Among the fossil remains of this family there are impressions, casts, and true petrifactions, so that we know both its external appearance and the internal anatomy of nearly every part of several species of the genus. For a long time the various fossil remains of the plant were not recognized as belonging to each other and together forming the records of one and the same plant—the broad, long leaves with their parallel veins were looked on as Monocotyledons (seefig. 61); the pith casts (seefig. 63) were thought to be peculiar constricted stems, and were calledSternbergia; while the wood, which was known from its microscopic structure, was calledAraucarioxylon—but the careful work of many masters of fossil botany, whose laborious studies we cannot describe in detail here, brought all these fragments together and proved them to belong toCordaites.
Fig. 61.—Leaf ofCordaites,l, attached by its broad base to a Stem,s
We now know thatCordaiteswere large trees, with strong upright shafts of wood, to whose branches large simple leaves were attached. The leaves were much bigger than those of any living Gymnosperm, even than those of the Kauri Pine (a member of the Araucariaceæ), and seem in some species to have exceeded 3 ft. in length. The trees branched only at the top of the main shaft, and with their huge sword-like leaves must have differed greatly in appearance from any plant now living. The leaves had many parallel veins, as can be seen infig. 61, and were attached by a broad base directly to the main stem; thus coming closer to theAraucarias than the other groups of Gymnosperms in their leaf characters.
Fig. 62A.—Microscopic Section of Part of a Leaf ofCordaites
V, Vascular bundle;W, wood of bundle;sh, its sheath;S1, large sclerenchyma mass alternating with bundles;S2andS3, sclerenchyma caps of bundle;P, soft tissue of leaf.
The internal anatomy is often well preserved, and there is a number of species of leaves whose anatomy is known. As will be expected from the parallel veins, in each section there are many vascular bundles running equidistantly through the tissue.Fig. 62Ashows the microscopic details from a well-preserved leaf. In all the species patches of sclerenchyma were developed, and everything indicates that they were tough and well protected against loss of water, even to a greater extent than are most of the leaves of living Gymnosperms.
In the stems the pith was much larger than that in living Gymnosperms (where the wood is generally very solid), and it was hollow in older stems, except for discs of tissue across the cavity. The internal cast from these stems has been described before, and is seen infig. 63.
Fig. 62B.—Much-magnified Wood Elements fromCordaitesStem seen in longitudinal section, the type known asAraucarioxylon. Note the hexagonal outlines of the bordered pits, which lie in several rows
The wood was formed in closely packed radiatingrows by a normal cambium (seep. 66), and the tracheæ so formed had characteristic rows of bordered pits (seefig. 62B). The wood comes nearer to that of the living Araucarias than any other, and indeed the numerous pieces of fossil wood of this type which are known from all the geological periods are calledAraucarioxylon.[11]A double strand goes out from the main mass of wood, which afterwards divides and subdivides to provide the numerous bundles of the leaf.
Fig. 63.—Cast of Hollow Pith ofCordaites, the constrictions corresponding to discs of solid tissue across the cavity
In the case of these fossils we are fortunate enough to have the fructifications, both male and female, in a good state of preservation. As in other Gymnosperms, the male and female cones are separate, but they differed less from each other in their arrangement than do those of any of the living types hitherto mentioned. They can hardly be described as true cones, though they had something of that nature; the seeds seem to be borne on special short stems, round which are also sterile scales. In the seed and the way it is borne perhaps the Cordaiteæ may be compared more nearly with the Taxeæ than with the other groups. A seed, not yet ripe, is shown in slightly diagrammatic form infig. 64, where the essential details are illustrated. The seeds of this family sometimes reached a considerable size, and had a fleshy layer which was thick in comparison with the stone, and externally comparable with a cherry—though, of course, of very different nature in reality, forCordaites, likeTaxus, is a Gymnosperm, with simple naked seeds, while a cherry is the fruit of an Angiosperm.
In a few words, these are the main characters of the large group ofCordaites, which held the dominant position among Gymnosperms in the Palæozoic era. They have relationships, or perhaps one should say likenesses, to many groups. Their stem- and root-anatomy is similar to the Coniferæ of the present day, the position of the ovules is like that in the Taxaceæ, the male cones in some measure recall those ofGinkgo, the anatomy of their leaves has points which are comparable with those of the Cycads, to which group also the large pith in the stem and the structure of some details in the seeds unite them. Their own specially distinctive characters lie in their crown of huge leaves, and unbranched shaft of stem, the similarity of their male and female inflorescences, and some points in their pollen grains which have not been mentioned. The type is a very complex one, possibly coming near the stock which, having branched out in various directions, gave rise to several of the living families.
