2 P. heterophyllus Schreb. Floating lvs. lanceolate or oblong, 5 to 7-veined. tapering to the petioles, scarcely coriaceous, subinersed lvs. long, narrowly linear, membranous, acute, 1-veined, slightly tapering to the sessile base; stip. nearly distinct, resembling the lvs.; spikes dense, on thickened peduncles; fr. compressed, suborbicular. - Ponds and slow waters, frequent. St. round, slender or filiform, often branched. Lower lvs. 3 to 6' by 1 1/2", remote, upper about 2 to 3' by 1/2. Spikes 1' long, peduncles 2 to 4'. (P. Claytonia Tuckerman.)

3 P. diversifdlius Bart. St. filiform, branching; upper lvs. oval or lance-oval, 5-veined, on short petioles, lower ones submerged, sessile, filiform, alternate, often densely fascicled, not at all reticulated, obtuse. - Common in pools and ditches A very slender and delicate species, only the upper lvs. arising to the surface. These are 6 to 10" by 2 to 4", acute at each end, on hair-like petioles 3 to 6" long. Spadices dense, short, 5 to 6-flowered. Jl. (P. setaccum Ph.) - Varies with the leaves nearly all of either kind.

4 P. hybridus Mx. Floating lvs. elliptic-oblong, coriaceous, scarcely veined, longer than their petioles; submersed lvs. long-linear, thin, sessile; stipules above equaling the petioles, those of the submersed lvs. very short or wanting; spikes cylindric, dense, on short, thickened ped.; fruit keeled on the back, seed coiled into a ring. - Pools and slow waters, S. ? and W. States. Sts. mostly simple, very slender, 1 to 3f long. Lower lvs. 3 to 5' long, alternate, upper opposite, 1' to 18". Spike about l. A handsome species.

5 P. fluitans Roth. Floating lvs. opposite, oval-lanceolate, coriaceous, acute at each end, shorter than the petioles; submersed lvs. larger than the floating, lanceolate, sessile, short-acuminate, strongly veined, wavy, thin, not shining, faintly reticulated; stip. large, connate; ped. thickened, cylindric. - In clear, deep waters, N. New Eng. and Can. Sts. simple or branched, several feet long. Submersed lvs. 5 to 7' long, a third as wide, the floating 2 to 3' long. Stip. 2 to 3' long. Spikes 2' long, rather dense-flowered. Aug.

6 P. lucens L. Lvs. shining, oblong-lanceolate, acuminate, flat, large, the short petioles continuing in a thick midvein; spikes long, cylindric, many-flowered; ped. thickened upward; fr. slightly keeled. -Order CXXXIV Naiadaceae Naiads Continued 1925 Can, N. Eng., etc. Rivers and lakes. Distinguished for its large leaves which are very pellucid, and, when dry, shining above, conspicuously cross-veined, 3 to 5' long, an inch or more wide, each with a lanceolate, double stipule above its base. Spadix 2' long, of numerous green flowers, on a peduncle 2 or 3 times as long, thick and enlarged upwards. Jn.

7 P. obrutus. Lvs. linear-lanceolate, sessile, rather acute, only the midvein conspicuous, alternate, approximate, the lower stip. wanting; spikes long, pedunculate; ped. not enlarged upwards; ach. inflated, margined on the back, beak incurved, both sides conspicuously umbilicate. - A remarkable species, first found at Lyndon, Vt., since seen southward to Ga.; in slow waters. St. round, slender, simple. Lvs. uniform, 3 to 4' by 4 to 6", tapering to the slightly clasping base, the two upper opposite. Spike dense, 1 1/2' long, ped 3'. Seed coiled into a ring as shown by the pits of the fruit.

8 P. praelongus Wolfg. Lvs. oblong or ovate, obtuse, many-veined, with three stronger veins, all reticulately connected, base amplexicaul; ped. very long; spike cylindrical, many-flowered; fr. ventricous, lunate, acutely carinate on the back. - Ponds and rivers, Northern States and Can. The plant is wholly submersed, sending up its spike to the surface on a very long stalk. We have gathered it in Niagara river, growing in depths of 6 or 8f. July, Aug.

