NAIADACE^E. (PONDWEED FAMILY.) 489 



18. P. K"iagar6nsiS, Tuckerman. Stem flattened, very branching; leaves 

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

 nerved, scarcely veiny (1^'- 3' long and at most 1" wide) ; midrib below dilated; 

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

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

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

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



19. P. pauciflorilS, Bursh. Stem filiform, flattish and very branching, 

 leaves narrow linear (1'- 2' long and seldom |" wide), acute, obscurely 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 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. — Fools 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 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, obsolete/y 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 filiform and very branching, leaves thicker, 

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

 with a proper midrib; stipules long (£'-1'), obtuse; spikes very few, always 

 interrupted, 3 - 6-flowcred, 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 

 gemma?, the fruit not maturing. 



21. P. Tuckermaili, Bobbins. 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 (1' -4' long and %" extreme width), obscurely 1 - 

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

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