Fig. 64.—Representation ofCordaitesSeed and its Axis with Scales, slightly diagrammatic, modified from Renault.
A, Axis withs, scales;c, coat of the seed, from which the inner parts have shrunk away;n, nucellus;p.c, pollen chamber containing pollen grains which enter throughm.
Plants which come very near to the Cordaiteæ are thePoroxyleæ. Of this group we have unfortunately no remains of fructifications in organic connection, so that its actual position must remain a little doubtful till they are discovered. There seems no doubt that they must have borne seeds.
Still, it has been abundantly demonstrated in recent years that the anatomy of the root, stem, and leavesindicates with considerable exactness the position of any plant, so that, as these are known, we can deduce from them, with a feeling of safety, the position thatPoroxylontakes in the natural system. In its anatomy the characters are those of the Cordaiteæ, with certain details which show a more primitive nature and seem to be characteristic of the groups below it in organization.
Poroxylonis not common, and until recently had not been found in the Lower Coal Measures of England. The plants appear to have been much smaller thanCordaites, with delicate stems which bore relatively large simple leaves. The anatomy of the root was that common in Gymnosperms, but the stem had a very large pith, and the leaves were much like those ofCordaitesin having parallel veins. An important character in the anatomy of the stem was the presence of what is known ascentripetal wood. This must be shortly explained. In all the stems hitherto considered, the first-formed wood cells (protoxylems, seep. 57) developed at the central point of the wood, towards the pith (seefig. 19,px,p. 49). This is characteristic of all Angiosperms and the higher Gymnosperms (except in a couple of recently investigated Pines), but among the lower plants we find that part of the later wood develops to the inner side of these protoxylem masses. The distinction is shown infig. 65.
Fig. 65.—A, Normal bundle of higher plant;x, protoxylem on inner side next the pithp, and the older woodwoutside it,centrifugalwood.B, Bundle with wood cellscdeveloped on inner side of protoxylem,centripetalwood; the arrow indicates the direction of the centre of the stem.
This point is one to which botanists have given much attention, and on which they have laid much weight in considering the affinities of the lower Gymnosperms and the intermediate groups between them and the ferns, which are found among the fossils. InCordaitesthispoint of connection with the lower types is not seen, but inPoroxylon, which has otherwise a stem anatomy very similar toCordaites, we find groups ofcentripetalwood developed inside the protoxylem of primary bundles. For this reason, principally, isPoroxylonof interest at present, as in its stem anatomy it seems to connect theCordaitestype with that of the group below it in general organization.
Ginkgoales.—Reference top. 44shows thatGinkgo, the Maidenhair tree, belongs to the Ginkgoales, a group taking equal rank with the large and complex series of the Coniferales. The Ginkgoales of the present day, however, have but one living representative.Ginkgostands alone, the single living species of its genus, representing a family so different from any other living family that it forms a prime group by itself.
Had the tree not been held sacred in China and Japan, it is probable that it would long since have been extinct, for it is now known only in cultivation. It is indeed a relic from the past which has been fortunately preserved alive for our examination. It belongs to the fossil world, as a belated November rose belongs to the summer.
Because of its beauty and interest the plant is now widely distributed under cultivation, and is available for study almost as freely as the other types of living Gymnosperms already mentioned, so that but a short summary of its more important features is needed here.
Old plants, such as can be seen growing freely in Japan (in Kew Gardens there is also a fine specimen), are very tall handsome woody trees, with noble shafts and many branches. The leaves grow on little side shoots and are the most characteristic external feature of the tree; their living form is illustrated infig. 66, which shows the typical simple shape as well as the lobed form of the leaf which are to be found, with all intermediate stages, on the same tree. No other plant (save afew ferns, which can generally be distinguished from it without difficulty) has leaves at all like these, so that it is particularly easy to identify the fossil remains, of which there are many.
Fig. 66.—A, Tuft ofGinkgoLeaves, showing their “maidenhair”-like shape.B, Single deeply-divided Leaf to be found on the same tree, usually on young branches.
