496 THE NAIAD FAMILY. [ Potamogeton. 
Leaves obtuse or scarcely acute. Spikes. usually 4 
inch long. s A é . P. obtusifolius, . 
Leaves very acute. Spikes alot and few-ftowered . 9. P. acutifolius. 
Leaves under 1 line broad. Nuts under 1 line long 10. P. pusillus, 
Leaves dilated at the base into a sheath, scarious at the 
edges : « 2 . ll. P. pectinatus. 
[The Pilaiogetens, ike SO may water Plaats are very difficult of dis- 
crimination, and as many more species as Bentham has described are 
generally regarded as British. An attempt is here made to express his 
view of the most distinct ef these, by giving their names and characters 
in brackets under their nearest allies. | 
1. P.natans, Linn. (fig. 956). Broad Pondweed.—One of the largest 
of ‘our Potamogetons. Leaves stalked, the upper ones floating on the sur- 
face of the water, of a thick, opaque texture, ovate or oblong, 2 to 4 
inches long by 1 to 13 broad, usually rounded at the base but sometimes 
cordate or tapering, marked by several longitudinal nerves, with a few 
cross veins often branched or slightly netted ; the submerged leaves thinner 
and narrower, but stalked like the floating ones or reduced to a mere stalk. 
Axillary stipules closely sheathing, often an inch long. Spike dense and 
cylindrical, often an inch long or more, on a stout peduncle of several inches. 
Nuts ovoid, above a line long, slightly compressed, 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 cf the world, and varies accordingly in the size, shape, and 
texture of the foliage, the size and number of the flowers, fruit, ete. 
Abundant in Britain. Fl. summer. 
[This includes the true P. natans with coriaceous floating leaves, no true 
submerged ones, and keeled nuts ; P. polygonifolius, Pourr.,with more mem- 
branous 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. 957). Various-leaved Pondweed. 
—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 like those of the 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. 958). Shining Pondweed.—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. Sl. summer. 
A variety with the upper leaves floating on the surface and shortly stalked, 
either lanceolate or oblong, has been distinguished under the name of P. 
