PAST HISTORIES OF PLANT FAMILIES
IX. Sphenophyllales
The group to which _Sphenophyllum_ belongs is of considerable interest
and importance, and is, further, one of those extinct families whose very
existence would never have been suspected had it not been discovered by
fossil botanists. Not only is the family as a whole extinct, it also
shows features in its anatomy which are not to be paralleled among living
stems. _Sphenophyllum_ became extinct in the Palæozoic period, but its
interest is very real and living to-day, and in the peculiar features of
its structure we see the first clue that suggests a common ancestor for
the still living groups of Lycopods and Equisetaceæ, which now stand so
isolated and far apart.
Before, however, we can consider the affinities of the group, we must
describe the structure of a typical plant belonging to it. The genus
_Sphenophyllum_ includes several species (for which there are no common
English names, as they are only known to science) whose differences are
of less importance than their points of similarity, so that one species
only, _S. plurifoliatum_, will be described.
We have a general knowledge of the external appearance of _Sphenophyllum_
from the numerous impressions of leaves attached to twigs which are found
in the rocks of the Carboniferous period. These impressions present a
good deal of variety, but all have rather delicate stems with whorls of
leaves attached at regular intervals. The specimens are generally easy to
recognize from the shape of the leaves, which are like broad wedges
attached at the point (see fig. 112). In some cases the leaves are more
finely divided and less fanlike, and it may even happen that on the same
branch some may be wedge-shaped like those in fig. 112, and others almost
hairlike. This naturally suggests comparison with water plants, which
have finely divided submerged leaves and expanded aerial ones. In the
case of _Sphenophyllum_, however, the divided leaves sometimes come at
the upper ends of the stems, quite near the cones, and so can hardly have
been those of a submerged part. The very delicate stems and some points
in their internal anatomy suggest that the plant was a trailing creeper
which supported itself on the stouter stems of other plants.
[Illustration: Fig. 112.—Impression of _Sphenophyllum_ Leaves attached
to the Stem, showing the wedge-shaped leaflets arranged in whorls]
The stems were ribbed, but unlike those of the Calamites the ribs ran
straight down the stem through the nodes, and did not alternate there, so
that the bundles at the node did not branch and fuse as they did in
_Calamites_.
The external appearance of the long slender cones was not unlike that of
the Calamite cones, though their internal details showed important
distinctions.
In one noticeable external feature the plants differed from those of the
last two groups considered, and that was in their _size_. Palæozoic
Lycopods and Equisetaceæ reached the dimensions of great trees, but
hitherto no treelike form of _Sphenophyllum_ has been discovered, and in
the structure-petrifactions the largest stems we know were less than an
inch in diameter.
In the internal anatomy of these stems lies one of the chief interests
and peculiarities of the plants. In the very young stage there was a
sharply pointed solid triangle of wood in the centre (fig. 113), at each
of the corners of which was a group of small cells, the protoxylems. The
structure of such a stem is like that of a root, in which the primary
wood all grows inwards from the protoxylems towards the centre, and had
we had nothing but these isolated young stems it would have been
impossible to recognize their true nature.
[Illustration: Fig. 113.—_Sphenophyllum_, Transverse Section of Young
Stem
_c_, Cortex, the soft tissue within which has decayed and left a space,
in which lies the solid triangle of wood, with the small protoxylem
groups _px_ at each corner. (Microphoto.)]
Such very young stems are rare, for the development of secondary wood
began early, and it soon greatly exceeded the primary wood in amount.
Fig. 114 shows a photograph of a stem in which the secondary wood is well
developed. The primary triangle of wood is still to be seen in the
centre, and corresponds to that in fig. 113, while closely fitting to it
are the bays of the first-formed secondary wood, which makes the wood
mass roughly circular. Outside this the secondary wood forms a regular
cylinder round the axis, which shows no sign of annual rings. The cells
of the wood are large and approximately square in shape, while at the
angles formed at the junction of every four cells is a group of small,
thin-walled parenchyma, see fig. 115. There are no medullary rays going
out radially through the wood, such as are found in all other zones of
secondary wood, and in this arrangement of soft tissue the plants are
unique.
[Illustration: Fig. 114.—_Sphenophyllum_, Transverse Section with
Secondary Wood W. At _c_ the cork formation is to be seen.
(Microphoto.)]
Beyond the wood was a zone of soft tissue and phloem, which is not often
preserved, while outside that was the cork, which added to the cortical
tissues as the stem grew (see fig. 114, _c_).
[Illustration: Fig. 115.—Group of Wood Cells _w_, showing their shape
and the small soft-walled cells at the angles between them _p_]
Petrified material of leaves and roots is rare, and both are chiefly
known through the work of the French palæobotanist Renault. The leaves
are chiefly remarkable for the bands of sclerized strengthening tissue,
and generally had the structure of aerial, not submerged leaves. The
roots were simple in structure, and, as in _Calamites_, had secondary
tissue like that in the stems.
