Potamogeton. | LXXVIII. NAIADEZ. 427 
rufescens, Schrad. LP. lanceolatus, Sm., appears to be a smaller state of 
the same species, which is always to be distinguished from P. natans and 
P. heterophylius by the sessile, many-nerved lower leaves. 
[P. Lonchites, Tuckerm., is an American form late'y found in the river 
Boyne, distinguished by its very long 7—9-nerved leaves. | 
4, P. prvelongus, Wulf. (fig. 959). Long Pondweed.—A large species, 
with the leaves all submerged and thin, with numerous longitudinal veins, 
and a few transverse reticulations, like P. lucens, but the leaves are 
broader, all closely sessile, and half-clasping the stem by their rounded 
base, usually 3 or 4 inches long, obtuse at éhe tip, and concave, so as to 
split in drying. Stipules very prominent. Peduncles long and stout, with 
the flowers and acutely keeled fruits usually larger than in P. natans, in a 
rather close spike. ; 
In pools and rivers of the north temperate zone. In Britain, not:so 
common as P. lucens. Fl. summer. 
5. P. perfoliatus, Linn. (fig. 960). Perfoliate Pondweed.—Leaves 
all submerged, thin and many-nerved as in P. prelongus and P. lucens, 
but much shorter, usually ovate, obtuse, completely clasping the stem; the 
auricles often united on the opposite side, so that the leaf appears to be 
pierced through ; from 1 to 14 inches long by fully an inch broad. Stipules 
as in the preceding species, but soon disappearing. Spike of flowers seldom 
above 6 or 8 lines long. 
In rivers and ponds, all over the northern hemisphere, and in Australia. 
Generally distributed in Britain. #7. summer. 
6. PB. crispus, Linn. (fig. 961). Curly Pondweed.—One of the most 
marked of the alternate-leaved species. Leaves all submerged and thin, 
narrow-oblong or broadly linear, obtuse, shortly tapering at the base, 1 to 
2 inches long, 3 to 5 lines broad, always waved and sinuated on their edges, 
and marked by 1 strong midrib and 2 parallel slender nerves at some 
distance from it, but connected with it by a few transverse veins. Spikes 
small, consisting of about 3 to 6 flowers, at some distance from each other. 
In ponds, streams, and ditches of the northern hemisphere, except the 
extreme north, and in Australia. Common in Britain. Fl. summer. 
7. ®. densus, Linn. (fig. 962). Opposite Pondweed.—Readily known 
by its numerous short leaves, all opposite, and arranged in two rows on the 
opposite sides of the stem; they are all submerged and thin, broadly 
lanceolate, 6 to 9 lines long, folded and clasping the stem at their base, 
with a strong midrib and 2 fainter parallel nerves, connected by a few 
transverse veins. Stipules only under the peduncles or branches. Pedun- 
cles very short, turned down after flowering, bearing a head of 2 or 3 
flowers only. Ripe carpels rather large, rounded, and smooth. 
In shallow pools, and ditches, of Kurope, except the extreme north, 
temperate Asia and America, Common in Britain, #7. summer. 
8. P. obtusifolius, Mert. and Koch. (fig. 963). Obdtuse Potamogeton. 
—Stems slender. Leaves all submerged, alternate except under the 
branches, 13 to 3 inches long, and 1 to 2 lines broad, obtuse or scarcely 
acute, usually 3-nerved, with a very few transverse veins. Sheathing 
scarious stipules often persistent under the branches, but sometimes mostly 
fallen away. fPeduncles not long. Spikes usually $ inch or rather longer, 
not very dense. Nuts broadly ovoid, about 13 lines long, nearly smooth 
or with a prominent often rugose dorsal rib and straight beak. Seed much 
curved. 
