QUILLWORTS (ISOETES).
[Illustration: Fig. 336. Isoetes, mature plant, sporophyte stage.]
=589.= The quillworts, as they are popularly called, are very
curious plants. They grow in wet marshy places. They receive their
name from the supposed resemblance of the leaf to a quill. Fig. 336
represents one of these quillworts (Isoetes engelmannii). The leaves
are the prominent part of the plant, and they are about all that can
be seen except the roots, without removing the leaves. Each leaf, it
will be seen, is long and needle-like, except the basal part, which
is expanded, not very unlike, in outline, a scale of an onion. These
expanded basal portions of the leaves closely overlap each other, and
the very short stem is completely covered at all times. Fig. 338 is
from a longitudinal section of a quillwort. It shows the form of the
leaves from this view (side view), and also the general outline of the
short stem, which is triangular. The stem is therefore a very short
object.
=590. Sporangia of isoetes.=—If we pull off some of the leaves of
the plant we see that they are somewhat spoon-shaped as in fig. 337. In
the inner surface of the expanded base we note a circular depression
which seems to be of a different texture from the other portions of the
leaf. This is a _sporangium_. Beside the spores on the inside of the
sporangium, there are strands of sterile tissue which extend across the
cavity. This is peculiar to isoetes of all the members of the class
of plants to which the ferns belong, but it will be remembered that
sterile strands of tissue are found in some of the liverworts in the
form of elaters.
[Illustration: Fig. 337. Base of leaf of isoetes, showing sporangium
with macrospores. (Isoetes engelmannii.)]
[Illustration: Fig. 338. Section of plant of Isoetes engelmannii,
showing cup-shaped stem, and longitudinal sections of the sporangia in
the thickened bases of the leaves.]
=591.= The spores of isoetes are of two kinds, small ones
(microspores) and large ones (macrospores), so that in this respect
it agrees with selaginella, though it is so very different in other
respects. When one kind of spore is borne in a sporangium usually all
in that sporangium are of the same kind, so that certain sporangia
bear microspores, and others bear macrospores. But it is not uncommon
to find both kinds in the same sporangium. When a sporangium bears
only microspores the number is much greater than when one bears only
macrospores.
=592.= If we examine some of the microspores of isoetes we see
that they are shaped like the quarters of an apple, that is they are of
the bilateral type as seen in some of the ferns (asplenium).
=593. Male prothallia.=—In isoetes, as in selaginella, the
microspores develop only male prothallia, and these are very
rudimentary, one division of the spore having taken place before the
spore is mature, just as in selaginella.
=594. Female prothallia.=—These are developed from the
macrospores. The latter are of the tetrahedral type. The development
of the female prothallium takes place in much the same way as in
selaginella, the entire prothallium being enclosed in the macrospore,
though the cell divisions take place after it has left the sporangium.
When the archegonia begin to develop the macrospore cracks at the three
angles and the surface bearing the archegonia projects slightly as in
selaginella. Absorbing organs in the form of rhizoids are very rarely
formed.
=595. Embryo.=—The embryo lies well immersed in the tissue of the
prothallium, though there is no suspensor developed as in selaginella.