290 PATAGONIAN EXPEDITIONS : BOTANY. 



2-staminate male flower and a solitary female flower at its base, dorsally 

 adnate to the spathe. 

 Only species : 



P. STRATIOTES Linn. 



In tropical fresh-water ponds and streams. From Texas by W. Indies 

 and Central Amer. to Argentina and Patagonia. (Not in Australia or the 

 Pacific Is.) 



(Fig. in Eng. & Prantl, ii, 3, p. 153.) 



Family 13. LEMNACE.E. Duckweed. 



Minute floating plants without differentiation of leaf and stem, con- 

 sisting, in the active stage, of a loosely cellular frond, with or without roots, 

 and extending by lateral branching, the new fronds soon separating. 

 Reproduction by minute unisexual flowers immersed, mostly with a deli- 

 cate spathe in the brood-pouch. Male flowers 1-2, having i stamen; 

 female flower i, with i carpel and i -few seeds, having endosperm. 



Species 28, widely distributed. 



A. Frond with roots and 2 reproductive pouches. 



b. Roots usually more than I, fascicled. Anthers dehiscing longitudinally. Spirodela. 



b2. Root solitary. Anthers dehiscing transversely. Lemna. 



A2. Frond rootless and nerveless, with i reproductive pouch. 



b. Frond thick, subglobular. Wolffia. 



b2. Frond thin, elongated. Wolffiella. 



i. SPIRODELA Schleiden. 



Stipe fixed peltately under the frond ; roots more than i . Reproductive 

 pouches 2, as cleft openings at base of the frond ; spathes sac-like. An- 

 thers 2-celled, with longitudinal dehiscence. Ovules 2, rarely i. 



S. PUNCTATA (Meyer sub Lemna] Thompson. 



Frond solitary to 2-4, cohering in a chain, elliptical to reniform, aver- 

 aging 2.5 by 1.4 mm., obscurely 3-nerved, convex above, punctate with 

 brown pigment-cells. Spathes with flowers protruding ; 2 staminate and 

 i pistillate flowers on the spadix. Filaments long ; friiits short, broad, 

 with wings. Seed i. 



Orange Harbor, near Cape Horn, collected by the Wilkes Exploring 

 Expedition, 1839, an d forwarded by the courtesy of Professor B. L. Rob- 



