PAST HISTORIES OF PLANT FAMILIES
III. The Bennettitales
This fascinating family is known only from the fossils, and is so remote
in its organization from any common living forms that it may perhaps be a
little difficult for those who do not know the Cycads to appreciate the
position of Bennettites. It would probably be better for one studying
fossil plants for the first time to read the chapters on the Cycads,
Pteridosperms, and Ferns before this chapter on the present group, which
has characters connecting it with that series.
Until recently the bulk of the fossils which are found as impressions of
stems and foliage of this family were very naturally classed as Cycads.
They are extremely common in the Mesozoic rocks (the so-called Age of
Cycads), and in the external appearance of both stems and leaves they are
practically identical with the Cycads.
A few incomplete fructifications of some species have been known in
Europe for many years, but it is only recently that they have been fully
known. This is owing to Wieland’s[12] work on the American species, which
has made known the complete organization of the fructifications from a
mass of rich and well-petrified material.
In the Lower Cretaceous and Upper Jurassic rocks of America these plants
abound, with their microscopic structure well preserved, and their
fructifications show an organization of a different nature from that of
any past or present Cycad.
Probably owing to their external appearance, Wieland describes the plants
as “Cycads” in the title of his big book on them; but the generic name he
uses, _Cycadeoidea_, seems less known in this country than the equally
well-established name of _Bennettites_, which has long been used to
denote the European specimens of this family, and which will be used in
the following short account of the group.
At the present time no family of fossils is exciting more interest. Their
completely Cycadean appearance and their unique type of fructification
have led many botanists to see in them the forerunners of the
Angiosperms, to look on them as the key to that mystery—the origin of the
flowering plants. This position will be discussed and the many facts in
its favour noted, but we must not forget that the _Bennettitales_ have
only recently been realized fully by botanists, and that a new toy is
ever particularly charming, a new cure particularly efficacious, and a
new theory all-persuasive.
From their detailed study of the flowering plants botanists have leaned
toward different groups as the present representatives of the primitive
types. The various claims of the different families to this position
cannot be considered here; probably that of the Ranales (the group of
families round Ranunculaceæ as a central type) is the best supported. Yet
these plants are most frequently delicate herbs, which would have stood
relatively less chance of fossilization than the other families which may
be considered primitive. They are peculiarly remote from the group of
Bennettiteæ in their vegetative structure, a fact the importance of which
seems to have been underrated, for in the same breath we are assured that
the Bennettites are a kind of cousin to the ancient Angiosperms, and that
the Ranales are among the most primitive living Angiosperms, and
therefore presumably nearest the ancient ones.
However, let us leave the charms of controversy on one side and look at
the actual structure of the group. They were widely spread in Lower
Mesozoic times, the plants being preserved as casts, impressions, and
with structure in great numbers. The bulk of the described structural
specimens have been obtained from the rocks of England, France, Italy,
and America, although leaf impressions are almost universally known. The
genus _Williamsonia_ belongs to this family, and is one of the best known
of Mesozoic plant impressions.
Externally the Bennettiteæ were identical in appearance with stumpy
Cycads, and their leaves it is which gave rise to the surmise, so long
prevalent, that the Lower Mesozoic was the “Age of Cycads”, just as it
was the Pteridosperm leaves that gave the Palæozoic the credit of being
the “Age of Ferns”. In the anatomy of both stem and leaf, also, the
characters are entirely Cycadean; the outgoing leaf trace is indeed
simpler in its course than that of the Cycads.
[Illustration: Fig. 71.—Half of a Longitudinal Section through a Mature
Cone of _Bennettites_
A, Short conical axis; _s_, enclosing bracts; S, seeds; _sc_, sterile
scales between the seeds.]
The fructifications, however, differ fundamentally from those of the
Cycads, as indeed they do from those of any known family. They took the
form of compact cones, which occurred in very large numbers in the mature
plants hidden by the leaf bases. In _Williamsonia_, of which we know much
less detail, the fructifications stood away from the main axis on long
pedicels.
In _Bennettites_ the cones were composed of series of sheathing scales
surrounding a short conical axis on which stood thin radiating stalks,
each bearing a seed. Between them were long-stalked sterile scales with
expanded ends. A part of a cone is illustrated diagrammatically in fig.
71. The whole had much the appearance of a complex fruit. In some
specimens these features alone are present in the cones, but in younger
cones from the American plants further structures are found attached.
Below the main axis of the seed-bearing part of the cone was a series of
large complex leaflike structures closely resembling fern leaves in their
much-divided nature. On the pinnæ of these leaves were crowded
innumerable large sporangia, similar to those of a fern, which provided
the pollen grains. The fossils are particularly well preserved, and have
been found with these male (pollen-bearing) organs in the young unopened
stages, and also in the mature unfolded condition, as well as the
ripening seed cones from which they have faded, just as the stamens fade
from a flower when the seeds enlarge.
[Illustration: Fig. 72.—Diagram of Complete Cone of _Bennettites_
A, Central axis of conical shape terminating in the seed-bearing cone
S. (After Wieland), and bearing successively Br., bracts, comparable
with floral leaves; M, large complex leaves with pollen sacs.]
