PAST HISTORIES OF PLANT FAMILIES
VIII. The Horsetails
The horsetails of to-day all belong to the one genus, _Equisetum_, among
the different species of which there is a remarkably close similarity.
Most of the species love swampy land, and even grow standing up through
water; but some live on the dry clay of ploughed fields. Wherever they
grow they usually congregate in large numbers, and form little groves
together. They are easily recognized by their delicate stems, branching
in bottle-brush fashion, and the small leaves arranged round them in
whorls, with their narrow teeth joined to a ring at the base. At the end
of some of the branches come the cones, with compactly arranged and
simple sporophylls all of one kind. In England most plants of this family
are but a few inches or a foot in height, though one species sometimes
reaches 6 ft., while in South America there are groves of
delicate-stemmed plants 20 ft. high.
The ribbed stems and the whorls of small, finely toothed leaves are the
most important external characteristics of the plants, while in their
internal anatomy the hollow stems have very little wood, which is
arranged in a series of small bundles, each associated with a hollow
canal in the ground tissue.
The family stands apart from all others, and even between it and the
group of Lycopods there seems to be a big gap across which stretch no
bonds of affinity. Has the group always been in a similar position, and
stood isolated in a backwater of the stream of plant life?
[Illustration: Fig. 103.—Impression of Leaf Whorl of _Equisetites_ from
the Mesozoic Rocks, showing the narrow toothed form of the leaves.
(Photo.)]
In the late Tertiary period they seem to have held much the same position
as they do now, and we learn nothing new of them from rocks of that age.
When, however, we come to the Mesozoic, the members of the family are of
greater size, though they appear (to judge from their external
appearance) to have been practically identical with those now living in
all their arrangements. In some beds their impressions are very numerous,
but unfortunately most are without any indication of internal structure.
Fossils from the Mesozoic are called _Equisetites_, a name which
indicates that they come very close to the living ones in their
characters. In the Lower Mesozoic some of these stems seem to have
reached the great size of a couple of feet in circumference, but to have
no essential difference from the others of the group.
When, however, we come to the Palæozoic rocks we find many specimens with
their structure preserved, and we are at once in a very different
position as regards the family.
First in the Permian we meet with the important genus of plant called
_Calamites_, which were very abundant in the Coal Measures. Many of the
Calamites were of great size, for specimens with large trunks have been
found 30 ft. and more long, which when growing must certainly have been
much taller than that. The number of individuals must also have been very
great, for casts and impressions of the genus are among the commonest
fossils. They were, in fact, one of the dominant groups of the period.
Like the Lycopods, the Equisetaceæ reached their high-water mark of
development in the Carboniferous period; at that time the plants were
most numerous, and of the largest size and most complicated structure
that they ever attained.
[Illustration: Fig. 104.—Small Branches attached to stouter Axis of
_Calamites_. Photo of Impression]
As will be immediately suspected from analogy with the Lycopods, they
differed from the modern members of the family in their strongly
developed anatomy, and in the strength and quantity of their secondary
wood. Yet in their external appearance they probably resembled the living
genus in all essentials, and the groves of the larger ones of to-day
growing in the marshes probably have the appearance that the palæozoic
plants would have had if looked at through a reversed opera glass.
Fig. 104 is a photograph of some of the small branches of a Calamite, in
which the ribbed stem can be seen, and on the small side twigs the fine,
pointed leaves lying in whorls.
In most of the fossil specimens, however, particularly the larger ones,
the ribs are not those of the true surface, but are those marked on the
_internal cast_ of the pith.
[Illustration: Fig. 105.—Transverse Section of _Calamites_ Stem with
Secondary Wood _w_ formed in Regular Radial Rows in a Solid Ring
_c_, Canals associated with the primary bundles; _p_, cells of the
pith, which is hollow with a cavity _l_, _cor_, Cortex and outer
tissues well preserved. (Microphoto.)]
Among tissue petrifactions there are many Calamite stems of various
stages of growth. In the very young ones there are only primary bundles,
and these little stems are like those of a living Equisetum in their
anatomy, and have a hollow pith and small vascular bundles with canals
associated. The fossil forms, however, soon began to grow secondary wood,
which developed in regular radial rows from a cambium behind the primary
bundles and joined to a complete ring.
A stem in this stage of development is seen in fig. 105, where only the
wood and internal tissues are preserved. The very characteristic canals
associated with the primary bundles are clearly shown. The amount of
secondary wood steadily increased as the stems grew (there appear to have
been no “annual rings”) till there was a very large quantity of secondary
tissue of regular texture, through which ran small medullary rays, so
that the stems became increasingly like those of the higher plants as
they grew older. It is the primary structure which is the important
factor in considering their affinity, and that is essentially the same as
in the other members of the family in which secondary thickening is not
developed. As we have seen already in other groups of fossils, secondary
wood appears to develop on similar lines whenever it is needed in any
group, and therefore has but little value as an indication of systematic
position. This important fact is one, however, which has only been
realized as a result of the study of fossil plants.
