Fig. 71B: Limnobium Spongia: a, staminate flower, X 10; b, section of capsule, 

 X 21/2; c, seed, X 42. (Courtesy of R. K. Godfrey). 



4(3). Middle and upper leaves in whorls of 4 to 6, averaging about 2.5 cm. long; 



staminate spathes 2- to 4-flowered; petals about 8 mm. wide 



.....3. Egeria 



4. Middle and upper leaves opposite or in whorls of 3 (rarely with some 4), 



rarely more than 2 cm. long; staminate spathes 1-flowered; petals 

 1.5 mm. wide or less 4. Elodea 



5(1). Leaves alternate on a short stout concealed stem, ribbonlike, more than 

 1 dm. long; spathe composed of 2 bracts connate at the base to 

 form a tube 5. Thalassia 



5. Leaves opposite at summit of slender nearly naked stem, mostly oblong- 



elliptic, less than 5 cm. long; spathe composed of 2 free bracts. 

 6. Halophila 



1. Limnobium Rich. American Frog's-bit 



Three species centered in tropical America. 

 1. Limnobium Spongia (Bosc.) Steud. Common frog's-bit. Figs. 71 A and 7 IB. 



Floating aquatic with pendent roots and stolons; leaves in a basal rosette, 

 erect or ascending, with petioles to 15 cm. long, ovate to suborbicular or the 

 earlier ones reniform, to 5 cm. broad, obtuse at the apex, truncate to cordate 

 at base, entire, faintly 5-nerved, purplish and spongy beneath; flowers unisexual; 

 staminate scapes to 1 dm. long, producing 3 or more flowers, filiform, the lance- 

 ovoid spathe 3-5 cm. long; pistillate scapes 2-leaved, with 1 or 2 short-pedicelled 

 flowers, about 25 mm. long in flower, stout, strongly recurved and elongated in 

 fruit; sepals 3, 7-10 mm. long; petals 3, 8-10 mm. long; stamens represented by 



159 



