NAIADACK-E. ( PON I> YVKED FAMILY.) 489 



18. P. Niagarensis, Tuckerman. Stem flattened, very branching; leaves 

 linear, very acute, mucronate and much attenuate at the nearly sessilt base, 3-5- 

 nerved, scarcely veiny (l^ / -3 , long and at most I"wide) ; midrib below dilated; 

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



peduncles short, club-shaped, compressed ; spikes few, capitate, 8- \2-fiowered; 

 fruit roundish, compressed, with a winged and toothed keel and angled Jim ; " seed 

 convolute-uneinate." — Rapids above Niagara Falls, Tuckerman. Aug. 



19. P. paucifldrus, Pursh. Stem filiform, flatfish and very branching ; 

 leaves narrow linear (1' -2' long- and seldom £" wide), acute, obscurely %-wrred; 

 stipules obtuse; spikes capitate, 1-4- usually 2-fiowered, 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 are approached by the preceding. 



20. P. pusillus, L. Stem slender, flattish or nearly cylindrical, branching ; 

 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 of embryo incurved and directed obliquely downwards. — Pools and ditches: 

 rather common, especially southward. — The principal forms are 



Var. major, Fries. Stem less branching; leaves broader (almost 1" wide), 

 often 5-nerved; spikes interrupted. (P. mucronatus, Schruder.) — This hardly 

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



Var. vulgaris, Fries. Slender, very branching ; leaves S-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, obsolettly 3-tierved ; 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 filiform and very branching, leaves thicker, 

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

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

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

 wanting. (P. gemmiparus, Robbins 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 

 gemmae, the fruit not maturing. 



21. P. Tuckermani, Kobbins. Very slender and delicate from a creeping 

 rootstock, of a fine light 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 (l'-4' long and J" extreme width), obscurely 1 — 

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

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



