9. P. fluitans, Roth. Stem often branching below; _floating leaves thinnish, lance-oblong or long-elliptical_, often acute, _long-petioled_, 17--23-nerved; _submersed leaves very long_ (3--12', by 2--12" wide), _lanceolate and lance-linear_, 7--15-nerved, coa.r.s.ely reticulated; peduncles somewhat thickened upward; fruit obliquely obovate, obscurely 3-keeled when fresh, and distinctly so when dry, the middle one winged above and sometimes with 3--5 shallow indentations; the rounded slightly curved face surmounted by the short style; nutlet with the sides scarcely impressed; upper part of the embryo circularly in-curved. (P. lonchites, _Tuckerm._)--In streams or rarely in ponds, N. Brunswick to N. J., west to Minn. and Iowa. Aug., Sept. (Eu.)

10. P. pulcher, Tuckerm. Stem simple, black-spotted; _leaves of three kinds_; floating ones becoming very large (4 by 3'), _roundish-ovate and cordate or ovate-oblong_, 25--37-nerved, _all alternate; upper submersed ones (3--5) usually lanceolate_, acute at base and very long-ac.u.minate, 10--15-nerved, very thin, cellular each side of the midrib, undulate, short-petioled; _lowest_ (2--4 near the base of the stem) _thicker, plane, oval or oblong_ with a rounded base, or spatulate-oblong, on longer petioles; stipules rather short and obtuse; peduncles thicker than the stem; fruit with a rounded back and angular face, pointed, distinctly 3-keeled when fresh, sharply so when dry; nutlet with two deep dorsal furrows, and a sinus below the angle in front; sides flat; embryo circularly much incurved above.--Ponds, Vt. to Ga. and Mo. July, Aug.

11. P. ampliflius, Tuckerm. Stems simple, of very variable length; _floating leaves_ (sometimes wanting) large, _oblong or lance-ovate_, sometimes slightly cordate at base, abruptly acutish, 30--50-nerved, on _rather long petioles_; submersed leaves often very large (reaching 7'

by 2'), lanceolate or oval, acute at each end, _usually much recurved, undulate_, mostly on short petioles; _stipules very long and tapering to a point_, soon becoming loose; peduncles thickened upward, in deep water much elongated; fruit very large (over 2" long), rather obliquely obovate, 3-keeled, with a broad stout beak; nutlet slightly impressed on the sides; upper part of the embryo curved into a ring.--Ponds and rivers, N. Eng. to N. J., west to Minn. and Kan. Aug., Sept.

12. P. Illinoensis, Morong. _Stem stout, branching_ towards the summit; floating leaves opposite, oval or ovate (2--5' long by 1--2' broad), 19--25-nerved, rounded or subcordate at base, with a short blunt point at apex, on short petioles, _submersed leaves rather few, oblong-elliptical_, acute at each end, _usually ample_ (largest 8' by 1'); _stipules coa.r.s.e, obtuse, strongly bicarinate_ (2' long); peduncles often cl.u.s.tered at the summit (2--4' long), thickening upward; _fruit roundish-obovate_ (1--2" long), 3-keeled on the back, middle keel prominent; nutlet flattened and slightly impressed on the sides, obtuse or pointed at base; apex of embryo directed transversely inward.--Streams and ditches, western N. Y. to Ill., Iowa, and Minn.

Very near the last.

13. P. heterophllus, Schreb. _Stem slender, very branching_ below; floating leaves mostly thin, variable, but with a short blunt point, 9--15-nerved, usually 1--2' long and 6--9" wide; submersed ones usually lanceolate or linear-lanceolate, ac.u.minate or cuspidate, narrowed toward the base, about 7-nerved on the stem and 3-nerved on the branches; upper ones petioled, lower sessile; _stipules obtuse_, loose; peduncles somewhat thickened upward; fruit small, roundish, compressed, scarcely keeled; embryo annular above. (P. gramineus, _Fries_.)--Still or flowing water, common. Varies exceedingly in its submersed leaves, peduncles, etc.; the var. GRAMINIFLIUS (Fries), growing in rapid streams, with stems much elongated and less branched, and the flaccid submersed leaves 2--7' long by 2--10" wide.

Var. (?) myriophllus, Robbins. Sending up from running rootstocks many short repeatedly dichotomous and densely leafy stems; fertile stems very slender; floating leaves small, delicate, lance-oblong, on long filiform petioles; submersed stem-leaves larger, early perishing; those of the branches (deep-green) linear-oblanceolate, very small (--1' long), acute, sometimes minutely serrulate; spike slender, loosely-flowered, much shorter than the thickened peduncle.--Apponaug Pond, R. I., without fruit.