In the case of the fructifications it is the English material which has
yielded the most illuminating specimens. The cones were long and slender,
externally covered by the closely packed tips of the scales, which
overlapped deeply. Between the whorls of scales lay the sporangia,
attached to their upper sides by slender stalks. A diagram will best
explain how they were arranged (see fig. 116). Two sporangia were
attached to each bract, but their stalks were of different lengths, so
that one sporangium lay near the axis and one lay outside it toward the
tip of the bract.
[Illustration: Fig. 116.—Diagram of Arrangement of Scales and Sporangia
in Cones of _Sphenophyllum_
A, Axis; _br_, bract; S, sporangium, with stalk _st_.]
In its anatomy the stalk of the cone has certain features similar to
those in the stem proper, which were among the first indications that led
to the discovery that the cone belonged to _Sphenophyllum_. There were
numerous spores in each of the sporangia, which had coats ornamented with
little spines when they were ripe (fig. 117, if examined with a
magnifying glass, will show this). Hitherto the only spores known are of
uniform size, and there is no evidence that there was any differentiation
into small (male) and large (female) spores such as were found in some of
the Lepidodendrons. In this respect _Sphenophyllum_ was less specialized
than either _Lepidodendron_ or _Calamites_.
In the actual sections of _Sphenophyllum_ cones the numerous sporangia
seem massed together in confusion, but usually some are cut so as to show
the attachment of the stalk, as in fig. 117, _st_. As the stalk was long
and slender, but a short length of it is usually cut through in any one
section, and to realize their mode of attachment to the axis (as shown in
fig. 116) it is necessary to study a series of sections.
[Illustration: Fig. 117.—Part of Cone of _Sphenophyllum_, showing
sporangia _sp_, some of which are cut so as to show a part of their
stalks _st_. B, Bract. (Microphoto.)]
Of the other plants belonging to the group, _Bowmanites Römeri_ is
specially interesting. Its sporangia were borne on stalks similar to
those of _Sphenophyllum_, but each stalk had two sporangia attached to
it. Two sporangia are also borne on each stalk in _S. fertile_. These
plants help in elucidating the nature of the stalked sporangia of
_Sphenophyllum_, for they seem to indicate a direct comparison between
them and the sporophylls of the Equisetales.
There is, further, another plant, of which we only know the cone, of
still greater importance. This cone (_Cheirostrobus_) is, however, so
complex that it would take far too much space to describe it in detail.
Even a diagram of its arrangements is extraordinarily elaborate. To the
specialist the cone is peculiarly fascinating, for its very complexity
gives him great scope for weaving theories about it; but for our purposes
most of these are too abstruse.
[Illustration: Fig. 118.—A, Diagram of Three-lobed Bract from Cone of
_Cheirostrobus_. _a_, Axis; _br_, the three sterile lower lobes of the
bract; _sp_, the three upper sporophyll-like lobes, to each of which
were attached four sporangia S. B, Part of the above seen in section
longitudinal to the axis. (Modified from Scott.)]
Its most important features are the following. Round the axis were series
of scales, twelve in each whorl, and each scale was divided into an upper
and a lower portion, each of which again divided into three lobes. The
lower three of each of these scale groups were sterile and bractlike,
comparable, perhaps, with the bracts in fig. 116; while the upper three
divisions were stalks round each of which were four sporangia. Each
sporophyll segment thus resembled the sporophyll of _Calamites_, while
the long sausage-shaped sporangia themselves were more like those of
_Lepidodendron_. In fig. 118 is a diagram of a trilobed bract with its
three attached sporophylls. Round the axis were very numerous whorls of
such bracts, and as the cone was large there were enormous numbers of
spore sacs.
A point of interest is the character of the wood of the main axis, which
is similar to that of Lepidodendron in many respects, being a ring of
centripetally developed wood with twelve projecting external points of
protoxylem.
This cone[13] is the most complex fructification of any of the known
Pteridophytes, whether living or fossil, which alone ensures it a special
importance, though for our purpose the mixed affinities it shows are of
greater interest.
To mention some of its characters:—The individual segments of the
sporophylls, each bearing four sporangia, are comparable with those of
_Calamites_, while the individual sporangia and the length of the
sporophyll stalk are similar in appearance to those of _Lepidodendron_.
The wood of the main axis also resembles that of a typical
_Lepidodendron_. The way the vascular bundles of the bract pass out from
the axis, and the way the stalks bearing the sporangia are attached to
the sterile part of the bracts, are like the corresponding features in
_Sphenophyllum_, and still more like _Bowmanites_.
Many other points of comparison are to be found in these plants, but
without going into further detail enough has been indicated to support
the conclusion that _Cheirostrobus_ is a very important clue to the
affinities of the Sphenophyllales and early Pteridophytes. It is indeed
considered to have belonged to an ancient stock of plants, from which the
Equisetaceæ, and _Sphenophylla_, and possibly also the Lycopods all
sprang.
_Sphenophyllum_, _Bowmanites_, and _Cheirostrobus_, a series of forms
that became extinct in the Palæozoic, remote in their structure from any
living types, whose existence would have been entirely unsuspected but
for the work of fossil botany, are yet the clues which have led to a
partial solution of the mysteries surrounding the present-day Lycopods
and Equisetums, and which help to bridge the chasm between these remote
and degenerate families.