b ALISMACEAE. 



6. P. perfoliatus L. Leaves all submerged; blades suborbicular to orbicular- 

 ovate, 2-4 cm. long, usually obtuse and serrulate at the apex, cordate-perfoliate, 

 13-23-veined: drupelets 2.5-3 mm. long, obscurely 3-keeled. Susquehanna and 

 tributaries. Bather rare, in flowing water. Sum. and fall. 



7. P. crispus L. Leaves 2-ranked; blades mainly oblong to linear-oblong, 3-10 

 cm. long, serrulate, obtuse, crisped, sessile or partly clasping, 3-7-veined: 

 drupelets 5-6 mm. long, acuminate-beaked, 3-keeled, the prominent even middle 

 keel ending in a spur at the base. Susquehanna and Conestoga Creek. Bare, 

 in slow-flowing water. Sum. 



8. P. foliosus Baf. Leaves not 2-ranked; blades linear or filiform, 1.5-5 cm. 

 long and 3-veined, acute: drupelets mostly 4 together, 1.5-2 mm. long, abruptly 

 short-beaked, the middle keel crest-like, sinuate-undulate and usually extended 

 into a shoulder at each end. Susquehanna. Bare, in slow-flowing water. 

 Sum. 



9. P. obtusifolius Mert. & Koch. Leaves 5-8 cm. long; blades linear, obtuse, 

 but often mucronate, mostly 3-veined, the midrib broad: mature spikes con- 

 tinuous: drupelets about 3 mm. long, 3-keeled, the middle keel often undulate. 



Susquehanna and tributaries. Bather rare, in slow-flowing water. Sum. 



10. P. pusillus L. Leaves 2-8 cm. long; blades very narrowly linear or linear- 

 filiform, 3-veined, acute: spikes globular, not interrupted: drupelets about 2 

 mm. long, the middle keel not crest-like, or obsolete. Susquehanna and tribu- 

 taries. Bather rare, in slow-flowing or still water. Sum. 



11. P. diversifolius Baf. Leaves of two kinds, the submerged ones linear- 

 setaceous and 1-veined, the floating ones elliptic, oval or oblong-lanceolate, 

 1-2.5 cm. long: drupelets about 1 mm. long, cochleate, the middle keel crest- 

 like, usually with 7 or 8 teeth. Susquehanna, creeks and ponds. Bather 

 common, in slow-flowing or still water. Spr. and sum. 



12. P. pectinatus L. Leaves 2.5-15 cm. long; blades setaceous or filiform, 

 attenuate, 1-veined, sometimes obscurely so: spikes interrupted: drupelets 3-4 

 mm. long, crestless. Susquehanna. Bather rare, in flowing water. Sum. 



13. P. Robbinsii Oakes. Leaves 7-13 cm. long; blades linear, acuminate, 

 many-veined: spikes interrupted: drupelets about 4 mm. long and fully as 

 wide, narrowed at the base, prominently beaked, 3-keeled. Susquehanna. 

 Bare, in flowing water. Sum, and fall. 



FAMILY 2. NAIADACEAE. NAIAS FAMILY. 



Mostly fresh- water herbs. Leaves mainly opposite: blades mostly 

 toothed, the bases dilated. Flowers dioecious, axillary : staminate, a double 

 perianth and a single stamen: pistillate, a gynoecium consisting of an 

 ovary, a style and 2 or 3 stigmas. Fruit drupaceous. 



1. NAIAS L. Submerged plants with usually much-branched stems. 

 Leaves numerous, the margins prominently or inconspicuously toothed. 



1. N. flexilis (Willd.) Bostk. & Schmidt. Stems slender: leaves many; blades 

 linear, mainly acuminate, minutely serrulate: drupelets 3-4 mm. long, oblong. 



Susquehanna and tributaries. Bare, in slow-flowing water. Sum. NAIAD. 



Order ALISMALES. 



Marsh or aquatic herbs. Leaves alternate, mere phyllodia, or with 

 dilated blades. Flowers regular. Perianth of sepals, and usually also of 

 petals. Androecium of 3-6 stamens or more. Gynoecium of 3-many 

 carpels. 



