NAIADACK.E. (PONDWEED FAMILY.) 489 



18. P. Niagarensis, Tuckerman. Stem flattened, t>en/ branchirifj ; leaves 

 linear, very acute, iiiiicronatc and much attenuate at the nearlij scusilc base,. 3 -5- 

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

 stipules acutisk (sometimes 8" lon;.^), the numerous nerves early becominr/ bristles; 

 peduncles short, club-shaped, comj)resscd ; spikes few, capitate, 8 - l2-flowered ; 

 fnut roundish, compressed, tcith a winc/ed and toothed keel and anrjkd flu:e ; " seed 

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



19. P. paueiflbrus, I'ursh. Stem Jiliform, flaltish and verij branching; 

 leaves narrow linear (I' -i' long and seldom ^" wide), acute, t>6>c(«-e/^ 3-nervcd ; 

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

 cles ; fruit roundish-lenticular ; the back more or less crested; upjier 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. Stem slender, flattish or nearly cylindrical, branching ; 

 leaves narroio- or setaceous-linear, obtuse or acute, furnished with translucent (/lands 

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

 2 - 8-flowcred, on rather long peduncles ; J'ruit 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, I'ries. Stem less branching; leaves broader (almost 1" wide), 

 often 5-nerved ; spikes interrupted. (P. mucronatus, Schmder.) — 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 cus\)'niat(i, obsoletely 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. 1 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'-T), obtuse; spikes very few, always 

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

 wanting. (P. gemnn'parus, 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 hy its 

 gemmffi, the fruit tiot maturing. 



21. P. Tuekerm^ni, Robbins. Vtry slender and delicate i\-om a crcQ-pm^ 

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

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

 ing to near the fineness of a hair (I' -4' long and ^" 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- 



