44 American Midland Naturalist Monograph No. 2 



ponds or slow streams, chiefly in the eastern part of the 



state, rare P. gramineus L. 



11. Submerged leaves linear or filiform, not more than 5 mm. wide. 



17. Submerged leaves linear, 2-5 mm. wide, conspicuously reticulate 



along the midvein; ponds and lakes, not common 



P. epihydrus Raf. 



17. Sumberged leaves filiform, 1-2 mm. wide. 



18. Spikes of 2 kinds: one emersed, cylindrical, many-flowered, the 

 other submerged, globose, few-flowered; ditches and slow 

 streams, not uncommon, chiefly in w. and s. III. [P. hybridus 



of Michx. and Am. auth.] P. diversifolius Raf. 



18. Spikes all alike, cylindrical. 



19. Blades of the floating leaves less than 1.5 cm. long, equalling 

 or longer than the petioles; spikes less than 1 cm. long; 



lakes, not common P. vaseyi Robbins 



19. Blades of the floating leaves 2.5 cm. or more in length, mostly 

 shorter than the petioles; spikes 1.5 cm. or more in length; 

 lakes, ponds, and ditches, not uncommon P. natans L. 



2. Zannichellia L. — Horned Pondweed 



Z.- palustris L. Ditches and ponds, not common. Peo*-ia and Henderson 

 counties. 



19. Juncaginaceae Lindl. — Arrow-grass Family 



I. Flowers numerous, greenish, in a long spike-like raceme; leaves all basal.. 1. Triglochin 

 1. Flowers few, white, in a loose raceme 2. Scheuchzeria 



1. Triglochin L. — Arrow-grass 



1. Carpels usually 6, in fruit ellipsoid, 3-6 mm. long; sandy or marly swales, 



or in swamps or along ditches, n. 111., not common. June- July 



T. mar'thma L. 



1. Carpels 3, in fruit clavate, 7-8 mm. long; calcareous soil, rare, Peoria, Kane, 

 and Lake counties. July-Sept T. palustris L. 



2. Scheuchzeria L. 



S. americana (Fern.) n. comb. Bogs, rare, n. 111. June-July. \_S. palustris 

 of Am. auth., not L.; S. palustris var. americana Fern., in Rhodora 25:177. 

 1923}. On the basis of shape and size of follicles, our plants appear .specifically 

 distinct from the European 5. palustris L. 



20. Alismaceae DC. — Water-plantain Family 



1. Flowers numerous, small, perfect, in a compound panicle; leaves oval or ovate; 

 stamens usually 6; carpels flattened, arranged in a ring on a small flat receptacle.... 

 I . Alisma 



I. Flowers in whorls, fewer; stamens 9-many; carpels in a head on a convex receptacle. 

 2. Leaves cordate or ovate, with 5-7 veins from the base; flowers [jerfect, 3-9 or more 



in each whorl ; plants annual 2. Echinodorus 



2. Leaves sagittate or lanceolate, more than 7-veined; plants perennial. 



