Ancient Plants – Past Histories of Plant Families: Bennettitales

CHAPTER X
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 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.

fig71

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.

fig72

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.

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.

fig73

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, 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, and left almost no trace of the endosperm. Reference 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 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.