98 NAIADACEAE 
toward the base and blending into the short leaf stem. Lower floating 
leaves gradually modified until they pass to the form of the submersed 
leaves. Submersed leaves pear-shaped or elliptic and generally with 
nerves diverging from the base. Ponds and streams, through our area. 
C. Upper leaves elliptic, lower leaves thread-like. 
10. P. hybridus, Michx. RAFINESQUE’S PONDWEED. (P. diversi- 
folius, Raf.) Floating leaves elliptic, sometimes narrowed to lance- 
shaped, about 1 in. long, 4 as wide. Submersed leaves 3 or 4 in. long, 
thread-like, stipules obtuse, less than 4 as long as submersed leaves. 
Spikes of flowers at surface of water, 4 in. long, those in the axils of 
submersed leaves with about 4 flowers on a flower stalk } in. long, dif- 
fering in this respect from No. 9. Still waters, throughout our area. 
11. P, dimorphus, Raf. (Fig. 8, pl. 5.) Sprran PonpweEep. (P. 
spirillus, Tuckerm.) Stem 6 in. to 2 ft. long. Floating leaves 2/3 to 
1 in. long, 4 as wide. Leaf stalk about as long as leaf. Near the float- 
ing leaves are the cylindric flower spikes on stems 4 to 1 in. long, while 
in the axils of the submersed leaves the small rounded flower masses are 
without flower stalks. Ponds and ditches, our area. 
12. P. Vaseyi, Robbins. Vasry’s PoNDWEED. Floating leaves 1 to 
4 pairs, opposite, on fruit-bearing stems, } to 4 in, long, more than 4 as 
wide, stipules long and narrow. Flower spikes only on the stem of floating 
leaves. Eastern Mass. and westward. 
D. Floating leaves both elliptic and very narrow grass-like. 
13. BP. lateralis, Morong. OprosITE-LEAVED PoNDWEED. Stem thread- 
like, branching, the branches with floating leaves have no flower spikes. 
Floating leaves elliptic, obtuse, 1/3 in. long, 4 as wide as linear. Sub- 
mersed branches with very narrow linear leaves bearing flower groups on 
flower stalks 1 to 2 in. long, flower spike with about 4 flowers. Lakes and 
streams, Eastern Mass. and westward. 
Groupe 2. Leaves all submersed or exceptionally a few floating. 
A. Leaves broad, rounded or heart-shaped at base, clasping the stem 
14. BP. perfoliatus, L. (Fig. 6, pl. 4.) CLASPING-LEAVED PONDWEED. 
Leaves broadly oval or rounded, nearly or quite as broad as long, cordate 
and clasping the stem, upper leaves opposite, lower alternate, Ponds 
and streams, our area. 
Var. P. lanceolatus, with longer lance-shaped leaves. Lake Champlain 
and northward. 
15. P. Richardsonii, (Benn.) Rybd. Leaves long, lance-shaped, base 
heart-shaped and clasping the stem, wavy, pale bright green. Quiet 
waters, New England and N. Y. 
B. Leaves broad lance-shaped not clasping at base. 
16. P. bupleuroides, Fernald. Leaves nearly orbicular to lance- 
shaped, obtuse at apex, flat not ecrisped, Stipules inconspicuous, not 
usually developed. Fruit rounded, somewhat 3-keeled, Brackish waters. 
