NAIADACE.E. 351 



Z. palustris, L. Common Homed Pondiceed. 



Stem, filiform, floating; style half as long as the fruit, which is flattish, some- 

 what incurved, even, more or leas toothed on the hack, nearly sessile. 



Ponds and slew streams. July, Aug. Stem 1 to 2 feet long, round, smooth. 

 leaves grass-like, 2 to 3 inches long, sessile. Flowers issuing from axillary bracts, 

 ►mall, 2 together, a sterile and fertile. 



3. POTOMAGETON, Tourn. Pondweed. 



f?r.potomos, a river, and gciton, near. 



Flowers perfect, spiked. Perianth single, 4-leaved. 

 Stamens 4, nearly sessile, opposite the perianth lobes. 

 Ov r aries 4, pedicellate : stigma sessile or nearly so. Fruit 

 4 sessile nutlets or drupes, flattened on one or two sides. 

 Seeds hook-shaped. — Mostly perennial aquatic and submersed 

 herbs, with creeping and rooting stems, two-ranked pellucid leaves , 

 united membraneous sheathing stipules, and small greenish flowers 

 3 to 10, in a pedunculate spike, rising above the water. 



* Leaves of two forms, the upper floating. 



1. P. natans, S. Broad-leaved Pondweed. 



Leaves all long-petioled, the floating ones coriaceous, oval, elliptical, or ovate, 

 chiefly rounded or a little heart-shaped at the baser, many-nerved; immersed ones 

 linear or lanceolate ; spikes rather dense, shorter than the peduncles ; fruit short- 

 pointed, more or less keeled on the back. 



Ponds and slow waters, common in the Susquehanna. July, Aug. Stem slender, 

 1 to 3 feet long, branched. Spike 1 to 2 inches long, 20 to 40-flowered. Varies with 

 the lower leaves all reduced to petioles. 



2. P. OBLONGUS, Viv., Fries. Oblong-leaved Pondweed. 



Leaves oblong-elliptical ; nutlets small, obtuse and pointless, always rounded at 

 the back. 



Pools and ditches. Floating haves oblong-elliptical or oblong-lanceolate. Fruit 

 rounded, not half as large ae in P. natans. 



3. P. heterophyllus, Schreb. Yarious-leaved Pondweed. 



Floating leaves elliptieal or oblong, or the lowest lance-spatulate, on long pe» 

 tioles; immersed leaves lanceolate or linear, sometimes elongated and grass-like, 

 flaccid, obscurely denticulate or roughish on the margins, the lower sessile ; pe- 

 duncles much thicker than the stem, elongated; spike cylindric, many-flowered. 



Pools and shallow slow streams, common. Aug. Stems numerous, branched, 

 filiform. Floating leaves 1 inch long, very variable. Peduncle 1 to 2 inches long. 

 yutlcls roundish, flattened on the sides, obtuse and rigid on the back. 



4. P. hybridus, Michx. Hybrid Pondweed. 



Floating leaves oval or lance-oblong, 5 to7nerved, on petioles; immersed leaves 

 capillary; spike globular, few-flowered, on a short somewhat club-shaped peduncle. 



Shallow pools and streams. Aug. A delicate species, with thread-like branching 

 Btems 1 foot or more in length, and the floating leaves % to % inch long, some- 

 times none. Fruit nearly round, flattened on the sides, somewhat keeled and 

 crested on the back. 



* * Leaves all submersed, uniform. 



5. P. lucens, L. Shining Pondweed. 

 Ltavet oval-lanceolate, flat, large, the ghort petioles continuing in a thick midrib* 



