NAIADACE^. (PONDWEED FAMILY.) 489 



18. P. ]!Tiagar^IlSiS, Tuckerman. Stem flattened, very branching; leaves 

 linear, very acute, mucronate and much attenuate at the nearli/ sessile base, 3-5- 

 nerved, scarcely veiny < li' ~ ■'*' '"ig and at most 1" widej ; midrib below dilated; 

 stipules acutish (sometimes 8" long), the numerous nerves early becoming bristles; 

 peduncles short, club-shaped, compressed ; spikes few, capitate, 8- 12-_/?(OTere(/; 

 fruit roundish, compressed, with a winged and toothed Iceel and angled face ; " seed 

 convolute-uncinate." — Kapids above Niagara Falls, Tuckerman. Aug. 



19. P. paueifldrUS, Pursh. Stem filiform, flatlish and very branching; 

 leaves narrow linear [V - i' \oug and seldom ^" wide), acute, ofisrare?^ 3-nerved; 

 stipules obtuse ; spikes capitate, 1-4- usually 2-flowered, on short club-shaped pedun- 

 cles ; fruit roundish-lenticular ; the back more or less crested; upper portion of the 

 embryo incurved in a circle. — Still or stagnant waters : common. Aug., 

 Sept. — Its largest forms arc approached by the preceding. 



20 P. pusillus, L. 5<cm sfcnder, flattish or nearly cylindrical, brandling ; 

 leaves narrow- or setaceous-linear, obtuse or acute, furnished with translucent glands 

 on each side at the base'; stipules at first obtuse; spikes interrupted or capitate, 

 2 - 8-flowered, on rather long peduncles ; fruit obliquely elliptical, scarcely keeled; 

 apex if embryo incurved and directed obliquely downwards. — Pools and ditches; 

 rather common, especially southward. — The principal forms are 



Var. m^or, Fries. Stem less branching ; /enwes irooffcr (almost 1" wide), 

 oflen 5-nerved ; spikes interrupted. (P. mucronatus, /Sc/irurfer.) — This hardly 

 passes into the following forms : rather rare. July. (Eu.) 



Var. vulgaris, T?ries. Slender, very branching ; leaves 3-nerved, often ob- 

 tuse ; spikes cylindrical and interrupted, or capitate and then but 1 -3-flowered. 

 — A rare form (E. Mass.) has sometimes lanceolate floating leaves of the length 

 of the petioles, with 5 nerves impressed beneath, as in P. hybridus. A Swedish 

 specimen in Fries. Herb. Norm, exhibits the same in the following variety, though 

 in a slighter degree. July, Aug. (Eu.) 



Var. tenuissimus, Mertens & Koch. Stem very slender and much 

 branched ; leaves almost setaceous, acute or cuspidate, ohsoktely 3-nerved ; spikes 

 interrupted or capitate. — New England and New York : rather rare. July, 

 Aug. — All three are rather sparingly furnished with reproductive buds : also 

 the last two fruit freely, — the reverse of the fact in the following. 



Var. ? gemmiparUS. Stem iiliform and very branching, leaves thicker, 

 perfectly setaceous and usually exeeedrngly attenuate to the finest point, scarcely 

 with a proper midrib ; stipules long (i'-l'), obtuse; spikes very few, always 

 interrupted, 3 - 6-flowered, long-peduncled ; propagating buds very numerous ; fruit 

 wanting. (P. gemmi'parus, Bobbins in herb.) — Pools and slow-flowing waters: 

 outlet of Mystic Pond, near Boston, Tuckerman ; valley of the Blackstone from 

 Worcester to Providence. — This plant is annual, propagated exclusively by its 

 gemmte, the fruit not maturing. 



21. P. Tuckermini, Bobbins, Very slenderand delicate from a creeping 

 rootstock, of a fine Hght green ; stem filiform with several short and repeatedly 

 dichotomous leaf-bearing branches ; leaves thin and flat, but setaceous and taper- 

 ing to near the fineness of a hair(\'-i' long and J" extreme width), obscurely I — 

 3-ncrved, with a few coarse reticulations ; stipules rather persistent below, J' 

 long, obtuse; peduncle solitary, very long, lather thickening upward; spike 4-8- 



