Leaves alternate or the uppermost opposite, often of 2 kinds, submerged and floating, the submerged mostly linear, the floating coriaceous, lanceolate, ovate or oval. -Spathes stipular, often ligulate, free or connate with the base of the leaf or petiole, enclosing the young buds and usually soon perishing after expanding. Peduncles axillary, usually emersed. Flowers small, spicate, green or red. Perianth none. Stamens 4. Anthers sessile, the connective dilated, perianth-like (Fig. 186). Ovaries 4, sessile, distinct, 1-celled, 1-ovuled, attenuated into a short erect or recurved style, or with a sessile stigma. Fruit of 4 ovoid or sub-globose drupelets, the pericarp usually thin and hard or spongy. Seeds crustaceous, campylo-tropous, with an uncinate embryo thickened at the radicular end. [Greek, in allusion to the aquatic habitat.] Water Spike.

About 65 well-defined species, natives of temperate regions. Besides the following. about 3 others occur in the southern parts of North America. Type species Potamogeton natans L.

Stipules axillary and free from the rest of the leaf.

With floating and submerged leaves.

Submerged leaves bladeless.

Nutlets more or less pitted.

1.

P. nutans.

Nutlets not pitted.

2.

P. Oakesianus.

Submerged leaves with a proper blade.

Submerged leaves of 2 kinds, lanceolate and oval or oblong

Uppermost broadly oval or elliptical, lowest lanceolate.

3.

P. amplifolius.

Uppermost lanceolate and pellucid, lowest oblong and opaque,

4.

P. pulcher.

Submerged leaves all alike, capillary or linear-setaceous.

1-nerved or nerveless.

25.

P. Vaseyi.

3-nerved.

26.

P. lateralis.

Submerged leaves all alike, linear.

* Text of this family and of the two following ones contributed to the first edition by the late Rev. Thomas Morong, revised for this edition by Mr. Norman Taylor.

Nearly the same breadth throughout, obtusely pointed, coarsely cellular-reticulated in

the middle.

5.

P. epihydrus.

Broader at base, acute, without cellular-reticulation.

9.

P. heterophyllus.

Submerged leaves all alike, lanceolate.

Uppermost leaves petioled, lowest sessile.

6.

P. alp in us.

All the leaves petioled.

Floating leaves large, broadly elliptic, rounded or subcordate at base.

11.

P. illinoensis.

Floating leaves narrowly elliptical, tapering at base.

7.

P. americanus.

Floating leaves mostly obovate or oblanceolate, tapering at base.

8.

P. Faxoni.

All the leaves sessile or subsessile.

Fruit only 1 line long, obscurely 3-keeled.

10.

P. varians.

Fruit 1 1/2 lines long, distinctly 3-keeled.

12.

P. angustifolius.

With submerged leaves only.

Without propagating buds and without glands.

Leaves with broad blades, mostly lanceolate or ovate, many-nerved.

Leaves subsessile or short-petioled, mostly acute or cuspidate.

13.

P. lucens.

Leaves semi-amplexicaul, obtuse and cucullate at the apex.

14.

P. praelongus.

Leaves meeting around the stem, very obtuse at the apex, not cucullate.

15.

P. perfoliatus.

Leaves with narrow blades, linear or oblong-linear, several-nerved.

Leaves oblong-linear, 5-7-nerved, obtuse at the apex.

16.

P. mysticus.

Leaves narrowly linear, 3-nerved, acute at the apex.

31.

P. foliosus.

Leaves with narrow blades, capillary or setaceous, 1-nerved or nerveless.

17.

P. confervoides.

With propagating buds or glands, or both.

With buds, but without glands.

Leaves serrulate, 3-7-nerved.

18.

P. crispus.

Leaves entire, with 3 principal and many fine nerves.

19.

P. compressus.

Commonly with glands, but no buds.

Stems long-branching from the base; leaves lax, flat, 3-nerved, abruptly acute or

cuspidate.

20.

P. Hillii.

Stems simple: leaves strict, revolute, 3-5-nerved, acuminate.

24.

P. rutilus.

With both buds and glands.

Glands large and translucent: buds rare.

22.

P. obtusifolius.

Glands small, often dull; buds common.

Leaves linear, 5-7-nerved.

23.

P. Friesii.

Leaves linear, 3-nerved.

27.

P. pusillus.

Leaves capillary, 1-nerved or nerveless.

28.

P. gemmiparus.

Stipules adnate to the leaves or petioles.

With both floating and submerged leaves.

Submerged peduncles as long as the spikes, clavate, often recurved.

29.

P. diversifolius.

Submerged peduncles none, or at most hardly a line long.

30.

P. dimorphus.

With submerged leaves only.

Stigma broad and sessile.

Sheath of stipule less than 4" long.

31.

P. filiformis.

Sheath of stipule more than 7" long.

32.

P. interior.

Style apparent; stigma capitate.

Fruit without keels or obscurely keeled.

33.

P. pectinatus.

Fruit strongly 3-keeled.

Leaves entire, 3-5-nerved.

34.

P. interruptus.

Leaves minutely serrulate, finely many-nerved.

35.

P. Robbinsii.