428 THE NAIAD FAMILY, | Potamogeton. 
In pools, ditches, and still fresh waters, over a great part of the globe. 
Common in Britain. 7. summer. The Linnean names P. gramineus and 
P. compressus have been so variously applied by himself, as well as others, 
to this and the following species, as well as to some states of P. hetero- 
phyllus, that the more definite names given by German botanists are now 
more generally adopted. 
9, P. acutifolius, Linn. (fig. 964). Acute Potamogeton.—Yery near 
P, obtusifolius, with the same habit and stipules. Leaves as in that 
species, narrow-linear, but semi-amplexicaul and very acute, with 1 promi- 
nent central nerve, and often ‘1 on each side not connected by transverse 
veins, but occasionally numerous exceedingly fine longitudinal ones may be 
seen underalens. Spikes usually shorter than in P. obtusifolius. Nuts 
the same, but with a recurved beak. | 
Appears to be as generally spread as P. obtusifolius, and perhaps more 
common in Britain. #7. summer. Both this and the preceding species 
were included in early editions as robust varieties of R. pusillus. 
[P. zosterifolius, Schum., is a very closely allied plant with broad 
almost winged stems, abruptly acuminate leaves, and 3-ribbed nuts. | 
10. P. pusillus, Linn. (fig. 965). Slender Pondweed,—Distinguished 
from all the preceding species by the thread-like stems, and very narrow 
linear leaves like those of Zannichellia or Ruppia, and from the follow- 
ing by the scarious sheathing stipules, always observable in the axils of 
those leaves at least which are under the branches or peduncles. Leaves 
veined as in P. pectinatus, 1 to 3 inches long and very seldom a line 
broad. Peduncles slender, with a short, close spike of small flowers. 
Nuts ovoid, under 1 line long, almost pointed, with a more or less strongly 
marked dorsal rib. 
In pools, ditches, and still waters, fresh or salt, generally spread at least 
in the northern hemisphere. Common iu Britain. 7. summer. 
[P. trichoides, Cham. and Schl., a European species found in the 
eastern counties of England and in Ireland, is a form with capillary stems, 
setaceous leaves, and very few flowers with solitary carpels. | 
ll. P. pectinatus, Linn. (fig. 966). Fennel Pondweed.—Stems 
thread-like, with very narrow, grass-like leaves, usually 2 or 3 inches long, 
most of them dilated at the base into a rather long sheath, which is scarious 
at the edge and often projecting at the top into two small scarious lobes 
(stipules adhering to the base of the leaf); the sheathing stipules of the 
other species either absent or very rare under the peduncles. The midrib 
of the leaf sometimes separates into longitudinal, netted veins, only visible 
when magnified, and there are usually 2 faint longitudinal nerves at some 
distance from it. Peduncles usually bearing several distant clusters of 2 
or 3 flowers, forming a slender interrupted spike, rarely reduced to a single, 
small terminal cluster. Nuts as in P. pusillus.. 
In pools, ditches, and still waters, fresh or salt, almost all over the world. 
Generally distributed over Britain. FV. summer. 
[P. filiformis, Nolte, a widely distributed species, in the north and 
south hemispheres, has capillary leaves, flowers in whorls, and much larger 
nuts. It is not uncommon in Scotland, and has also been found in Angle- 
sea and Ireland. | 
