maricopa, 

 Mohave, Navajo, Pima, Pinal and Yavapai cos.); distributed throughout the 

 world. 



Those plants whose thallus is not or is scarcely inflated and is 3-6 mm. long 

 are placed here. It has never been made entirely clear, even by the most recent 

 monographer of the genus, as to how this plant really differs from the so-called 

 "flattened form" of L. gib ha. 



8. Lemna gibba L. Inflated duckweed, wind-bags. Fig. 294. 



Thallus solitary or few in a group, orbicular-obovate, 2-5 mm. long, 2-4 mm. 

 wide, thick, with 2 layers of air spaces within, dark-green above and often suf- 

 fused with red or purple, with a slight ridge and 1- to 3-nerved above and con- 

 spicuously round, usually inflated-gibbous on the ventral side because of the 

 enlargement of the lower tier of air spaces or these not much enlarged and the 

 thallus merely convex below. 



In ponds, marshes and slow streams in w. Tex., N.M. (Grant and Hidalgo cos.) 

 and Ariz. (Cochise, Maricopa, Mohave, Pima, Pinal, Santa Cruz and Yavapai 

 cos.); widely distributed throughout most of the world. 



3. Wolffia HoRKEL Water-meal 



Diminutive floating rootless herbs, scarcely visible to the naked eye as indi- 

 viduals and often forming uninterrupted green masses on the surface of the water 

 as a thin green scum; thallus spheroid to ellipsoid, sometimes flattened above, with 

 a single funnel-shaped reproductive pouch at one end that bears asexually succes- 

 sive daughter-fronds, sometimes bearing masses of red pigment bodies in each 

 epidermal cell; inflorescence breaking through the upper surface of the frond, 

 composed of 1 staminate flower consisting of a single stamen and 1 pistillate 

 flower consisting of a single pistil; utricle spherical, smooth. 



About 10 species, mainly in the tropics and subtropics. 



The thallus, about the size of a pinhead, is the smallest seed plant known. 

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

 able. Although the highest power of a dissecting microscope can be used in study- 

 ing the flowers and fruits, best results can be obtained by using a higher magni- 

 fication. 



1. Thallus mostly globular, the dorsal surface strongly convex and without a 

 papilla, without pigmented cells in epidermis 1. W. columbiana. 



1. Thallus typically ellipsoidal or broadly ovoid, commonly punctate on all sur- 



faces, with brown or reddish-brown pigment cells in epidermis (2) 



2(1). Thallus with a prominent conical papilla in center of dorsal surface 



2. W. papulifera. 



2. Thallus with dorsal surface flat or slightly rounded and without a papilla 



3. W. punctata. 



Fig. 293: Lemna. a-i, L. minor: a and b, shouldered fruits, a showing remains of 

 spathe, X 16; c, seed, X 16; d, frond (longitudinal section), X 8; e, group habit, show- 

 ing the papules along median ridge and the solitary roots, X 2; f and g, roots with and 

 without rootcap, X 28; h, pistillate and staminate flowers in short saclike spathe, X 36; i, 

 habit, top view showing young, flowering, and fruiting fronds, X 4. j-u, L. perpusilla: 

 j and k, roots, with and without rootcap, X 12; 1 habit, showing top view of fruiting 

 fronds, X 4; m, pistillate and staminate flowers, the surrounding spathe open on one 

 side, X 40; n and o, fronds (longitudinal section), the air spaces in a single layer, X 8; 

 p-r, mature seeds, ribbed, cross-striate between ribs, X 20; s and t. fruits asymmetrical, 

 X 20; u, group habit, the roots long, X 2. (From Mason, Fig. 161). 



573 



