=The sexual generation.= The prothallium is green and leaf-like, as in
the majority of Ferns, but irregularly branched and curled. It is often
unisexual. The male prothallia bear antheridia only, and are smaller
and less branched (Fig. 222 _A_) than the female; the latter may attain
a diameter of ½ an inch, and bear archegonia only (Fig. 222 _B_). The
antheridia and the archegonia resemble those of the Ferns, but the
spermatozoids (Fig. 223) are larger and less twisted. On the last curve
is situated a more or less elongated appendage of cytoplasm (Fig. 223
_c_).
=The asexual generation.= The embryo is similar to that of the Ferns.
The fully developed _Equisetum_ is a perennial herb, with widely
creeping (in some species tuberous) rhizome, from which extend erect,
aerial, most frequently annual shoots.
[Illustration: FIG. 224.--_Equisetum arvense_: _a_ fertile branch
with cone; _b_ vegetative shoot; _c_ cone; _d_ sporophylls.]
The vegetative aerial STEMS are divided into a number of
internodes by the whorls of leaves (Fig. 224). The internodes are
hollow, the cavities being separated from each other by the transverse
partitions of the solid nodes. The lower portion of the internode,
which is encased by the leaves, has much thinner and softer cell-walls,
so that the stem is easily separated into segments just above the
nodes. Each internode has a large number of ridges and furrows,
and bears at its apex a whorl of leaves whose number and position
correspond to the ridges of the internode. As in the case of other
verticillate plants, the whorls are placed alternately, one above
the other; the same arrangement is also found in the ridges on two
successive internodes. In addition to the large air-cavity in the
centre of each internode (the central cavity), a whorl of tubular
air-passages is found in the cortex of the stems, opposite the furrows
(vallecular canals). There is also a similar air-passage (carinal
canals) in each of the vascular bundles, which are placed in a ring,
one opposite each ridge, and therefore alternating with the vallecular
canals. The vascular bundles are _collateral_ as in the majority
of Flowering-plants, but poorly developed. The xylem of each bundle
consists of two groups of annular or spiral vessels, close to the outer
border of the carinal canal, and two groups of scalariform tracheides,
each placed on a radius passing through a group of spiral vessels.
The phloëm is placed between these four groups, each of which has
only a few vessels. The stiffness of the stems is mainly due to the
large amount of silica in the cell-walls of the epidermis, and to the
sclerenchymatous cells of the ridges.
All LEAVES are situated in _whorls_. The VEGETATIVE are simple,
undivided, 1-nerved, and are united into toothed sheaths (Fig. 224 _a_,
_b_). The branching of the stems in some species (_E. arvense_) is very
abundant. The branches break through the base of the leaf-sheaths (Fig.
224 _b_), and generally _alternate with the teeth_ (leaves).
The FERTILE LEAVES (_sporophylls_) are different from the barren ones.
They are _free, shield-like_, each one having a short stalk bearing
usually an hexagonal plate (Fig. 224 _d_), and closely compressed into
an ear or cone (Fig. 224 _a_, _c_). The _Equisetums_ thus present an
advance in development distinctly beyond that of the Ferns, which is
further emphasized by the circumstance that a transition from the
sheath-leaves to the fertile-leaves is found in the involucre or
annulus, a “collar” of specially modified leaves situated at the base
of the cone (Fig. 224 _a_ and _c_). The cone may be considered as a
very rudimentary flower, and the annulus may be regarded as a very
early stage in the formation of a flower (perianth). See page 235.
The SPORANGIA are situated on the underside of the sporophylls, one
at each angle; they are sac-like, and open inwardly by a longitudinal
cleft (Fig. 224 _d_). An annulus is wanting; but in the wall of the
sporangium, as in the pollen-sacs of the Flowering-plants, a layer of
cells, with annular or spiral thickenings, is developed, which assists
in the dehiscence of the sporangium.
The SPORES are green; the walls composed of four distinct layers,
of which the outer is gradually separated, except at one point, and
becomes split into four long bands (_elaters_) (Fig. 225). The elaters
are extremely hygroscopic, coiling round the spore when moistened, and
expanding as soon as dry, presenting a most lively object under the
microscope when breathed upon and allowed to dry. The second layer,
when germination commences, becomes detached from the inner wall, which
is formed of the exospore and endospore.
The order has become much reduced, and at the present time includes
only one genus, _Equisetum_, with about twenty-five species, which are
distributed over the entire globe, particularly in damp situations. In
SOME SPECIES the barren shoots are green and very much branched, but
the fertile ones are unbranched, pale brown, and possess no chlorophyll
(_E. arvense_, Field-Horsetail, Fig. 224, and _E. maximum_). IN
OTHERS the fertile and barren shoots are alike green, and either both
unbranched (_E. hiemale_), or branched (_E. palustre_, _E. limosum_,
etc). The fertile shoots of _E. silvaticum_, up to maturity, resemble
those without chylorophyll of _E. arvense_, but after that period they
produce green branches, and thus resemble the barren ones.
[Illustration: FIG. 225.--Spores of _Equisetum_: _A_ damp, with
elaters (_e_) coiled round the spore; _B_ dry, with elaters
expanded.]
EXTINCT ISOSPOROUS EQUISETINÆ. In addition to several true
species of fossilized _Equisetums_, the order of the CALAMITES,
which no doubt is closely allied to the Equisetinæ, is also
found in the fossil state. These were gigantic forms, attaining
about twenty times the size of those of the present day, and
stems of nearly 10–12 metres in height are known. They reached
the culminating point of their development in the Carboniferous
period, and died out towards the close of the Palæozoic. The
stems had hollow internodes and alternating grooves, similar
to their relatives of the present day. The leaves must either
have been absent or very perishable, since they have not been
identified with certainty. If the determinations of certain
remains of cones which of late have been discovered are correct,
they were heterosporous and had two kinds of sporangia as in
the following sub-class. A cambium formation and an increase in
thickness has been found in the stems.
Their USES are very limited. A few species, such as _E. hiemale_
are used for polishing on account of the hard siliceous
cell-walls of the epidermis, found in all species of _Equisetum_.