The wood is compact and fine grained, the rings of secondary tissue being developed from a normal cambium as in the case of the higher Gymnosperms, and the individual tracheæ have round bordered pits. There are small male cones, but the seeds are not borne in cones. They develop on special stalks on which are no scales, but a small mass of tissue at the base of the seed called the “collar”. Usually there are two young ovules, of which often only one ripens to a fleshy seed, though both may mature.
Fig. 67.—Ripe Stage ofGinkgoSeeds attached to their Stalk.c, “Collar” of seed.
The ripe seed reaches the size shown in the diagram, and is orange coloured and very fleshy; within it is astone encasing the endosperm, which is large,green, and starchy, and contains the embryo with two cotyledons. This embryo is small compared with the endosperm, cf.fig. 57,p. 76, which is somewhat similar to that ofGinkgoin this stage.
Of the microscopic characters of the reproductive organs the most remarkable is the male cell. This is not a passive nucleus, as in the plants hitherto considered, but is anactively swimmingcell of some size, provided with a spiral of cilia (hairlike structures) whose movements propel it through the water. In the cavity of the unripe seed these swim towards the female cell, and actively penetrate it. The arrangements of the seed are diagrammatically shown infig. 68, which should be compared with that ofCycas,fig. 76, with which it has many points in common.
Fig. 68.—Section through Seed ofGinkgo
p.c, Pollen chamber in the nucellusn, which is fused to the coatcto the levell;sc, stony layer in coat;S, the big spore, filled with endosperm tissue (in this case green in colour);e, egg cells, one of which will produce the embryo after fertilization.
The nature of the male cell inCordaitesis not yet known, but there is reason to suspect it may have been actively swimming also. As this is uncertain, however, we may considerGinkgothe most highly organized plant which has such a primitive feature, a feature which is a bond of union between it and the ferns, and which, when it was discovered about a dozen years ago, caused a considerable sensation in the botanical world.
To turn now to the fossil records of this family. Leaf impressions ofGinkgoare found in rocks of nearly all ages back even to the Upper Palæozoic. They show a considerable variety of form, and it is certain that they do not all belong to the samespeciesas the living plant,but probably they are closely allied.Fig. 69shows a typical impression from the Lower Mesozoic rocks. In this specimen, the cells of the epidermis were fortunately sufficiently well preserved to be seen with the microscope, and there is a distinct difference in the size and shape of the cells of living and fossil species, seefig. 70; but this difference is slight as compared with the great similarity of form and appearance, as can be seen on comparing figs.69and66,B, so that the fossil is at the most a different species of the genusGinkgo. Among the fossil leaves there is greater variety than among the living ones, and some which are very deeply lobed so as to form a divided palm-like leaf go by different names, e.g.Baiera, but they are supposed to belong to the same family. Fossil seeds and male cones are also known as impressions, and are found far back in the Mesozoic rocks. From the fossil impressions it is certain thatGinkgoand plants closely allied to it were very widespread in the past, as they are found all over Europe as well as the other continents. Particularly in the Lower Mesozoic rocksGinkgoseems to have been a world-wide type growing in great abundance.
Fig. 69.—Leaf Impression ofGinkgofrom Mesozoic Rocks of Scotland
Fig. 70.—Showing Epidermis with Stomates from the lower side of the Leaf seen infig. 69
e, Epidermis cells;s, stomates;v, long cells of epidermis lying over the veins.
In the Palæozoic the records are not so undoubted, but there is strong evidence which leads us to suppose that if the genus now living were not then extant, at least other closely related genera were, and there seems to be good grounds forsupposing thatGinkgoandCordaitesmay have both arisen from some ancient common stock.
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 ofBennettites, 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 theBennettitaleshave 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 genusWilliamsoniabelongs 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.
Fig. 71.—Half of a Longitudinal Section through a Mature Cone ofBennettites
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. InWilliamsonia, of which we know much less detail, the fructifications stood away from the main axis on long pedicels.
InBennettitesthe 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 infig. 71. The whole had much the appearance of a complex fruit. In some specimens these features alone are present in the cones, but in youngercones 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.
Fig. 72.—Diagram of Complete Cone ofBennettites
A, Central axis of conical shape terminating in the seed-bearing coneS. (After Wieland), and bearing successivelyBr., 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 (seefig. 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 partabovethe 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.