13^a. P. Zizii, Mert. & Koch. Resembling P. lucens, but smaller, much branched at base; _upper leaves coriaceous or subcoriaceous, long-petioled_ and _sometimes emersed_, the others subsessile, all usually numerous, undulate and _shining_; peduncle elongated. (P.

lucens, var. minor, _Nolte_. Also P. gramineus, var. (?) spathulaeformis, _Robbins_; P. spathaeformis, _Tuckerm._; "P. varians, _Morong_.")--N. Eng to Fla., and westward. Connecting with the next section. (Eu.)

-- 2. _Leaves all submersed and similar, mostly sessile, membranaceous and dilated, lanceolate, oblong, or oval; stipules obtuse, becoming loose._

14. P. lucens, L. Stem thick, branching, sometimes very large; _leaves more or less petioled_, oval or lanceolate, _mucronate_, often rough serrulate, _frequently shining_; peduncles often elongated; _fruit_ roundish and compressed, _with obtuse margins, slightly keeled_; embryo circularly incurved above.--Ponds, N. Eng. to Fla., west to the Pacific.

Aug., Sept. (Eu.)

Var. (?) Connecticutensis, Robbins. Stem flexuous; _leaves_ all submersed, nearly sessile, lanceolate, _ac.u.minate, crisped, not shining nor serrulate; fruit larger, distinctly keeled_; nutlet thick and hard.--Lake Saltonstall, East Haven, Conn.

15. P. praelongus, Wulf. Stem very long, branching, flexuous; _leaves_ lance-oblong or lanceolate (sometimes 7' long), _half-clasping, obtuse with a boat-shaped cavity_ at the extremity, thence splitting on pressure; stipules scarious, very obtuse; spikes rather loose-flowered; _peduncles very long_ (sometimes reaching 20'); _fruit_ obliquely obovate, compressed, _sharply keeled_ when dry; style terminating the nearly straight face; curve of the embryo oval and longitudinal.--Ponds and large rivers, N. Scotia to Ma.s.s., west to Minn. and Iowa. Sept., Oct.--Stem white; foliage bright green. (Eu.)

16. P. perfoliatus, L. Stem branching; _leaves...o...b..cular, ovate or lanceolate from a cordate-clasping base_, usually obtuse and often minutely serrulate; peduncles short, cylindrical; _fruit_ irregularly obovate, _obtusely margined_; embryo incurved in an oval.--Ponds and slow streams, common. N. Scotia to Fla., west to Minn. and Iowa. Sept., Oct. (Eu.)

Var. lanceolatus, Robbins. Larger; _leaves long-lanceolate_ from a cordate clasping base and _ac.u.minate_, wavy, 3--4' long; _peduncles thickened upward_.--Same range as the species, and extending west to the Pacific.

17. P. crispus, L. _Stem compressed; leaves linear-oblong_, half-clasping, obtuse, _serrulate, crisped-wavy, 3-nerved; fruit long-beaked_; upper portion of the embryo incurved in a large circle.--Flowing and stagnant waters, Ma.s.s. to N. J. and Va., west to western N. Y. June, July. (Eu.)

18. P. Msticus, Morong. _Stem very slender_ and irregularly branching, nearly filiform (1--3 high); _leaves oblong-linear_ (--1' long by 2--3" wide), 5--7-nerved, _finely undulate and entire, obtuse or bluntly pointed, abruptly narrowing at base, sessile or partly clasping_; spikes few, capitate (4--6-flowered). on erect peduncles (1--2' long); fruit (immature) obovate, small (hardly " long), _obscurely 3-keeled on the back_, a little beaked by the slender recurved style.--Mystic Pond, Medford, Ma.s.s.

-- 3. _Leaves all submersed and similar, mostly membranaceous and sessile, linear or setaceous._

[*] _Stipules free from the sheathing base of the leaf._

19. P. zosteraeflius, Schum. _Stem branching, wing-fattened; leaves_ linear and gra.s.s-like (commonly 4' by 1"), _abruptly pointed, with many fine and 3 larger nerves_; stipules (seen young) oblong, very obtuse; _spikes cylindrical_, 12--15-flowered, _not half as long as the peduncle_; fruit obliquely obovate, somewhat keeled and with slight teeth on the back, the sides not impressed, the face arching and terminated by the short style; _summit of the large embryo lying transverse to the fruit_. (P. compressus, _Fries_; not _L._?)--Still and slow-flowing waters, N. Eng. to N. J., Iowa, and Minn. Aug., Sept. (Eu.)

20. P. Hillii, Morong. Stem slender, _widely branching, flattish_; leaves linear, _acute_ (1--2' long by --1" wide), _3-nerved_, the lateral nerves delicate and near the margin; stipules whitish, striate, obtuse (3--5" long); _spikes capitate_ (3--6-fruited), _on short spreading or recurved peduncles_; fruit as in the last.--Mich. and western N. Y.

