Order 13i.— NAIADACKffi. 675 



7 P. obr&tuB. Lvs. linear-lanceolate, sessile, rather acute, only the inidvein con- 

 spicuous, alternate, approximate, tho lower stip. wanting; spikes long, peduncu- 

 late; ped. not enlarged upwards; ach. inflated, margined on the back, beak in- 

 curred, both sides conspicuously umbilicate. — A remarkable species, first found 

 at Lyndon, Vt., since seen southward to Ga.; in slow waters. St. round, slender, 

 simple. Lvs. uniform, 3 to 4' by 4 to 6", tapering to tho slightly clasping base, 

 the two upper opposite. Spike dense, IV long, ped 3'. Seed coiled into a ring 

 as shown by the pits of the fruit. 



8 P. praelongus "Wolfg. Lvs. oblong or ovate, obtuse, many-veined, with three 

 stronger veins, all reticulately connected, baso amploxieaul ; ped. very long; spik» 

 Cylindrical, many-flowered ; fr. ventricous, lunate, acutely carinato on the back.- — 

 Ponds and rivers, Northern States and Can. Tho plant is wholly submersed, 

 sending up its spike to the surface on a very long stalk. Wo have gathered it ia 

 Niagara river, growing in depths of G or Sf. Jul}', Aug. 



9 P perfoliatus L. Lvs. cordate, clasping tho stem, uniform, all immersed; 

 spikes terminal ; fls. alternate ; fruit not keeled. — A common species growing in 

 ponds and slow waters, wholly below tho surfaco except the purplish flowers. 

 Stem dichotomous, very leafy, 6 — 10' long. L2aves alternate, apparently per- 

 foliate near t'.ie base, IV long, J as wide, obtuse, pellucid. Spadix on a short 

 peduncle (1 — 2'), few-flowered. Jl. 



10 P. paucifloras Pursfa. St. dichotomous, slightly compressed, filiform; lvs 

 linear, alternate, sessile ; fls. few in the spike, ped. short ; fruit distinctly crested 

 on the back. — A. delicate species, in rivers, &c. Leaves numerous, obtuse, taper- 

 ing to the stipulate base, 2 — 3' long, a line wide, 3-veined, of a bright green 

 color. Peduncle an inch long, terminal, bearing 3 — 5 greenish fls. above tha 

 water, but ripening tho seeds below. (P. gramiueum Mx.) 



11 P. compressus L. St. compressed, ancipital, flexuous ; lvs. broad-linear, ob- 

 tuse ; spike short, pedunclo elongated. — A very distinct species in ponds and 

 rivers. Stem 1 — 2f long, branching, weak, flattened, green, with sheathing 

 stipules above tho nodes. Leaves 3 — k' in length, 2'' wide, closely sessile, re- 

 mote, the margins perfectly parallel, ending in an abrupt point. Spadix terminal,, 

 i — 1' long, on a peduncle 1 — 2' long, and bearing 5 — 25 flowers. Jl. (P. zos- 

 terilblium helium.) 



12 P. pusilhi3 L. St. filiform, flexuous, branched; lvs. linear-subulate, mem- 

 branaceous, very acute, sessile, not narrower than tho stipules, spikes capitate, 

 few-flowered; fr. ovoid-cumpressed, umbilicate each side. — Shallow waters, N. 

 Eng. to Ohio and Can. A very delicate species, wholly submersed. Leaves 1 

 — 2' by £", a little longer than the internodes. Spikes 3 — 5-flowered, the pe- 

 duncles £' long. Fruit with sharp pits, as in P. obrutus. and rather inflated. 



13 P. Tuckermani Robbing ? St filiform, with capillary branches ; lvs. few, all 

 capillary and confervoid, with minute, membranous stipules; spikes few (6 to 9)- 

 flo\vered, oblong, on a long, filiform pedunclo, which is slightly thicker than tha 

 stem ; (/ruit immature). — In clear water, Uxbridge, Mass. (Rieard) (White Mts., 

 Alleghany Mts. Tuckerman? in Gray's Manual.) An exceedingly delicate species. 

 Tho leaves taper to tho fineness of cobwebs. Spiko 4" long, the ped. about b' 

 laug. 



14 P. pectinatus (and P. marinu3 L.) St. slender, branched, striate, flexuous , 

 lvs. numerous aud fascicled in tho axils, long, narrowly linear, acuminaie, oa 

 sheathing stipules: spike3 cylindrical, the lower fls. remote; peel filiform, long.. 

 — Plant submersed in deep water, bushy and very leafy, N. Eng. ? Middle States I 

 W. to Wis. (Lapham ?) Leaves 4- — 7' by (les3 than) 1 , thin, tho midvein scarcely 

 perceptible. Fruit large, purplish, rough, a littlo compressed, neither carinate, 

 nor umbilicate. Jn. 



15 P. Robbinsii Oakes. Lvs. lance-linear, approximate, sheathing the stem 

 with the adnate stipules, lamina auriculate at base, margin minutely ciliate-ser- 

 rulate; spikes oblong, small and few-flowered; pod. shorter than the leaves. — 

 First discovered by Dr. Robbins in Pondicherry Pond, Jefferson, N. IT. Sinca 

 found in many other ponds in N. H., Mass. W. to Ohio. St. long, branched, 

 almost wholly enclosed in tho sheaths. Lvs. 2 to 4' by 2 to 3", very acute, some- 

 what crowded. 



