POTAMOGETON. CXXXVII. NAIADACE^E. 525 



1 3f long according to the depth of the water, branched. Upper leaves 3 4' 

 by 8 ]6", petioles 2 8', submersed about as wide. Spike 1 2' long, 

 20 40-flowered. Jl. Aug. Varies with the lower leaves all reduced to peti- 

 oles. In the Wisconsin plant the leaves are all subcordate, fruit acutely cari- 

 nate but not lunate. 



2. P. CLAYTONII. Tuckerman. (P. fluitans'? Ph., J3w., Torr., fyc.) 

 Floating; Ivs. lanceolate or oblong, tapering to long petioles (sometimes 



on short petioles, E. T.), scarcely coriaceous, submersed leaves long, narrowly 

 linear, membranaceous, acute, 1-veined, slightly tapering to the sessile base; 

 spikes rather loose, on long peduncles; fr. compressed, suborbicular. Ponds and 

 slow waters, frequent. Stems round, slender or filiform, often branched. Lower 

 leaves 3 6' by 1", remote, upper about 2 3' by \'. Spikes 1' long, pedun- 

 cles 24' 



1 (P. heterophyllus. Torr.) Lower Ivs. approximate, lowest slightly petio- 

 late ; ped. shorter. Uxbridge, Mass., Richard ! Probably common. A beauti- 

 ful variety. Fruit not seen. 



3. P. DIVERSIFOLIUS. Bart. (P. setaceum. PA.) Setaceous-leaved Pond-weed. 



Upper Ivs. lanceolate, opposite, 5-veined, on short petioles, lower ones sub- 

 merged, sessile, filiform, alternate, dense, axillary. Common in pools and 

 ditches. A very slender and delicate species, only the upper leaves arising to 

 the surface. These are 6 10" by 2 4", acute at each end, on hair-like peti- 

 oles 5 6" long. Spadices dense, short, 5 6 flowered. July. 



/?. Submersed leaves few, not fascicled in the axils. 



2. Leaves all submersed, uniform. 



4. P. LUCENS. S/iining Pond-weed. 



Lvs. lanceolate, flat, large, the short petioles continuing in a thick mid- 

 vein; spikes long, cylindric, many-flowered. 1\. Can.,N. Eng., &c. Rivers and 

 lakes. Distinguished for its large leaves which are very pellucid, and, when 

 dry, shining above, beautifully veined, 3 5' long, acuminate, J 1' wide, each 

 with a lanceolate bract above its base. Spadix 2' long, of numerous, green 

 flowers, on a peduncle 2 or 3 times as long, thick and enlarged upwards. June. 



5. P. OBRUTUS. Wood. Lyndon Pond-weed. 



Lvs. glossy, linear-lanceolate, sessile, rather acute, only the midvein con- 

 spicuous, alternate, approximate, the lower stipules wanting ; spikes long-pedun- 

 culate ; achenia inflated, subhemispherical, margined on the back, beak incurved 

 both sides, conspicuously umbilicate. Passumpsic river, Lyndon, Vt. ! A 

 remarkable species, differing widely from any other with which I am acquainted. 

 Stem round, slender, simple. Leaves uniform, 3 4' by ', tapering to the 

 slightly clasping base, the two upper opposite. Spike dense, !' long, peduncle 

 3' in length. Fruit with 2 little pits. 



6. P. PRJELONGUS. Wolff. Long-stalked Pond-weed. 



Lvs. oblong or ovate, obtuse, many- veined, with 3 stronger veins, all reticu- 

 lately connected, base amplexicaul; ped. very long; spike cylindrical, many- 

 flowered; fr. ventricose, lunate, acutely carinate on the back. Ponds and 

 rivers, Northern States and Can. The plant is wholly submersed, sending up 

 its spike to the surface on a very long stalk. I have gathered it in Niagara 

 river, growing in depths of 6 or 8f. July, Aug. 



7. P. PERFOLIATUS. Perfoliate or Clasping Pond-weed. 



Lvs. cordate, clasping the stem, uniform, all immersed ; spikes terminal; fls. 

 alternate, A common species growing in ponds and slow waters, wholly below 

 the surface except the purplish flowers. Stem dichotomous, very leafy, 6 10' 

 long. Leaves alternate, apparently perfoliate near the base, 1 J' long, | as wide, 

 obtuse, pellucid. Spadix on a short peduncle (12'), few flowered. Jl. 



8. P. ROBBINSII. Oakes. Robbing Pond-weed. 



_ Lvs. lance-linear, approximate, sheathing the stem with the adn ate stipules, 

 lamina auriculate at base, margin minutely ciliate-serrulate ; spikes oblong, 

 small and few-flowered ; ped. shorter than the leaves. First discovered by Dr. 

 Rabbins! in Pondicherry Pond, Jefferson, N. H. Since found in many other 



