217
(liating lines, wliicli appear to have been produced by the
pressure of the three other spores that originally helped to
constitute the quaternary union. The spores of some of
the other tribes being apparently developed in similar unions,
it is probable that similar lines, indicating a mode of
opening by valves, also exist on them, and this is certainly
the case in some instances, as in Isoetes, Lycopodium, and
Osmunda ; and in those cases in which such a structure is
not visible, it is probably owing to a thickening of the membrane,
or a deposition of opake matter on its surface, as in
Pilularia, in the mature spores of which they can only be
discovered by dissection, and in the abortive ones they cannot
be discovered at all after the earlier stages of growth.
It, however, does appear that in these plants, after the protrusion
of the germ, it is immaterial from what part of that
body the first leaves, root, or stem shall arise.
The Pillwort is widely distributed throughout the United
Kingdom, but is apparently more abundant in England and
Wales, than in Scotland and Ireland. It usually grows on
the margins of lakes or pools, where it is covered by the
water in winter, and more or less exposed during the summer;
but it is also sometimes, though rarely, met with
entirely submerged.
THE BRITISH HORSETAILS.
This race of plants bears an aspect altogether dffierent from
that of the groups in whose company they are placed in
books, and indeed they have no very obvious affinity to any
existing order of plants. In their mode of growth they
have a certain resemblance to a small group of plants known
by the name of Ppliedra, and belonging to the order Gneta-
cem, and also to another limited set called Casuarina; but
this resemblance is confined to their general aspect, and is
in great measure owing to the peculiar jointing of the stems
and branches. With Eerns and Club-mosses they have
little in common, though so frequently associated with them
in books. Their most direct relationship is probably with
a smaU group called Liverworts [Marchantiaceai); and they
have also some analogy with the aquatic group, OJiaracem.
The Horsetails are distinguished from other plants by the
following characteristics. They are leafless, branching plants,
with fistular jointed stems, separable at the joints, where
they are solid, and at these points surrounded by membranous