ZANNICHELLIACEAE. 



perfect, spicate. Stamens 4, with appendaged anthers, 

 together, spicate, sessile. PONDWEED. 



Stipules axillary, free from the rest of the leaf. 

 Plants with both submerged and floating leaves. 

 Submerged leaves without blades. 

 Submerged leaves with blades. 



Submerged leaves with linear or nearly linear blades. 

 Leaf-blades with coarse cellular reticulation in 



the middle. 



Leaf-blades with inconspicuous cellular reticula- 

 tion. 

 Submerged leaves with lanceolate, oblong or oblance- 



olate blades. 



Submerged leaves with petioles, the blades nar- 

 rower than those of the floating ones. 

 Submerged leaves sessile or nearly so, the blades 



broader than those of the upper. 

 Plants with submerged leaves only. 

 Leaf-blades broad. 



Leaf-blades perfoliate, entire; drupelets abruptly 



short-beaked. 

 Leaf-blades sessile, serrulate : drupelets gradually 



long-beaked. 

 Leaf-blades narrow, linear or filiform. 



Leaf-blades linear, over 1 mm. wide : drupelets 



crested. 



Leaf -blades 3-veined : plants without, propagat- 

 ing buds. 



Leaf-blades many-veined : plants with propagat- 

 ing buds. 

 Leaf-blades narrowly linear or linear-filiform, less 



than 1 mm. wide : drupelets crestless. 



Stipules adnate to the blades, or to the petioles, of the sub- 

 merged leaves. 

 Plants with submerged and floating leaves: drupelets 



crested. 



Plants with submerged leaves only : drupelets crestless. 

 Leaf-blades setaceous or filiform, entire : druplets 



obscurely beaked. 



Leaf -blades linear, serrulate : drupelets prominently 

 beaked. 



Carpels 4. Drupelets 4 



1. P. natans. 



2. P. epihydrus. 



3. P. heterophyllus. 



4. P. americanus. 



5. P. alpinus. 



6. P. perfoliatus. 

 1. P. crisp us. 



8. P. compressus. 



9. P. obtusifolius. 

 10. P. pusillus. 



11. P. diversifolius. 



12. P. pectinatus. 



13. P.RobUnsii. 



1. P. natans L. Leaves of two kinds, the submerged ones phyllodia, the float- 

 ing ones with ovate to oval blades 4-9 cm. long, rounded or subcordate at the 

 base: drupelets obovoid, 4-4.5 mm. long, scarcely keeled. Susquehanna and 

 tributaries. Eather common, in still water. Sum. 



2. P. epihydms Eaf. Leaves of two kinds, the submerged ones with linear 

 5-nerved blades, the floating ones with oblong, elliptic, or obovate blades 3-8 

 cm. long: drupelets globose-obovoid, 2.5-3 mm. long, with an elevated shoulder 

 back of the beak, the middle keel sharp. Susquehanna and tributaries. 

 Eather rare. Sum. 



3. P. heterophyllus Schreb. Leaves of two kinds, the submerged ones narrowed 

 upward, the floating with oval or elliptic blades, 1.5-4 cm. long, abruptly 

 pointed: drupelets 2-3 mm. long, with a horizontal shoulder back of the beak, 

 indistinctly 3-keeled. Creeks and Susquehanna. Eather common, in still 

 water. Sum. 



4. P. americanus Cham. & Schlecht. Leaves of two kinds, the submerged ones 

 with elongate blades, the floating ones with oblong to elliptic long-petioled 

 blades 4-15 cm. long, usually acute: mature spikes 2.5 cm. long, terminating 

 peduncles 5-8 cm. long : drupelets 3.5-4 mm. long, the middle keel prominent. 

 Susquehanna and tributaries. Eather common, in slow-flowing water. Sum. 

 and fall. 



5. P. alpinus Balbis. Leaves of two kinds, the submerged ones with mostly 

 linear-oblong to linear-lanceolate blades, the floating ones with mostly spatu- 

 late to oblanceolate blades 2-12 cm. long: peduncles thinner than the stem: 

 drupelets about 2.5 mm. long, 3-keeled, the middle keel sharp. Susquehanna. 

 Eather rare, in still water. Sum. 



