to bolster their diet. The myriad plants often form a floating sheet that smothers 

 out much of the submerged plant life. 



1. Thallus with 1 or more roots and 2 lateral reproductive pouches; inflorescence 

 of 2 staminate and 1 pistillate flowers surrounded by a membrana- 

 ceous spathe (2) 



1. Thallus rootless, each with a single basal reproductive pouch; inflorescence 



of 1 staminate and 1 pistillate flowers with a spathe (3) 



2(1). Roots usually 2 or more on each thallus; mature thallus usually obscurely 

 or conspicuously 3- to 11 -nerved, the ventral surface usually 

 reddish-purple 1. Spirodela 



2. Roots solitary on each thallus; mature thallus 1- to 3-nerved or apparently 



nerveless, the ventral surface typically green or rarely streaked or 

 tinged with brown 2. Lemna 



3(1). Thallus globose to ellipsoid, more or less obviously 3-dimensional, usually 

 only mother- and daughter-thalluses connected; stipe attachment 

 within the reproductive pouch 3. IVolffia 



3. Thallus flat, thin. Ungulate or ligulate, usually falcate, appearing 2-dimensional, 



solitary or united in stellate colonies; stipe attachment on one side 

 of the reproductive pouch 4. Wolffiella 



1. Spirodela Schleid. Duck-meat 



Thallus floating, solitary or usually in clusters of 2 to 5 or more, orbicular 

 to obovate or oblong-elliptic, sometimes slightly curved, with 2 to numerous roots 

 fascicled on ventral surface, palmately 3- to II -nerved; reproductive pouches 2, 

 one on either side of the basal end; inflorescence arising from a pouch that con- 

 sists of a saclike spathe enclosing 1 pistillate and 2 or 3 staminate flowers; 

 ovary somewhat winged on the shoulders, 1- to 4-ovuled; fruit a utricle, slightly 

 winged; seed longitudinally ribbed and transversely striate or smooth with a 

 spongy outer layer. 



About 6 species, cosmopolitan. 



1. Thallus broadly obovate, usually almost as wide as long, with 5 or more nerves 

 radiating from above the root attachment 1. S. polyrhiza. 



1. Thallus oblong-elliptic to narrowly obovate, longer than wide, sometimes 

 slightly curved, seemingly nerveless or usually with a lateral branch 

 on each side of a central nerve just below the middle at the root 

 attachment 2. S. oUgorhiza. 



1. Spirodela polyrhiza (L.) Schleid. Fig. 288. 



Thallus with 4 to 12 fascicled roots, conspicuously orbicular-obovate, 3-10 

 mm. long, almost as broad as long, dark-glossy-green above, usually reddish-purple 

 beneath, with 5 to I 1 conspicuous radiating nerves that create a peltate appear- 

 ance, the stipe marginal or submarginal and the reproductive pouches on either 

 side; roots provided with a single vascular strand and a long pointed rootcap; 

 forming turions at all seasons but abundantly so in the fall. 



In ponds, lakes, bayous and sluggish streams throughout Okla. and Tex., 

 through N. M. (Bernalillo Co.) and Ariz. (Navajo Co.); cosmospolitan but 

 apparently lacking in S. A. 



This is the largest of the surface-floating duckweeds. It is often present as 

 scattered, large thalluses in masses of Lemna. WolffieUa and Azolla, and occa- 

 sionally in almost pure stands. The plants winter by producing buds that are 

 dense and sink to the bottom of the pond. 



2. Spirodela oligorhiza (Kurtz) Hegelm. Fig. 289. 



Thallus with 2 to 5 roots, oblong-elliptic to oblong-obovate or somewhat 

 elliptic-reniform, 2.5-5 mm. long, 1.5-3 mm. broad, obscurely 3- to 5-nerved, 



564 