Fig. 73.—Diagram of Cross Section ofBennettites, 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 anembryo. In this they differ from all known seeds of an earlier date, which, as has been already noted (seep. 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 (seefig. 73), and left almost no trace of the endosperm. Reference top. 112will show that this is an advance on the Cycad seed, which has a small embryoembedded 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” ofBennettitesis 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 fromCordaites, 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, anda 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 thoughsomeAngiosperms 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 ofplants, 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.
The group of theCycadales, which has a systematic value equivalent to theGinkgoales, contains a much larger variety of genera and species than does the latter. There are still living nine genera, with more than a hundred and fifty species, which form (though a small one compared with most of the prime groups) a well-defined family. They are the most primitive Gymnosperms, the most primitive seed-bearing plants now living, and in their appearance and characters are very different from any other modern type. Their external resemblance to the group of the Bennettitales, however, is very striking, and indeed, without the fructifications it would be impossible to distinguish them.
The best known of the genera is that ofCycas, of which an illustration is given infig. 74. The thick, stumpy stem and crown of “palm”-like leaves give it a very different appearance from any other Gymnosperm. Commonly the plants reach only a few feet in height, but very old specimens may grow to the height of 30 ft. or more. The other genera are smaller, and some have short stems and a very fern-like appearance, as, forexample, the genusStangeria, which was supposed to be a fern when it was first discovered and before fruiting specimens had been seen.
The large compound leaves are all borne directly on the main stem, generally in a single rosette at its apex, and as they die off they leave their fleshy leaf bases, which cover the stem and remain for an almost indefinite number of years.
The wood of the main trunks differs from that of the other Gymnosperms in being very loosely built, with a large pith and much soft tissue between the radiating bands of wood. There is a cambium which adds zones of secondary tissue, but it does not do its work regularly, and the cross section of an old Cycad stem shows disconnected rings of wood, accompanied by much soft tissue. The cells of the wood have bordered pits on their walls, and in the main axis the wood is usually all developed in a centrifugal direction, but in the axis of the cones some centripetal wood is found (refer toc,fig. 65,p. 97).
Fig. 74.—Plant ofCycas, showing the main stem with the crown of leaves and the irregular branches which come on an old plant
In their fructifications the Cycads stand even further apart from the rest of the Gymnosperms. One striking point is the enormous size of theirmalecones. The male cones consist of a stout axis, round which are spiral series of closely packed simple scales covered with pollen-bearingsacs (which bear no inconsiderable likeness to fern sporangia), the whole cone reaching 1½ ft. in length in some genera, and weighing several pounds. All the other Gymnosperms, except the Araucareæ, where they are an inch or two long, have male cones but a fraction of an inch in length.
Fig. 75.—Seed-bearing Scale ofCycas, showing its lobed and leaflike character
s, Seeds attached on either side below the divisions of the sporophyll.
In all the members of the family, exceptingCycasitself, the female fructifications also consist of similarly organized cones bearing a couple of seeds on each scale instead of the numerous pollen sacs. InCycasthe male cones are like those of the other genera, and reach an enormous size; but there are no female cones, for the seeds are borne on special leaflike scales. These are illustrated infig. 75, which shows also that there are not two seeds (as in the other genera with cones) to each scale, but an indefinite number.
The leafy nature of the seed-bearing scale is an important and interesting feature. Although theoretically botanists are accustomed to accept the view that seeds are always borne on specially modified leaves (so that to a botanist even the “shell” of a pea-pod and the box of a poppy capsule are leaves), yet inCycasalone among living plants are seeds really found growing on a large structure which has the appearance of a leaf. Hence, from this point of view (seep. 45, however, for a caution against concluding that the whole plant is similarly lowly organized),Cycasis the most primitive of all the living plants that bear seeds, and hence presumably the likest to the fossil ancestors of the seed-bearing types. In this character it is more primitive than the fossil group oftheCordaiteæ, and comes very close to an intermediate group of fossils to be considered in the next chapter.
Fig. 76.—Seed ofCycascut open
n, The nucellus, fused at the levellto the coatc;sc, stony layer of coat;p.c, pollen chamber in apex of the nucellus;S, “spore”, filled with endosperm, in which lies the embryoe.
