42 American Midland Naturalist Monograph No. 2 



1. Achenes fusiform, stipitate; stigma 1. 



2. Pistillate heads all strictly axillary, 2-2.5 cm. in diameter at maturity; 

 beak of the mature achene 2-3 mm. long; leaves 6-12 mm. wide. 



3. Inflorescence branched; stigmas 2-4 mm. long; ditches 



^S". androcladum (Engelm.) Morong 



3. Inflorescence usually simple; stigmas 1-2 mm. long; ditches 



S. americanum Nutt. 



2. Pistillate heads usually supra-axillary; leaves 3-7 mm. wide. 



4. Fruiting heads 2-2.5 cm. in diameter at maturity; beak of the mature 



achene 4 mm. long; ditches and ponds, not common 



S. chlorocarpum Rydb. 



4. Fruiting heads 1.5 cm. in diameter at maturity; beak of the mature 



achene 1-2 mm. long; ditches and ponds, not common 



S. acaule (Beebe) Rydb. 



17. Naiadaceae Lindl. — Naiad Family 

 1. Naias L.— Naiad 



1. Fruit glossy, with 30-50 longitudinal lines; style 1-2 mm. long; leaves with 

 20-30 minute teeth on each margin; ponds and slow streams, widely dis- 

 tributed, but not common. June-Aug N. flexilis (Willd.) R. & S. 



1. Fruit dull, with 10-20 rows of distinct reticulations; style 0.2 0.6 mm. long; 

 leaves with 40-50 minute teeth on each margin; ponds and shallow lakes, 



rare; Peoria and Macoupin counties. July-Sept 



N. giiadalupensis (Spreng.) Morong 



18. PoTAMOGETONACEAE Engler — Pondweed Family 



1. Flowers perfect, in spikes; leaves alternate, or the upper sometimes opposite 



1 . Poiamogelon 



1. Flowers unisexual, axillary; leaves opposite, filiform, I -veined, entire.. ..2. Zannichcllia 



1. PoTAMOGETON L. — Pondweed 



1 . Leaves uniform, all submerged. 

 2. Leaves linear to filiform. 



3. Stipules free from the petioles and blades. 



4. Leaves 9-35-veined; fruits 3.5-5 mm. long; lakes, n.e. 111. [P. 

 compressus Am. auth., not L.; P. zosterifolitis Am. auth., not 



Schum.] P. zosterijormis Fern. 



4. Leaves 1-7-veined; fruits 1.5-3 mm. long. 



5. Leaves 5-7-veined, with a pair of basal glands; stagnant water, not 

 common. [P. mucronatus Schrad.} P. jriesii Rupr. 



5. Leaves 1-3-veined. 



6. Blades usually without basal glands; fruiting spikes subcapitate, 



2-8 mm. long; ponds, ditches, and streams, chiefly in the n. 



half of 111 P. foliosus Raf. 



6. Blades usually with a pair of small translucent glands at the 



base; lakes, etc. [P. panormitanus Biv.} P. pusillus L. 