21. P. obtusiflius, Mertens & Koch. _Stem flattened, very branching; leaves_ linear, tapering toward the base, obtuse and mucronate or very acute, _3- (rarely 5-) nerved; stipules elongated_, very obtuse; _spike ovate, continuous_, 5--8-flowered, _about the length of the peduncle_; fruit oval, apiculate with the style, not keeled when fresh, _upper portion of embryo_ coiled inward and _lying transverse to the fruit_.--Slow streams and ponds, Canada and N. Eng. to western N. Y. and Mich. Sept., Oct. (Eu.)

22. P. pauciflrus, Pursh. _Stem filiform, flattish and very branching; leaves narrowly linear_ (1--2' long and seldom "

wide), acute, _obscurely 3-nerved; stipules obtuse; spikes_ capitate, 1--4- _(usually 2-) flowered_, on short club-shaped peduncles; _fruit roundish-lenticular_; the _back_ more or less _crested_; upper portion of the embryo incurved in a circle.--Still or stagnant waters, N.

Brunswick to Ga., Iowa, Minn., and westward.

Var. Niagarensis, Gray. Stem often longer (1--3); leaves larger (1--3' long by 1" wide or less), 3--5-nerved at base, very acute and mucronate, narrowed to the subpetiolate base. (P. Niagarensis, _Tuckerm_.)--Running water, Great Lakes to S. C.; also in Cal.

23. P. pusillus, L. _Stem slender_, flattish or nearly cylindrical, often very branching; _leaves narrow- or setaceous-linear_, ac.u.minate, _acute or subacute, 1--3-nerved_, 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 downward_.--Pools and ditches, N. Scotia to N. J., west to Minn. and Mo., and westward.--Leaves sometimes almost setaceous (var.

tenuissimus, _Koch_).

Var. polyphllus, Morong. Dwarf form (3--5' high), divaricately branching from the base, very leafy throughout; leaves very obtuse, not cuspidate, 3-nerved; non-flowering but abundantly provided with propagating buds which are formed on the thickened and hardened ends of the branches and closely invested by imbricated leaves.--In a shallow pool, S. Natick, Ma.s.s.

24. P. mucronatus, Schrad. Resembling P. pusillus, but stem less branching; _leaves broader_ (almost 1" wide), _often 5-nerved; spikes interrupted_. (P. pusillus, var. major, _Fries_.)--N. Brunswick to western N. Y., Mich., and Minn. July. (Eu.)

25. P. gemmiparus, Robbins. Stem filiform, branching, terete, varying greatly in height; _leaves hair-like_, sometimes not as broad as the stem, often with no apparent midrib, _tapering to the finest point_ (1--3' long), bi-glandular at base; stipules --1' long; spikes few (3--6-flowered), _interrupted_, on long filiform peduncles; _propagating buds very numerous_; fruit like that of P. pusillus, very rare. (P.

pusillus, var.? gemmiparus, _Robbins_.)--Slow-moving streams and still water, Ma.s.s. Aug., Sept.

26. P. Tuckermani, Robbins. _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 tapering to near the fineness of a hair_ (1--4' long and {1/3}" extreme width), obscurely 1--3-nerved, with a few coa.r.s.e reticulations; stipules rather persistent below, {1/3}' long, obtuse; _peduncle solitary, very long_, rather thickened upward; _spike 4--8-flowered, in fruit continuous, oblong; fruit thick-lenticular_, obscurely 3-keeled; _nutlet slightly impressed on the sides; sh.e.l.l thick and hard_; embryo nearly annular.--Cold ponds, White Mountains of N. H., N. Y., and N. J.

[*][*] _Stipules united with the sheathing base of the leaf._

27. P. pectinatus, L. _Stem filiform_, repeatedly dichotomous; _leaves very narrowly linear_, attenuate to the apex, 1-nerved with a few transverse veins; _spikes interrupted_, on long filiform peduncles; fruit obliquely broad-obovate, compressed, _bluntly keeled_; sh.e.l.l of nutlet very thick; _embryo spirally incurved_.--N. Brunswick to Fla., westward across the continent. Aug.--Oct. (Eu.)

28. P. marnus, L. Resembling narrow-leaved forms of the last species, _low and very leafy_; peduncles much elongated; _fruit much smaller_ (1" long) and thinner, round-obovate, _not keeled_ upon the rounded back, tipped with the broad sessile stigma; _embryo annular_.--Western N. Y., Ill., Mich., and southward. Probably the range of this species is much more extensive than indicated, as it has been confounded with P.

pectinatus.

