32 Naiadaceae 
stalked; stigmas peltate, sessile or not. Mature fruits on long umbellate pedicels which 
recoil after the fruits drop. (Honor of H. B. Ruppius, a German botanist. ) 
A.  Stipular sheath 10 mm. or less long, its free part shorter; fruit 2 mm. long. W. 
R. maritima L. (Marine Ditch-grass) 
AA. Stipular sheath 15 mm. or more long, its free part as long; fruit 3—4 mm. long. 
Ge R. occidentalis Wats. (Western Ditch-grass) 
POTAMOGETON POND-WEED 
Stems submerged or floating, the end often thickening and forming a propagating 
bud which falls off and strikes root. Leaves either all submerged or some floating and 
some submerged, in some species varying greatly in length and width; floating leaves 
coriaceous or semi-coriaceous, rarely semi-pellucid, usually expanded into a proper blade; 
submerged leaves sometimes greatly reduced, or with broad or capillary leaf-blades; stipules 
present. Flowers usually thickly clustered on the simple or branched spike. Stamens 4. 
Pistils 4. Fruit sessile. In fresh water (except sometimes P. pectinatus). (Gk. pota- 
mos==a river, geiton—a neighbor; referring to the habitat.) 
A. With both floating and submerged leaves (floating rarely wanting in P. heterophyllus 
and P. alpinus), floating leaves broad. 
B. Mature submerged leaves narrowly linear to linear-lanceolate, never more than 
4 mm. wide. 
C. Stipules more than 3 cm. long, acute; submerged leaves 10—-25 cm. long, 
terete, scarcely 1.5 mm. thick. W. C. E. 
P. natans L. (Common Pond-weed) 
CC. Stipules less than 3 cm. long, acute, submerged leaves 6—I4 cm. long, 
linear to linear-lanceolate, 2.5—4 mm. wide. W. (P. nuttallii.) 
P. epihydrus Raf. 
BB. Mature submerged leaves wider or spatulate, never linear nor less than 4 mm. 
wide (except in forms of P. heterophyllus). , 
D. Floating leaves 32—-40-veined; submerged leaves of 2 kinds, the one falcate 
and the other oval. W. C. E. 
P. amplifolius Tuck. 
DD. Floating leaves fewer-veined; submerged leaves of | kind only (except in 
P. pulcher, and in that none falcate). 
E. Submerged leaves more than 7-veined, all petioled. 
F. Bases of floating leaves merely rounded or acute, not cordate; submerged 
leaves as wide or nearly as wide as the floating; style 1.5 mm. long. W. E. 
(P. americanus.) P. lonchitis Tuck. 
FF. Bases of floating leaves cordate or subcordate; submerged leaves much 
narrower than the floating; style 2 mm. or more long. C. 
P. pulcher Tuck. 
EE. Submerged leaves mostly 7-veined, at least the lowest sessile. 
G. Plants green; submerged leaves narrower than the floating ones; 1—2 
carpels of each flower maturing. C. E. 
P. heterophyllus Schreb. (Variable Pond-weed ) 
GG. Plants red; submerged ‘leaves as wide or wider than the floating ones; 
3—4 carpels of each flower maturing. C. E. 
FP. alpinus Balb. (Mountain Pond-weed) 
AA. With submerged leaves only. 
H: Leaves lanceolate or wider, not linear. 
I. Leaves peticled or merely sessile, not clasping at base; spike 4 cm. or more long. 
E. P. lucens ley (Shining Pond-weed) 