It appears that these huge complex leaflike structures were really
stamens, but nevertheless they were rolled up in the circinate form as
are young fern leaves, and as they unrolled and spread out round the
central cone they must have had the appearance of a whorl of leaves (see
fig. 72).
This, in a few words, is the main general character of the
fructification. The most important features, on which stress is laid, are
the following. The association of the male and female structures on the
same axis, with the female part _above_ the male. This arrangement is
found only in the flowering plants; the lower plants, which have male and
female on the same cone, have them mixed, or the female below, and are in
any case much simpler in their entire organization. The conical form of
the axis is also important, as is the fact that it terminates in the
seed-bearing structures.
[Illustration: Fig. 73.—Diagram of Cross Section of _Bennettites_,
Seed, with Embryo
_c_, Double-layered seed coat; _n_, crushed nucellus; _cot._, two
cotyledons which practically fill the seed.]
The position of the individual seeds, each on the end of a single stalk,
is remarkable, as are the long-stalked bracts whose shield-like ends join
in the protection of the seeds. These structures together give the cone
much of the appearance of a complex fruit of a flowering plant, but the
structure of the seeds themselves is that of a simple Gymnosperm.
In the seeds, however, was an _embryo_. In this they differ from all
known seeds of an earlier date, which, as has been already noted (see p.
77), are always devoid of one. This embryo is one of the most important
features of the plant. It had two cotyledons which filled the seed space
(see fig. 73), and left almost no trace of the endosperm. Reference to p.
112 will show that this is an advance on the Cycad seed, which has a
small embryo embedded in a large mass of endosperm, and that it
practically coincides with the Dicotyledonous type.
The seed with its embryo suggested comparison with the Angiosperms long
before the complete structure of the fructification was known.
The fern-like nature of the pollen-bearing structures is another very
important point. Were any one of these leaflike “stamens” found isolated
its fern-like nature would not have been questioned a year or two ago,
and their presence in the “flower” of _Bennettites_ is a strong argument
in favour of the Fern-Pteridosperm affinities of the group.
Had the parts of this remarkable fructification developed on separate
trees, or on separate branches or distinct cones of the same one, they
would have been much less suggestive than they are at present, and the
fructifications might well have been included among those of the
Gymnosperms, differing little more (apart from the embryo) from the other
Gymnosperm genera than they do from each other. In fact, the extremely
fern-like nature of the male organs is almost more suggestive of a
Pteridosperm affinity, for even the simplest Cycads have well-marked
scaly cones as their male organs. The female cone, again, considered as
an isolated structure, can be interpreted as being not vitally different
from _Cordaites_, where the seeds are borne on special short stalks
amidst scales.
The embryo would, in any case, point to a position among advanced types;
but it is so common for one organ of a plant to evolve along lines of its
own independently, or in advance of the other organs, that the embryo
structure alone could not have been held to counterbalance the Cycadean
stems and leaves, the Pteridosperm-like male organs, and the Gymnospermic
seeds.
But all these parts occur on the same axis, arranged in the manner
typical of Angiosperms. The seed-bearing structures at the apex, the
“stamens” below them, and a series of expanded scales below these again,
which it takes little imagination to picture as incipient petals and
sepals; and behold—the thing is a flower!
And being a “flower”, is in closest connection with the ancestors of the
modern flowering plants, which must consequently have evolved from some
Cycadean-like ancestor which also gave rise to the Bennettitales. Thus
can the flowering plants be linked on to the series that runs through the
Cycads directly to the primitive ferns!
It is evident that this group, of all those known among the fossils,
comes most closely to an approximation of Angiospermic structure and
arrangement. Enough has been said to show that in their actual nature
they are not Angiosperms, though they have some of their characters,
while at the same time they are not Cycads, though they have their
appearance. They stand somewhere between the two. Though many botanists
at present hold that this mixture of characters indicates a relationship
equivalent to a kind of cousinship with the Angiosperms, and both groups
may be supposed to have originated from a Cycadean stock, this theory has
not yet stood the test of time, nor is it supported by other evidence
from the fossils. We will go so far as to say that it appears as though
_some_ Angiosperms arose in that way; but flowering plants show so many
points utterly differing from the whole Cycadean stock that a little
scepticism may not be unwholesome.
It is well to remember the Lycopods, where (as we shall see, p. 141)
structures very like seeds were developed at the time when the Lycopods
were the dominant plants, and we do not find any evidence to prove that
they led on to the main line of seed plants. Similarly, Cycads may have
got what practically amounted to flowers at the time when they were the
dominant group, and it is very conceivable that they did not lead on to
the main line of flowering plants.
Whatever view may be held, however, and whatever may be the future
discoveries relating to this group of plants, we can see in the
Bennettitales points which throw much light on the potentialities of the
Cycadean stock, and structures which have given rise to some most
interesting speculations on the subject of the Angiosperms. This group is
another of the jewels in the crown of fossil botany, for the whole of its
structures have been reconstructed from the stones that hold all that
remains of this once extensive and now extinct family of plants.