9 P. perfoliatus L. Lvs. cordate, clasping the stem, uniform, all immersed; spikes terminal; fls. alternate; fruit not keeled. - A common species growing in ponds and slow waters, wholly below the surface except the purplish flowers. Stem dichotomous, very leafy, 6 - 10' long. Leaves alternate, apparently perfoliate near the base, 1 1/2' long, 1/3 as wide, obtuse, pellucid. Spadix on a short peduncle (1 - 2'), few-flowered. Jl.

10 P. pauciflorus Pursh. St. dichotomous, slightly compressed, filiform; lvs linear, alternate, sessile; fls. few in the spike, ped. short; fruit distinctly crested on the back. - A delicate species, in rivers, etc. Leaves numerous, obtuse, tapering to the stipulate base, 2 - 3' long, a line wide, 3-veined, of a bright green color. Peduncle an inch long, terminal, bearing 3 - 5 greenish fls. above the water, but ripening the seeds below. (P. gramineum Mx.)

11 P. compressus L. St compressed, ancipital, flexuous; lvs. broad-linear, obtuse; spike short, peduncle elongated. - A very distinct species in ponds and rivers. Stem 1 - 2f long, branching, weak, flattened, green, with sheathing stipules above the nodes. Leaves 3 - 4' in length, 2" wide, closely sessile, remote, the margins perfectly parallel, ending in an abrupt point Spadix terminal, 1/2 - 1' long, on a peduncle 1 - 2' long, and bearing 5 - 25 flowers. Jl. (P. zos-terifolium Schum.)

12 P. pusillus L. St. filiform, flexuous, branched; lvs. linear-subulate, membranaceous, very acute, sessile, not narrower than the stipules; spikes capitate, few-flowered; fr. ovoid-compressed, umbilicate each side. - Shallow waters, N. Eng. to Ohio and Can. A very delicate species, wholly submersed. Leaves 1 - 2' by 1/2', a little longer than the internodes. Spike3 3 - 5-flowered. the peduncles 1/4' long. Fruit with sharp pits, as in P. obrutus, and rather inflated.

13 P. Tuckermani Robbins ? St filiform, with capillary branches; lvs. few, all capillary and confervoid, with minute, membranous stipules; spikes few (6 to 9)-flowered, oblong, on a long, filiform peduncle, which is slightly thicker than the stem; (fruit immature). - In clear water, Uxbridge, Mass. (Ricard) (White Mts., Alleghany Mts. Tuckerman ? in Gray's Manual.) An exceedingly delicate species. Tho leaves taper to the fineness of cobwebs. Spike 4" long, the ped. about 5' long.

14 P. pectinatua (and P. marinus L.) St slender, branched, striate, flexuous; lvs. numerous and fascicled in the axils, long, narrowly linear, acuminate, on sheathing stipules; spikes cylindrical, the lower fls. remote; ped. filiform, long. - Plant submersed in deep water, bushy and very leafy, N. Eng. ? Middle States W. to Wis. (Lapham ?) Leaves 4 - 7' by (less than) 1", thin, the midvein scarcely perceptible. Fruit large, purplish, rough, a little compressed, neither carinate, nor umbilicate. Jn.

15 P. Robbinsii Oakes. Lvs. lance-linear, approximate, sheathing the stem with the adnate stipules, lamina auriculate at base, margin minutely ciliate-ser-rulate; spikes oblong, small and few-flowered; ped. shorter than the leaves. - First discovered by Dr. Robbins in Pondicherry Pond Jefferson, N. H. Since found in many other ponds in N. H, Mass. W. to Ohio. St long, branched, almost wholly enclosed in the sheathe Lvs. 2 to 4' by 2 to 3', very acute, somewhat crowded.

Cohort 6, FLORIDEae.

Endogenous plants with the Flowers usually perfect and complete, the perianth double, 3-parted, the outer often, and sometimes both, green.