Potamogeton. | LXXVIII. NAIADEX. 427 
pressed, nearly straight, the inner edge rounded outwards, with 1 or 
sometimes 3 dorsal ribs. 
In stagnant or running waters, deep or shallow, sunny or shaded, in 
almost all parts of the world, and varies accordingly in the size, shape, 
and texture of the foliage, the size and number of the flowers, fruit, &c. 
Abundant in Britain. Fl. summer. 
[This includes the true P. natans with coriaceous floating leaves, no 
true submerged ones, and keeled nuts; P. poylgonifolius, Pourr., with 
more membranous floating leaves, lanceolate submerged ones, and nuts 
with a rounded back; and P. plantagineus, Du Croz, with most of the 
leaves broader, and all submerged and translucent, and short broad 
obtuse stipules. | 
2. P. heterophyllus, Schreb. (fig. 959). Various-leaved P.—Usually 
much smaller than P. natans, which it resembles in the long stalk and 
the ovate or oblong shape of its floating leaves, but these are only 1 to 
2 inches long, and the submerged leaves are all narrow-lanceolate or 
linear, with the few veins of P. pusillus, tapering at both ends but not 
distinctly stalked. Spikes and fruits as in smaller forms of P. natans. 
Chiefly a North American species, not common in Europe, where it 
appears to be rather a western plant. Occurs in many parts of Britain. 
Fl. summer. Sometimes the floating leaves are not developed, and then 
it is scarcely to be distinguished from P. pusillus except by its larger 
size, with a denser spike, and generally a firmer consistence. 
3. P. lucens, Linn. (fig. 960). Shimimg P.—A large species, the leaves 
usually all thin and under water, sessile or nearly so, tapering at both 
ends or scarcely obtuse, 2 or 3 to near 6 inches long, seldom above half 
‘an inch broad, marked with 2 or 3 well-defined longitudinal nerves on 
each side of the midrib, besides several intermediate fainter ones, and 
a few transverse reticulations. Flowers as in P. natans. 
In ponds and rivers, usually rather deep, generally distributed over 
the globe, except the extreme north. Not uncommon in Britain. 4. 
summer. P.rufescens, Schrad., is a variety with the upper leaves floating 
on the surface and shortly stalked, either lanceolate or oblong. P. 
lanceolatus, Sm., appears to be a smaller state, which is always to be 
distinguished from P. natans and P. heterophyllus by the sessile, many- 
nerved lower leaves. 
[P. Lonchites, Tuckerm., is an American form lately found in the river 
Boyne, distinguished by its very long 7-9-nerved leaves. ] 
4. P. prelongus, Wulf. (fig. 961). Long P.—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 the 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. 
P. perfoliatus, Linn. (fig. 962). Perfoliate P.—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 