To enter into the detailed anatomy of the seeds would lead us too far into the realms of the specialist, but we must notice one or two points about them. Firstly, their very large size, for ripe seeds ofCycasare as large as peaches (and peaches, it is to be noted, are fruits, not seeds), and particularly the large size they attainbeforethey are fertilized and have an embryo. Among the higher plants the young seeds remain very minute until an embryo is secured by the act of fertilization, but in the Cycads the seeds enlarge and lay in a big store of starch in the endosperm before the embryo appears, so that in the cases in which fertilization is prevented large, sterile “seeds” are nevertheless produced. This must be looked on as a want of precision in the mechanism, and as a wasteful arrangement which is undeniably primitive. An even more wasteful arrangement appears to have been common to the “seeds” of the Palæozoic period, for, though many fossil “seeds” are known in detail from the old rocks, not one is known to have any trace of an embryo. A general plan of theCycasseed is shown infig. 76, which should be compared with that ofGinkgo(fig. 68). The large size of the endosperm and the thick and complex seed-coats are characteristic features of both these structures. Another point that makes the Cycad seeds of special interest is the fact that the male cells (as inGinkgo) are developed as active, free-swimmingsperms, which swim towards the female cell in the space provided for them in the seed (seep.c,fig. 76).
The characters of the Cycads as they are now living prove them to be an extremely primitive group, and therefore presumably well represented among the fossils; and indeed among the Mesozoic rocks there is no lack of impressions which have been described as the leaves of Cycads. There is, however, very little reliable material, and practically none which shows good microscopic structure. Leaf impressions alone are most unsafe—more unsafe in this group, perhaps, than in any other—for reasons that will be apparent later on, and the conclusions that used to be drawn about the vast number of Cycads which inhabited the globe in the early Mesozoic must be looked on with caution, resulting from the experience of recent discoveries proving many of these leaves to belong to a different family.
There remain, however, many authentic specimens which show thatCycascertainly goes back very far in history, and specimens of this genus are known from the older Mesozoic rocks. We cannot say, however, as securely as used to be said, that the Mesozoic was the “Age of Cycads”, although it was doubtless the age of plants which had much of the external appearance of Cycads.
From the Palæozoic we have no reliable evidence of the existence of Cycads, though the plants of that time included a group which has an undoubted connection with them.
Indeed, so far as fossil evidence goes, we must suppose that the Cycads, since their appearance, possibly at the close of the Palæozoic, have never been a dominant or very extensive family, though they grew in the past all over the world, and in Europe seem to have remained till the middle of the Tertiary epoch.
This group consists entirely of plants which are extinct, and which were in the height of their development in the Coal Measure period. As a group they are the most recently discovered in the plant world, and but a few years ago the name “Pteridosperm” was unknown. They form, however, both one of the most interesting of plant families and one of the most numerous of those which flourished in the Carboniferous period.
To mention first the vital point of interest in their structure, they showleaves which in all respects appear like ordinary foliage leaves, and yet bear seeds. These leaves, which we now know bore the seeds, had long been considered as typical fern leaves, and had been named and described as fern leaves. There are two extremely important results from the discovery of this fossil group, viz. that leaves, to all appearance like ordinary foliage, can directly bear seeds, and that the leaves, though like fern leaves, bore seeds like those of a Cycad.
As the namePteridospermindicates, the group is a link between the ferns and the seed-bearing plants, and as such is of special interest and value to botanists.
The gradual recognition of this group from among the numerous plant fragments of Palæozoic age is one of the most interesting of the accumulative discoveries of fossil botany. Ever since fossil remains attracted the attention of enquiring minds many “ferns” have been recognized among the rich impressions of the Coal Measures. Most of them, however, were not connected with any structural material, and were given many different names of specific value. So numerous were these fern “species” that it was supposed that in the CoalMeasure period the ferns must have been the dominant class, and it is often spoken of even yet as the “Age of Ferns”. From the rocks of the same age, preserved with their microscopical structure perfect, were stems which were calledLyginodendron. In the coal balls associated with these stems (which were the commonest of the stems so preserved) were also roots, petioles, and leaflets, but they were isolated, like the most of the fragments in a coal ball, and to each was given its name, with no thought of the various fragments having any connection with each other. Gradually, however, various fragments from the coal balls had been recognized as belonging together; one specimen of a petiole attached to a stem sufficed to prove that all the scattered petioles of the same type belonged also to that kind of stem, and when leaves were found attached to an isolated fragment of the petiole, the chain of proof was complete that theleaves belonged to the stem, and so on. By a series of lengthy and painstaking investigations all the parts of the plant now calledLyginodendronhave been brought together, and the impressions of its leaves have been connected with it, these being of the fernlike type so long calledSphenopteris, illustrated infig. 77.