EQUISETACEÆ, OR HORSETAILS.
THE Equiseta are leafless, herbaceous plants, annually renewed from a
creeping rhizome, and growing in marshy land, in pools and ditches, on
the banks of rivulets, and in rivers, from Lapland and Siberia to within
the tropics. There is but one genus, and few species. The largest of the
ten or eleven species, which are indigenous in Great Britain, is not
more than five or six feet high, but they are of greater size in warm
climates. The Horsetails begin their lives precisely like the Ferns;
for, when a spore begins to germinate, it forms a marchantioid leaf or
prothallus lying flat on the ground, upon which are produced antheridia,
full of cells, in each of which there is a spermatozoid with numerous
cilia. Archegonia are also formed on the prothallus. These, after
fertilization, give rise to the perfect plant, which throws out a
rhizome, whence new shoots are produced. ‘The structure of the rhizome
is very different from that of ferns. In an early stage it consists of a
central column of cellular tissue, sending off about eight radiating
plates, which connect it with an external cylinder of the same tissue,
and opposite to each of which there is, in the central column, a
vascular bundle consisting of annular vessels passing into spiral. At a
later period, tissue grows from the walls into the cavities, in such
wise that they are more or less perfectly obliterated.’[75] From the
rhizome, which often extends to a great length, the stems of the
Equisetums rise in the form of rough, rigid, hollow cylinders, striated
longitudinally, and articulated at intervals by separable joints. Each
articulation is invested at its base by a toothed membranaceous sheath,
from beneath which, in the greater number of the species, whorls of
branches spring, jointed like the stem, and similar to it even to the
number of teeth in the sheaths and striæ on the surface, but unbranched.
[Illustration: Fig. 70. Equisetum giganteum:—_a_, fragment of stem with
branches; _b_, cone or spike of fructification; _c_, one of the scales
of the cone; _d_, spore with its elastic filaments.]
The fructification is occasionally on separate stems, which make their
appearance before the barren ones, and are in general unbranched and
succulent, bearing a cone on their apex. The surface of the cone is at
first smooth or indistinctly reticulated, but it eventually splits into
numerous octagonal, brown, shield-like discs, spirally arranged, which,
separated from the stem, are found to have a stalk, and to bear on the
under-side four or eight pendent sac-like bodies of a whitish hue. These
are the sporangia, which open on the inside by a slit for the discharge
of the spores. The inner coat of the cells of the sporangium is composed
of beautifully spiral tissue. The spores arise by cell division, each
being covered by a separate membrane, which ultimately forms a pair of
elastic fibres attached by their middle, closely coiled round the spore,
as it is formed within the cell; but, when the spore is liberated, they
extend; the least moisture, however, even the breath, makes them
contract.
The Equisetaceæ differ from the Ferns in having spiral instead of
scalariform vessels in their structure; but they agree in having only
one period of fertilization, after which they produce a crop of spores
year after year, and these, when sown, give rise to the primary leaf
with its antheridia and archegonia—a perpetual cycle.
The whole substance of the stems of the Equisetums is so completely
penetrated with silex, that a silicious skeleton remains after the
herbaceous part is destroyed, and in some species as much as thirteen
per cent. of the whole plant, and fifty per cent. of its ashes are found
to be pure silex. On account of the crystallization of the silex, the
Horsetails form some of the most beautiful of microscopic objects. If a
fragment of the cuticle is magnified and viewed by polarized light, the
colours are seen to be intense and vivid, and the arrangement of the
silicious particles so elaborate and symmetrical as to resemble
necklaces of diamonds and coloured gems. The rows of crystals run in
lines parallel to the axis of the stem, the greater number being so
disposed, but the rest are grouped so as to form ovals joined together
by a chain of particles forming a sort of curvilinear quadrangle, these
rows of oval combinations being arranged in pairs. The effect is
brilliant when they are seen in polarized light. According to Sir David
Brewster every particle has a regular axis of double refraction; but
Professor Bailey, of the United States of America, states that the
effect under polarized light is not produced by the silicious particles,
but by the organized tissues, since, after these have been destroyed,
the silex shows no double refraction.
The vascular tissue of the Equisetums shows them to be of a higher class
than the Ferns. The recent forms vary much in size, but even the
Equisetum giganteum of Brazil, which is eighteen feet high and
three-quarters of an inch in diameter, is incomparably less than the
Calamites and other fossil Equiseta, which appear in the coal measures
and new red sandstone. Recent species are found in Iceland and the high
northern latitudes of America, in the tropics, and in most parts of the
world, except Australia and New Zealand.