28. Potamogeton Gemmiparus (Robbins) Morong. Thread-Like Pondweed

Fig. 201

Potamogeton pusillus var. (?) gemmiparus Rob-bins in A. Gray, Man. Ed. 5, 489. 1867.

Potamogeton gemmiparus Morong, Coult. Bot. Gaz. 5: 51. 1880.

Stems filiform, terete, branching, 5'- 4º long. Leaves capillary, sometimes not as wide as the stem, often with no perceptible midrib, tapering to the finest point, 1'-3' long, 2-glandular at the base; stipules 1/2'-1' long, acute or obtuse, mostly deciduous; spikes interrupted, 3-6-flow-ered; peduncles filiform or sometimes slightly thickened, ¥-2' long; fruit seldom formed, similar to that of P. pusillus.

In ponds, eastern Massachusetts and Rhode Island. It is commonly propagated by its abundant buds, the leaves and stems are often alike in thickness so that the plant seems to consist of threads. Aug.-Sept. A very slender form of the preceding and doubtfully distinct from it.

29. Potamogeton Diversifolius Raf. Rafinesque's Pondweed

Fig. 202

Potamogeton hybridus Michx. FI. Bor. Am. 1: 101. 1803. Not Thuill. 1790.

Potamogeton diversifolius Raf. Med. Rep. (II.) 5: 354. 1808.

Potamogeton diversifolius multidenticulatus Morong, Mem. Torr. Club, 3: Part 2, 48. 1893.

Potamogeton diversifolius trichophyllus Morong, Mem. Torr. Club, 3: part 2, 49. 1893.

Stems flattened or sometimes terete, much branched. Floating leaves coriaceous, the largest 1' long by 1/2 wide, oval or elliptic and obtuse, or lanceolate-oblong and acute; petioles generally shorter, but sometimes longer than the blades, filiform or dilated; submerged leaves setaceous, seldom over 1/4" wide, 1' - 3' long; stipules obtuse or truncate, 3"-5" long, those of the floating leaves free, those of the submerged leaves sometimes adnate; emersed peduncles 3"-7" long; submerged peduncles 2"-3" long, clavate, as long as the spikes; emersed spikes 3"-$" long, occasionally interrupted; fruit cochleate, rarely over 1/2" long, 3-keeled, the middle keel narrowly winged and usually with 7-12 knob-like teeth on the margin, the lateral keels sharp or toothed; embryo coiled 1 1/2 times.

In still water, Maine to Florida, west to California and Texas. June-Sept. A common and well-marked species which often covers large areas of water, practically to the exclusion of everything else. From P. dimorphus, its nearest relative, it may readily be distinguished by its distinctly stalked submerged spikes of flowers.

30. Potamogeton Dimorphus Raf. Spiral Pondweed

Fig. 203

Potamogeton dimorphus Raf. Month. Mag. Crit. Rev.

1: 358. 1817.. Potamogeton Spirillus Tuckerm. Am. Journ. Sci. (II.)

6: 228. 1848. Potamogeton Spirillus curvifolius Peck, N. Y. Stale Mus. Rep. 49: 28. 1896.

Stems compressed, branched, 6'-2o' long, the branches often short and recurved. Floating leaves oval or elliptic, obtuse, the largest about 1' long and ¥ wide, with 5-13 nerves deeply impressed beneath, their petioles often 1' long; submerged leaves linear, 1 1/2"-2" long, about ¥' wide, mostly 5-nerved; stipules of the upper floating leaves free; those of the submerged leaves adnate to the blade or petiole; spikes above water 3"-5" long, continuous, the lower mostly sessile, capitate and 1-10-fruited; fruit cochleate. roundish, less than 1" long, flat and deeply impressed on the sides, 3-keeled on the back, the middle keel winged and rarely 4-5-toothed; style deciduous; embryo spiral, about 1 1/2 turns.

In ponds and ditches, Nova Scotia and Ontario to Minnesota, south to Virginia, Missouri and Nebraska. June-Aug.