Yale University has noticed in some of the Cycadean stems from
the Black hills of Dakota and Wyoming that the wood appears to
possess a similar structure, differing in its narrower medullary rays
from the wood of modern Cycads. The lozenge-shaped areas
external to the axis of the stem represent the sections of petioles,
some of which are shown in fig. 13, A, attached to the stem. The
majority of Mesozoic stems agree in external appearance with those
of recent species of Encephalartos, Macrozamia, and some other
genera; the trunk is encased in a mass of persistent petiole-bases
separated from one another by a dense felt or packing of scaly
ramenta. The structure of the leaf-stalks is like that of modern
Cycads, but the ramenta, instead of having the form of long unicellular
hairs like those on the petioles and bud-scales of existing
species are exactly like the paleae or ramental scales characteristic
of the majority of ferns. This fern-like character affords an interesting
survival of the close relationship between Cycads and Ferns.
Some examples of Jurassic Cycadean stems from Wyoming are
characterized by an unusually rich development of ramental
scales; the ramenta from the old leaf-bases form an almost complete
covering over the surface of the trunk. Professor Lester Ward
has instituted a new generic name,
Cycadella, for these woolly forms. In
a few cases the fossil stems show no
trace of any lateral flowering shoots,
and in that respect agree with modern
forms: an instance of this is afforded
by a large Cycadean trunk discovered
some years ago in one of the Portland
quarries, and named Cycadeoidea gigantea
(fig. 14). In this stem the flowers
may have been terminal, as in existing
Cycads. As a rule, however, the
fossil stems show a marked difference
from modern forms in the possession
of lateral shoots given off from the
axils of leaves, and terminating in a
flower of complex structure containing
numerous orthotropous seeds. These
reproductive shoots differ in many
important respects from the flowers
of recent Cycads, and chiefly on this
account it is customary to include the
plants in a separate genus, Bennettites,
and in a separate group—the
Bennettitales—distinct from that of
the Cycadales including the existing
Cycads. The best preserved specimens
of the true Bennettites type so far
described are from the Lower Greensand
and Wealden of England, and
from Upper Mesozoic strata in North
America, Italy and France. A study
of the anatomical structure of the
vegetative stem, which on the whole
is very similar to that of recent
Cycads (fig. 15, 1 and 2), reveals
certain characters which are not met
with in modern Cycads. The chief distinguishing feature
afforded by the leaf-traces; in recent species (see Gymnosperms)
these pursue a somewhat complicated course as they pass
from the petiole towards the vascular cylinder of the stem,
but in Bennettites the vascular bundles from the leaves followed
a more direct course through the cortex of the stem (fig. 15, 3).
Among existing types the genus Macrozamia appears
to show the nearest approach to this simpler structure of the
leaf-traces. In a Floridan species of Zamia the leaf-traces
are described as characterized by a more direct course from the
stele of the stem to the leaves than in most modern genera, thus
agreeing more closely with the extinct Bennettites. The typical
Bennettites female flower (fig. 15, 4 and 7), as investigated in English,
French, Italian, and American specimens, may be briefly described
as a short lateral shoot or peduncle, arising in a leaf-axil and terminating
in a bluntly rounded apex, bearing numerous linear bracts
enclosing a central group of appendages, some of which consist of
slender pedicels traversed by a vascular strand and bearing a
single terminal ovule enclosed in an integument, which forms a
distal canal or micropyle. Associated with these seminiferous
pedicels occur sterile appendages consisting of slender stalks,
terminating in distal expansions, which form a fleshy covering over
the surface of the flower, leaving small apertures immediately above
the micropyles for the entrance of the pollen-grains. It has been
suggested by some authors that the almost complete investment of
the small Bennettites seeds by the surrounding swollen ends of the
interseminal scales (fig. 15, 7) represents an approach to the angiospermous
ovary. In Bennettites the ovules are left exposed at the
apex, but they are by no means so distinctly gymnospermous as
in recent Cycads and Conifers. The seeds have in some cases been
preserved in wonderful perfection, enabling one to make out the
structure of the embryo, with its bluntly conical radicle and two
fleshy cotyledons filling the exalbuminous seed (fig. 15, 11).
|
|
| ||
|
Fig. 12.—Podozamites lanceolatus. Inferior Oolite, England. |
Fig. 13.—Cycadean stem, from Upper Gondwana rocks, India. A, Surface view; B, Transverse section of stem. |
Fig. 14.—Cycadeoidea gigantea. Portland rocks, England. |
Our knowledge of the reproductive organs of the Bennettitaceae has until recently been confined to the female flowers, as described by Carruthers, Solms-Laubach, Lignier, and others. The fortunate discovery of several hundred Cycadean stems in the United States, of Lower Cretaceous and Upper Jurassic age, has supplied abundant material which has lately been investigated and is still receiving attention at the hands of Mr Wieland. This investigator has already published a well-illustrated account of his discoveries, which give valuable information as to the morphology of the male organs, and lead us to expect additional results in the future of the greatest importance and interest. On some of the American stems flowers have been found, borne at the apex of lateral shoots, which possess fully developed male organs consisting of sporangia with spores (pollen-grains), surrounding a conical central receptacle bearing numerous small and probably functionless or immature ovules (fig. 15, 10). The structure of this type of flower may be briefly described as follows. In shape and size the flower is similar to that long known as the female flower of Bennettites and Williamsonia. A number of hairy linear bracts enclose the whole; internal to these occur 12 to 20 crowded pinnate leaves (sporophylls), with their apical portions bent over towards the axis of the flower, the bases of the petioles being fused laterally into a disk surrounding the base of the conical receptacle. Numerous pairs of pinnules are attached to the rachis of each sporophyll, and the larger pinnules bear 20 to 30 synangia (sori or plurilocular sporangia) (fig. 15, 8 and 9). The synangia consist of a stout wall composed of thick-walled cells, succeeded by a layer of more delicate and smaller elements; and internal to the wall occur two rows of sporangial loculi containing microspores. When the synangia are ripe dehiscence takes place along a median line between the two rows of loculi. In size, position, arrangement, and manner of dehiscence the sporangia bear a striking resemblance to those of Marattia and Danaea among recent Marattiaceae. The most important point elucidated by this discovery is the very close correspondence of the male organs of the Bennettites flower with the sporophylls and synangia of Marattiaceous ferns—a further relic of the common origin of Cycads and Ferns. It remains to be seen if the ovuliferous cone in the centre of the flower represents simply a functionless gynoecium, as in Welwitschia and abnormal cones of certain Coniferae, or if the flowers were hermaphrodite, with both male and female organs fully developed. We have a combination in the same flower of stalked ovules, the structure of which has already been described, and interseminal scales constituting a complex gynoecium, which exhibits in certain features an approach to the angiospermous type, and differs in structure from other Gymnosperm flowers, associated with male organs constructed on a plan almost identical with that of the sporophylls in Marattiaceae. In many of the flowers described by Mr Wieland the structure is identical in essential features with that of the female flowers of Bennettites Gibsonianus described by Carruthers and by Solms-Laubach, and with that of a French Liassic species described by Lignier: the whole consists of a convex receptacle bearing mature seeds at the tips of pedicels associated with interseminal scales (fig. 15, 7) as already described. Mr Wieland’s researches have, however, demonstrated the existence in flowers of this type of the remains of a disk at the base of the receptacle, between the receptacle and the surrounding bracts, to which staminate leaves were originally attached. The flowers hitherto regarded as female were in some cases at least hermaphrodite,
