MARSILEACEÆ, OR RHIZOSPERMÆ.
[Illustration: Fig. 71. Pilularia minuta:—_a_, mature plant, natural
size; _b_, receptacle, slightly magnified.]
THE Marsileaceæ, a natural tribe of small perennial aquatic herbs, have
a filiform creeping rhizome with alternate erect leaves, curled in
vernation like those of the ferns. The sporangia are enclosed in oval or
spherical leathery capsules, or receptacles, which contain two
dissimilar forms of reproductive organs—sporangia and antheridia, and
are sessile, or nearly so, on the rhizome at the base of the leaves,
whence the general name of Rhizospermæ. The Pilularia globulifera, or
Pillwort, the only British species of its genus, may be taken as a type.
Its rhizome creeps over sand or mud, at the margins of lakes and pools,
where it is always submerged, or in sandy or gravelly places, which are
only occasionally overflowed. At regular intervals the rhizome sends off
a tuft of roots and a tuft of leaves opposite to them; the leaves are
smooth, erect, and very slender, in deep water almost hair like, varying
from four to five inches in height. The solitary globular receptacles,
brown, hairy, and about the size of a small peppercorn, spring from the
axils of the leaves, supported on so short a stem that they appear to be
sessile. Fig. 71 shows an Algerian species of its natural size. The
receptacles are divided by cross partitions into two or more cells, and
separate at maturity into four equal valves. Each cell has a sort of
placenta to which the sporangium and antheridia are attached; and its
upper half is lined with three minute, sessile, obovate, yellowish
bodies, which are the antheridia; the other half is occupied by a
larger, roundish, or oblong sessile sporangium, containing one spore,
which has a firm outer coat, tapering to a point, and leaving a cavity
at the top of the nucleus. According to Hofmeister, this cavity becomes
filled with cellular tissue, constituting a conical prothallus confluent
with the nucleus. A single archegonium is formed in the centre, the
orifice of which corresponds with the apex of the prothallus. The
antheridia contain numerous granules, from which long spiral delicate
spermatozoids are ultimately developed.[76] The embryos of the young
plants push forth their radicle in one direction to fix them to the
soil, and a frond or leaf in the opposite direction. Thus these minute
plants, approaching so nearly to a monocotyledon, exhibit a high
organization, only inferior to that of the Lycopods.
The group contains the genera Marsilea, which has leaves made up of
cuneiform lobes, and resembling those of some leguminous plant; and
Salvinia and Azolla, both of which consist of small floating plants,
which mostly occur in tropical or subtropical countries.