[Illustration: Fig. 106.—Diagram of the Arrangement of the Bundles at
the Node of a _Calamite_, showing how those of consecutive internodes
alternate
_n_, Region of node]
[Illustration: Fig. 107.—Leaf of _Calamites_ in Cross Section
_v_, Vascular bundles; _s_, cells of sheath, filled with blackened
contents; _p_, palisade cells; _e_, epidermis.]
The longitudinal section of the stems, when cut tangentially, is very
characteristic, as the bundles run straight down to each node and there
divide, the neighbouring halves joining so that the bundles of each node
alternate with those of the ones above and below it (see fig. 106).
The leaves which were attached at the nodes were naturally much larger
than those of the present Equisetums, though they were similarly simple
and undivided. Their anatomy is preserved in a number of cases (see fig.
107), and was simple, with a single small strand of vascular tissue lying
in the centre. They had certain large cells, sometimes very black in the
fossils, which may have been filled with mucilage.
[Illustration: Fig. 108.—Transverse Section of Young Root of
_Calamites_
_w_, Wood of axis; _l_, spaces in the lacunar cortex, whose radiating
strands _r_ are somewhat crushed; _ex_, outermost cells of cortex with
thickened wall.]
[Illustration: Fig. 109.—Diagram of Cone of _Calamites_
A, Main axis; _br_, sterile bracts; _sp_, sporophylls with four
sporangia S attached to each, of which two only are seen.]
The young roots of these plants have a very characteristic cortex, which
consists of cells loosely built together in a lacelike fashion, with
large air spaces, so that they are much like water plants in their
appearance (see fig. 108). Indeed, so unlike the old roots and the stems
are they, that for long they were called by another name and supposed to
be submerged stems, but their connection with _Calamites_ is now quite
certain. As their woody axis develops, the secondary tissue increases and
pushes off the lacelike cortex, and the roots become very similar in
their anatomy to the stems. Both have similar zones of secondary wood,
but the roots do not have those primary canals which are so
characteristic of the stems, and thereby they can be readily
distinguished from them.
The fructifications of the Calamites were not unlike those of the living
types of the family, though in some respects slightly more complex. Round
each cone axis developed rings of sporophylls which alternated with
sterile sheathing bracts. Each sporophyll was shaped like a small
umbrella with four spokes, and stood at right angles to the axis, bearing
a sporangium at each of the spokes. A diagram of this arrangement is seen
in fig. 109.
[Illustration: Fig. 110.—Longitudinal Section of Part of _Calamites_
Cone
_br_, Sterile bracts attached to axis; _sp_, attachment of sporophylls;
S, sporangia. At X a group of four sporangia is seen round the
sporophyll, which is seen at _a_. (Microphoto.)]
A photograph of an actual section of such a cone, cut slightly obliquely
through the length of the axis, is seen in fig. 110, where the upper
groups of sporangia are cut tangentially, and show their grouping round
the sporophyll to which they are attached.
A few single tetrads of spores are enlarged in fig. 111, where it will be
seen that the large spores are of a similar size, but that the small ones
of the tetrads are very irregular. They are aborting members of the
tetrad, and appear to have been used as food by the other spores. In each
sporangium large numbers of these tetrads develop and all the ripe spores
seem to have been of one size.
In a species of _Calamites_ (_C. casheana_), otherwise very similar to
the common one we have been considering, there is a distinct difference
in the sizes of the spores from different sporangia. The small ones,
however, were only about one-third of the diameter of the large ones, so
that the difference was very much less marked than it was between the
small and large spores of the Lycopods.
Among the palæozoic members of the group are other genera closely allied
to, but differing from _Calamites_ in some particulars. One of these is
_Archæocalamites_, which has a cone almost identical with that of the
living Equisetums, as it has no sterile bracts mingled with the
umbrella-like sporophylls. Other genera are more complex than those
described for _Calamites_, and even in the simple coned _Archæocalamites_
itself the leaves are finely branched and divided instead of being simple
scales.
But no genus is so completely known as is _Calamites_, which will itself
suffice as an illustration of the palæozoic Equisetaceæ. Though the
genus, as was pointed out above, shows several important characters
differing from those of Equisetum, and parallel to some extent to those
of the palæozoic Lycopods, yet these features are more of a physiological
nature than a systematic one, and they throw no light on the origin of
the family or on its connection with the other Pteridophytes. It is in
the extinct family dealt with in the next chapter that we find what some
consider as a clue to the solution of these problems.
[Illustration: Fig. 111.—Tetrads of Spores of _Calamites_
S, Normal-sized spores; _a_, _b_, &c., aborting spores.]