100 NAIADACEAE 
28. P. rutilus, Wolfg. SrenpER PoNnpwEED. Stems almost thread- 
like, 1 to 2 ft. long; leaves about 1 in. long, very slender grass-like and 
narrowing all the way to a sharp point. Most northern parts of our 
area. 
29. P, interruptus, Kitaibel. INTERRUPTED PONDWEED. Leaves 
erassy, in clusters sheathed by the stipules. Flowers on interrupted 
spikes. 
30. P. Robbinsii, Oakes. Rogssrn’s Ponpweep. Stems rigid, much 
branching; leaves rigid, in 2 opposite rows closely investing the stem, 
3 to 5 in. long, acute at outer extremity; stipules sheathing the stem, 
but at length free, very acute. Flowers very few in interrupted spikes. 
Lakes and ponds, N. J. and northward. 
D. Leaves all thread-like. 
31. P. pectinatus, L. (Fig. 2, pl. 5.) FENNEL-LEAVED PONDWEED. 
Spikes on long flower stalks with interrupted rounded spikes of flowers. 
Stem much branching. Stipules in contact with leaves half the length 
of the former. Some obscure raised lines on the back of the seed. Fresh 
or salt waters, our area. 
32. P. filiformis, Pers. (Fig. 7, pl. 5.) FiILtrorM PoNDWEEp. Stems 
thread-like, 3 to 20 ft. long; leaves 2 to 12 in. long. Differing from No. 
31 by absence of the lines on back of seed. Ponds, western N. Y. 
33. P. gemmiparus, Morong. CaprmLLARy PonpWeEp. The stipules 
are lance-shaped, acute at outer, broad at inner extremity. Ponds, east- 
ern Mass. and R. I. 
34. P. pusillus, L. SmaL~L PonpweEp. Resembles Nos. 31 and 32 
but flower groups are small rounded masses, each spike containing from 
3 to 8 flowers in a nearly globular mass. Ponds, lakes and streams, 
our area. 
35. BP. strictifolius. Leaves much more rigid than those of P. pusillus, 
the edges turned back. In other respects strongly resembles P. pusillus. 
36. P. confervoides, Reichb. AtGA-L1kE PonpWEED. Stems very slen- 
der, thread-like, 6 to 18 in. high. Leaves thread-like or flat bristle- 
formed, about 1 in. long, the point very fine, l-nerved or nerveless. 
Stipules nearly or quite + in. long, blunt. Flower stem long and erect. 
Flowers in short spikes or heads, about } in. long. In cold mountain 
ponds throughout most of our region. 
2. ZANNICHELLIA, Mitchell 
Submersed water plant with creeping roots, slender stems and thread- 
like leaves each broadened to a sheathing base, In the axils of the leaves 
spring the naked flowers which consist each of a single stamen and a 
group of 4 or less or more pistils, which expand toward the base into 
oval-shaped ovaries. Fruit directly on the receptacle or on a very short 
pedicel. 
Z. palustris, L. (Fig. 3, pl. 5.) Zannicnernia. Stems thread-like; 
leaves 1 to 3 in. long. Fruits 2 to 5 in a cluster. Fresh water, pools 
and ditches throughout our area. - 
