FLORA OF THE ' TSTRICT OF COLUMBIA. 68 



Leases all alike, auhmersed. 

 Leaves lanceolate or broader. 



Leaves cordate-clasping 5. P. perfoliatus. 



Leaves sessile or short-petioled, not clasping. 



Leaves oblong, sessile, minutely serrate 6. P. crispus. 



Leaves oval or lanceolate, short-petioled, the margin often crisped but not 



minutely serrate 4- P. lucens. 



Leaves linear to setaceous. 

 Leaves 2 mm. wide or more, with a l)road, coarsely cellular-reticulate space each 



side of the midrib 1- P- epihydrus. 



Leaves narrower or not differentially reticulated. 

 Stipules united with the leaves. 



Leaves threadlike; fruit abundant " 11. P. pectinatus. 



Leaves 4-8 mm. wide; plants rarely fruiting 12. P. robbinsii. 



Stipules free from the leaves. 



Stems much flattened; leaves 2-4 mm. wide 7. P. zosteraefolius. 



Stems little or not at all flattened; leaves less than 2 mm. wide. Spikes 

 usually few-flowered. 

 Plants with translucent wartlike glands on each side of the stem at the 



insertion of the leaves; fruiting spikes long-stalked 8. P. pusillus. 



Plants without such glands; fruiting spikes short-stalked. 

 Spikes borne at the ends of the branchlets; fruits plump. .9. P. foliosus. 

 Spikes borne in the leaf axils; fruits appearing as if coiled, hollowed out 

 on the sides 10. P. divecsifolius. 



1. Potamogeton epihydrus Raf. 



Occasional in pools or at margins of slow streams. June- July. Widely distributed 

 in N. Amer. (P. daytonii Tuckerm.; P. nuttallii Schlecht. & Cham.) 



2. Potamogeton americanus Schlecht. & Cham. 



Common in streams and the canal. Aug. Widely distributed in N. Amer. (P. 

 lonchites Tuckeiin.) 



3. Potamogeton amplifolius Tuckerm. 



Common in mouths of large creeks below Washington. Fr. June until fall. Eastern 

 and northern N. Amer. 



4. Potamogeton lucens L. 



One specimen collected by Ward near Custis Sj^ring. N. Amer. and Eurasia. 



5. Potamogeton perfoliatus L. Redhead grass. 

 Common in the river and creek mouths below Washington. June-Aug. N. Amer. 



and Eurasia. 



6. Potamogeton crispus L. 



Frequent in the canal, and in the Potomac and its tributaries below Washington. 

 Eastern U. S. ; also in Eur. 

 Propagates mainly by modified branchlets. 



7. Potamogeton zosteraefolius Schumacher. 



Common in mouths of large creeks below Washington. X. Amer. and Eurasia. 

 Fruit rare; propagation chiefly by winter buds. 



8. Potamogeton pusillus L. 



River and creek mouths below Washington : apparently scarce. Almost cosmopolitan . 

 Propagates extensively by winter buds. 



9. Potamogeton foliosus Raf. 



Frequent in pools and slow streams. July. Nearctic Amer. (P. pauciflorus 

 Pursh.) 



