long; sepals 3, herbaceous, elliptic-oblong, 3-4 mm. long; petals 3, white, obovate 

 to suborbicular, membranous, about 1 cm. long and 8 mm. wide; stamens 9, 

 distinct; anthers loculicidal; filaments glandular-papillose above; nectary central, 

 3-lobed, small; pistillate plants not seen. Elodea densa (Planch.) Casp., Anacharis 

 densa (Planch.) Vict. 



In lakes, ponds, pools, ditches and quiet streams in Okla. (Comanche Co.) and 

 in cen. and e. Tex., also Ariz. (Cochise and Santa Cruz cos.), Apr. -Oct.; a nat. 

 of S.A. that has escaped from cult, in various places in the U.S. and Eur. 



The relationship of this species to animal life is similar to that for species 

 of Elodea. 



4. Elodea Michx. Waterweed. Ditchmoss 



Submersed dioecious (ours) perennials, adapted for cross-pollination at the 

 surface of water, with dichotomously branching and usually nodally rooting slender 

 stems; leaves sessile, opposite or in whorls of 3, 1 -nerved, usually minutely dentic- 

 ulate; flowers mostly unisexual or occasionally in part perfect, borne in sessile to 

 pedunculate bilobed spathes, the sepals and petals 3 each; staminate flowers 1 from 

 a somewhat globose spathe, sessile or with a very short pedicellate hypanthium, 

 when sessile deciduous from the plant at anthesis and floating on surface of water; 

 stamens 3 to 9: pistillate flowers solitary in the tubular spathes, the pedicel-like 

 hypanthium elongated to carry the rest of the flower to the water surface; stigmas 

 3, simple or bilobed, tending to float, the styles slender; fruit ovoid to cylindric, 

 several-seeded. 



About 12 species in temperate and tropical America; one introduced into 

 Europe. 



The young, tender leafy stems of species in this genus are apparently only in- 

 cidentally eaten by ducks, beaver and muskrats. The dense herbage that is fre- 

 quently developed provides a sheltered habitat for small aquatic life but at the 

 same time it may suppress the growth of more desirable species. 



1 . Middle and upper leaves opposite or occasionally with at least some in whorls 

 of 3, rounded to broadly obtuse at apex (2) 



1. Middle and upper leaves always in whorls of 3, obtuse-apiculate to acute at 



apex (3) 



2(1). Largest leaves usually 1 cm. long or less; pistillate spathe 2-2.2 cm. long, 

 the apical teeth erect; in New Mexico in our region....!. E. bifoliata. 



2. Largest leaves usually 1.5 cm. long or more; pistillate spathe 3-7 cm. long, the 



apical teeth divergent; in Arizona in our region. ...2.E. longivaginata. 



3(1). Leaves rarely less than 1.5 mm. wide, obtuse-apiculate at apex; staminate 

 flowers with a slender stalk (hypanthium), not deciduous at an- 

 thesis; pistillate sepals 2-3 mm. long 3. E. canadensis. 



3. Leaves rarely more than 1.5 mm. wide, acute at apex; staminate flowers sessile, 



deciduous from plant at anthesis and floating on the surface of the 

 water; pistillate sepals about 1 mm. long 4. E. Nuttallii. 



1. Elodea bifoliata St. John. 



Stems slender, dichotomously branched; middle and upper leaves opposite or 

 occasionally with at least some in whorls of 3, linear to lance-linear, rounded 

 to broadly obtuse at apex, finely serrulate, the longer ones 6-10 mm. long and 

 1-1.5 mm. wide, bright green and flaccid; staminate spathe narrowed below into 

 a slender petiolelike base 6-8 mm. long, the upper portion ellipsoid-inflated and 

 5 mm. long; flower peduncled by the slender threadlike hypanthium; sepals and 

 petals 3.5 mm. long; stamens 9, raised on a very short common stalk; pistillate 

 spathe 2-2.2 cm. long, the flower exserted by the very slender threadlike elongated 

 hypanthium that becomes 9-12 cm. in length; sepals L4 mm. long and petals L8 



163 