29. P. Robbinsii, Oakes. _Stem_ ascending from a creeping base, _rigid_, very branching, _invested by the bases of the leaves and stipules; leaves crowded in two ranks, recurved-spreading_, narrow-lanceolate or linear (3--5' long and 2--3" wide), ac.u.minate, _ciliate-serrulate_ with translucent teeth, many-nerved; stipules obtuse when young, their nerves soon becoming bristles; _spikes numerous, loosely few-flowered_, on short peduncles; fruit oblong-obovate (2" long), _keeled with a broadish wing_, acutely beaked; embryo stout, ovally annular.--In ponds and slow streams, N. Brunswick to N. J., the N. sh.o.r.e of L. Superior, and far westward.

4. RuPPIA, L. DITCH-GRa.s.s.

Flowers perfect, 2 or more approximated on a slender spadix, which is at first enclosed in the sheathing spathe-like base of a leaf, entirely dest.i.tute of floral envelopes, consisting of 2 sessile stamens, each with 2 large and separate anther-cells, and 4 small sessile ovaries, with solitary campylotropous suspended ovules; stigma sessile, depressed. Fruit small obliquely ovate pointed drupes, each raised on a slender stalk which appears after flowering; the spadix itself also then raised on an elongated thread-form peduncle. Embryo ovoid, with a short and pointed plumule from the upper end, by the side of the short cotyledon.--Marine herbs, growing under water, with long and thread-like forking stems, and slender almost capillary alternate leaves, sheathing at the base. Flowers rising to the surface at the time of expansion.

(Dedicated to _H. B. Ruppius_, a German botanical author of the early part of the 18th century.)

1. R. maritima, L. Leaves linear-capillary; nut ovate, obliquely erect, 1" long; fruiting peduncles capillary (3--6' long); stipes 1--12"

long.--Shallow bays, along the entire coast; also occasionally in saline places in the interior. (Eu., Asia, etc.)

5. ZANNICh.e.l.lIA, Micheli. HORNED PONDWEED.

Flowers moncious, sessile, naked, usually both kinds from the same axil; the sterile consisting of a single stamen, with a slender filament bearing a 2--4-celled anther; the fertile of 2--5 (usually 4) sessile pistils in the same cup-shaped involucre, forming obliquely oblong nutlets in fruit, beaked with a short style, which is tipped by an obliquely disk-shaped or somewhat 2-lobed stigma. Seed orthotropous, suspended, straight. Cotyledon taper, bent and coiled.--Slender branching herbs, growing under water, with mostly opposite long and linear thread-form entire leaves, and sheathing membranous stipules.

(Named in honor of _Zannich.e.l.li_, a Venetian botanist.)

1. Z. pal.u.s.tris, L. Style at least half as long as the fruit, which is flattish, somewhat incurved, even, or occasionally more or less toothed on the back (not wing-margined in our plant), nearly sessile; or, in var. PEDUNCULaTA, both the cl.u.s.ter and the separate fruits evidently peduncled.--Ponds and slow streams, throughout N. America, but not common. July. (Eu., Asia.)

6. ZOSTeRA, L. GRa.s.s-WRACK. EEL-GRa.s.s.

Flowers moncious; the two kinds naked and sessile and alternately arranged in two rows on the midrib of one side of a linear leaf-like spadix, which is hidden in a long and sheath-like base of a leaf (spathe); the sterile flowers consisting of single ovate or oval 1-celled sessile anthers, as large as the ovaries, and containing a tuft of threads in place of ordinary pollen; the fertile of single ovate-oblong ovaries attached near their apex, tapering upward into an awl-shaped style, and containing a pendulous orthotropous ovule; stigmas 2, long and bristle-form, deciduous. Utricle bursting irregularly, enclosing an oblong longitudinally ribbed seed (or nutlet). Embryo short and thick (proper cotyledon almost obsolete), with an open c.h.i.n.k or cleft its whole length, from which protrudes a doubly curved slender plumule.--Gra.s.s-like marine herbs, growing wholly under water, from a jointed creeping stem or rootstock, sheathed by the bases of the very long and linear, obtuse, entire, gra.s.s-like, ribbon-shaped leaves (whence the name, from ??st??, _a band_).

1. Z. marna, L. Leaves obscurely 3--5-nerved.--Common in shoal water of bays along the coast, from Newf. to Fla. (Eu.)

7. NaIAS, L. NAIAD.

Flowers dicious or moncious, axillary, solitary and sessile; the sterile consisting of a single stamen enclosed in a little membranous spathe; anther at first nearly sessile, the filament at length elongated. Fertile flowers consisting of a single ovary tapering into a short style; stigmas 2--4, awl-shaped; ovule erect, anatropous. Fruit a little seed-like nutlet, enclosed in a loose and separable membranous epicarp. Embryo straight, the radicular end downward.--Slender branching herbs, growing under water, with opposite and linear leaves, somewhat crowded into whorls, spinulose-toothed, sessile and dilated at base.