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226 NOTES ON PONDWEEDS,
occupied space on tlie surface of the water, the leaves become
crisped and undulated to some extent ; but further observations
are needed to decide the question of state or variety. In relation
to this question it may be explained that Pondweeds have two
usual modes of defence against competing vegetation : one is by
producing narrow grass-like leaves which can exist in a crowd, the
other by developing broad coriaceous floating leaves, which, involute
in vernation, are enabled to pierce through the dense surface
growths, and thtn, expanding, are able to smother such plants as
are incapable of rising above the surface of the water. In the first
mode they imitate F. pccthiatus, in the second P. natans, the two
extremes of the genus in most arrangements. In another respect,
too, F. ciispm has some resemblance (or perhaps some reversion)
to the pectinatus-gron]), that is, in early growths, made from
November to January ; many of the lower leaves are adnate, being
sessile on the backs of the stipules. These adnate leaves are flat
and narrow, but in other respects resemble the ordinary leaves of
the species, and are of common though not universal occurrence ;
as many as two to six are to be met with on the main stem, and
from one to three on the branches. Like the stipules they are
very fugitive, and therefore cannot be found on the more mature
growths. Although most frequent in the winter, they are occasionally
produced throughout the year, especially on the young
branchlets springing from the axils of the lower leaves. These
adnate lower leaves have already been noticed in P. Friesii, P.
perfoluitus, P. proilomjus, P. GriJJithii, P. decipiens, P. Zizii, P.
coriaceus, P. heterophyllus, P. nitens, P. ritfescexs, P. phmta/^ineus,
and P. fiuitans. Probably they will be found to occur in all the
other British species ; they should be looked for when the plants
are growing under adverse conditions ; forms which are probable
hybrids seem especially liable to produce them.
I would also call attention to the remarkable fact of the fruitspikes
being borne above the surface of the water. As a rule, the
fruits of Potaviogeton seem to be matured under water, in some
species even after the decay of the stems in autumn ; full observations
on this point are much needed, and require to be made year
after year, as there are some indications that the season and
manner of ripening the fruit is not constant in the same species.*
Chamisso and Schlechtendal (' Linnea,' v. 2, p. 187) remark that
the fruits are often abortive ; this species, however, possesses a
peculiar way of propagation by means of thyrsiform winter-buds,
which form in the axils of the leaves, and which are produced in
such numbers as to render the plant able to diffuse itself through a
large sheet of water in less time than any other species can do by
seed. These winter-buds are horny in texture, and have probably
greatly aided the
the world.
dispersion of P. crispus over the greater part of
There is no species of Potamogeton which will better reward
careful investigation ; easily recognised by the beginner, there is
* Probably the fruit of P. crispun is matured under water, but it seems to be
less deeply submerged than that of most species, and liable to ireijuent exposure
to the air, a condition wliicli the thick eoiky drupelets can sustain without injury.