HORSETAILS.
=571.= Among the relatives of the ferns are the horsetails, so
called because of the supposed resemblance of the branched stems of
some of the species to a horse’s tail, as one might infer from the
plant shown in fig. 325. They do not bear the least resemblance to the
ferns which we have been studying. But then relationship in plants does
not depend on mere resemblance of outward form, or of the prominent
part of the plant.
[Illustration: Fig. 321. Portion of fertile plant of Equisetum arvense
showing whorls of leaves and the fruiting spike.]
=572. The field equisetum. Fertile shoots.=—Fig. 321 represents
the common horsetail (Equisetum arvense). It grows in moist sandy
or gravelly places, and the fruiting portion of the plant (for this
species is dimorphic), that is the portion which bears the spores,
appears above the ground early in the spring. It is one of the first
things to peep out of the recently frozen ground. This fertile shoot
of the plant does not form its growth this early in the spring. Its
development takes place under the ground in the autumn, so that with
the advent of spring it pushes up without delay. This shoot is from
10 to 20 _cm_. high, and at quite regular intervals there are slight
enlargements, the nodes of the stem. The cylindrical portions between
the nodes are the internodes. If we examine the region of the
internodes carefully we note that there are thin membranous scales,
more or less triangular in outline, and connected at their bases into a
ring around the stem. Curious as it may seem, these are the leaves of
the horsetail. The stem, if we examine it farther, will be seen to
possess numerous ridges which extend lengthwise and which alternate
with furrows. Farther, the ridges of one node alternate with those of
the internode both above and below. Likewise the leaves of one node
alternate with those of the nodes both above and below.
[Illustration: Fig. 322. Peltate sporophyll of equisetum (side view)
showing sporangia on under side.]
=573. Sporangia.=—The end of this fertile shoot we see possesses
a cylindrical to conic enlargement. This is the _fertile spike_, and we
note that its surface is marked off into regular areas if the spores
have not yet been disseminated. If we dissect off a few of these
portions of the fertile spike, and examine one of them with a low
magnifying power, it will appear like the fig. 322. We see here that
the angular area is a disk-shaped body, with a stalk attached to its
inner surface, and with several long sacs projecting from its inner
face parallel with the stalk and surrounding the same. These elongated
sacs are the _sporangia_, and the disk which bears them, together with
the stalk which attaches it to the stem axis, is the _sporophyll_, and
thus belongs to the leaf series. These sporophylls are borne in close
whorls on the axis.
=574. Spores.=—When the spores are ripe the tissue of the
sporangium becomes dry, and it cracks open and the spores fall out.
If we look at fig. 323 we see that the spore is covered with a very
singular coil which lies close to the wall. When the spore dries this
uncoils and thus rolls the spore about. Merely breathing upon these
spores is sufficient to make them perform very curious evolutions by
the twisting of these four coils which are attached to one place of the
wall. They are formed by the splitting up of an outer wall of the spore.
=575. Sterile shoot of the common horsetail.=—When the spores are
ripe they are soon scattered, and then the fertile shoot dies down.
Soon afterward, or even while some of the fertile shoots are still in
good condition, sterile shoots of the plant begin to appear above the
ground. One of these is shown in fig. 325. This has a much more slender
stem and is provided with numerous branches. If we examine the stem of
this shoot, and of the branches, we see that the same kind of leaves
are present and that the markings on the stem are similar. Since the
leaves of the horsetail are membranous and not green, the stem is green
in color, and this performs the function of photosynthesis. These green
shoots live for a great part of the season, building up material which
is carried down into the underground stems, where it goes to supply the
forming fertile shoots in the fall. On digging up some of these plants
we see that the underground stems are often of great extent, and that
both fertile and sterile shoots are attached to one and the same.
[Illustration: Fig. 323. Spore of equisetum with elaters coiled up.]
[Illustration: Fig. 324. Spore of equisetum with elaters uncoiled.]
[Illustration: Fig. 325. Sterile plant of horsetail (Equisetum
arvensis).]
=576. The scouring rush, or shave grass.=—Another common species
of horsetail in the Northern States grows on wet banks, or in sandy
soil which contains moisture along railroad embankments. It is the
scouring rush (E. hyemale), so called because it was once used for
polishing purposes. This plant like all the species of the horsetails
has underground stems. But unlike the common horsetail, there is but
one kind of aerial shoot, which is green in color and fertile. The
shoots range as high as one meter or more, and are quite stout. The new
shoots which come up for the year are unbranched, and bear the fertile
spike at the apex. When the spores are ripe the apex of the shoot dies,
and the next season small branches may form from a number of the nodes.
=577. Gametophyte of equisetum.=—The spores of equisetum have
chlorophyll when they are mature, and they are capable of germinating
as soon as mature. The spores are all of the same kind as regards size,
just as we found in the case of the ferns. But they develop prothallia
of different sizes, according to the amount of nutriment which they
obtain. Those which obtain but little nutriment are smaller and develop
only antheridia, while those which obtain more nutriment become larger,
more or less branched, and develop archegonia. This character of an
independent prothallium (gametophyte) with the characteristic sexual
organs, and the also independent sporophyte, with spores, shows the
relationship of the horsetails with the ferns. We thus see that these
characters of the reproductive organs, and the phases and fruiting of
the plant, are more essential in determining relationships of plants
than the mere outward appearances.