Fig. 77.—SphenopterisLeaf Impression, the fernlike foliage ofLyginodendron
Fig. 78A.—Diagram of the Transverse Section of Stem of theLyginodendron
p, Pith;P, primary wood groups;W, secondary wood;l.t, leaf trace;s, sclerized bands in the cortex;S, longitudinal view of wood elements to show the rows of bordered pits.
The anatomy of the main stem is very suggestive of that of a Cycad. The zones of secondary wood are loosely built, the quantity of soft tissue between the radiating bands of wood, and the size of the pith being large, while from the main axis double strands of wood run out to the leaf base. The primary bundles, however, are not like those of a Cycad stem, but have groups ofcentripetalwood within the protoxylem, and thus resemble the primary bundles ofPoroxylon(seep. 97), which are more primitive in this respect than those of the Cycads.
The roots ofLyginodendron, when young, were like those of the Marattiaceous ferns, their five-rayed mass of wood being characteristic of that family, and different from the type of root found in most other ferns (cf.fig. 78Bwithfig. 35onp. 60). Unlike fern roots of any kind, however, they have well-developed zones of secondary wood, in which they approach the Gymnospermic roots (seefig. 78B,s).
Fig. 78B.—Transverse Section of Root ofLyginodendron
w, Five-rayed mass of primary wood;s, zone of secondary wood;c, cortical and other soft tissues.
A further mixture of characters is seen in the vascular bundles of the petioles. A double strand, like that in the lower Gymnosperms, goes off to the leaf base from the main axis, but in the petiole itself the bundle is like a normal fern stele, and shows no characters in transverse section which would separate it from the ferns. Such a petiole is illustrated infig. 79, with itsV-shaped fernlike stele. On the petioles and stems were certain rough, spiny structures of the nature of complex hairs. In somecases they are glandular, as is seen inginfig. 79, and as they seem to be unique in their appearance they have been of great service in the identification of the various isolated organs of the plant.
As is seen fromfig. 77, the leaves were quite fern-like, but in structural specimens they have been found with the characteristic glandular hairs of the plant.
The seeds were so long known under the name ofLagenostomathat they are still called by it, though they have been identified as belonging toLyginodendron. They were small (about ¼ in. in maximum length) when compared with those of most other plants of the group, or of the Cycads, with which they show considerable affinity. They are too complex to describe fully, and have been mentioned already (seep. 76), so that they will not be described in much detail here. The diagrammatic figure (fig. 56) shows the essential characters of their longitudinal section, and their transverse section, as illustrated infig. 80, shows the complex and elaborate mechanism of the apex.
Fig. 79.—Transverse Section through Petiole ofLyginodendron
v, Fern-like stele;c, cortex;g, glandular hairlike protuberances.
Round the “seed” was a sheath, something like the husk round a hazel nut, which appears to have had the function of a protective organ, though what its real morphological nature may have been is as yet an unsolved problem. On the sheath were glandular hairslike those found on the petiole and leaves, which were, indeed, the first clues that led to the discovery of the connection between the seed and the plantLyginodendron.
The pollen grains seem to have been produced in sacs very like fern sporangia developed on normal foliage leaves, each grain entered the cavitypcin the seed (seefig. 56), but of the nature of the male cell we are ignorant. In none of the fossils has any embryo been found in the “seeds”, so that presumably they ripened, or at least matured their tissues, before fertilization.
These, in a few words, are the essentials of the structures ofLyginodendron. But this plant is only one of a group, and at least two other of the Pteridosperms deserve notice, viz.Medullosa, which is more complex, andHeterangium, which is simpler than the central type.
Fig. 80.—Diagram of Transverse Section ofLagenostomaSeed near the Apex, showing the nine flutingsfof the coatc;v, the vascular strand in each;nc, cone of nucellar tissue standing up in the fluted apex of the nucellusn;pc, the pollen chamber with a few pollen grains;s, space between nucellus and coat. Compare withdiagram 56.
Heterangiumis found also in rocks rather older than the coal series of England, though of Carboniferous age, viz. in the Calciferous sandstone series of Scotland, it occurs also in the ordinary Coal Measure nodules. It is in several respects more primitive thanLyginodendron, and in particular in the structure of its stele comes nearer to that of ferns. The stele is, in fact, a solid mass of primary wood and wood parenchyma, corresponding in some degree to the protostele of a simple type (seep. 61,fig. 36), but it has towards the outside groups of protoxylem surrounded by wood in both centripetal and centrifugal directions, which arejust like the primary bundles inLyginodendron. Outside the primary mass of wood is a zone of secondary wood, but the quantity is not large in proportion to it (seefig. 81), as is common in Lyginodendron.
Though the primary mass is so fernlike in appearance the larger tracheids show series of bordered pits, as do most of the tracheids of the Pteridosperms, in which they show a Gymnosperm-like character.
Fig. 81.—Heterangium
A, Half of the stele of a stem, showing the central mass of woodSmixed with parenchymap. The protoxylem groupsp. x.lie towards the outside of the stele. Surrounding it is the narrow zone of small-celled secondary woodW.B, A few of the wood cells in longitudinal view:p. x., Protoxylem;p, parenchyma.S, Large vessels with rows of bordered pits.
The foliage ofHeterangiumwas fernlike, with much-divided leaves similar to those ofLyginodendron. We have reason to suspect, though actual proof is wanting as yet, that small Gymnosperm-like seeds were borne directly on these leaves.
Medullosahas been mentioned already (seep. 72) because of the interesting and unusually complex type of its vascular anatomy. Each individual stele of the group of three in the stem, however, is essentially similar to the stele of a Heterangium.
Though the whole arrangement appears to differ so widely from other stems in the plant world, careful comparison with young stages of recent Cycads has indicated a possible remote connection with that group, while in the primary arrangements of the protosteles a likeness may be traced to the ferns. The roots, even in their primary tissues, were like those of Gymnosperms, but the foliage with its compound leaves was quite fern-like externally. A small part of a leaf is shown infig. 83, and is clearly like a fern in superficial appearance. The leaves were large, and the leaf bases strong and well supplied with very numerous branching vascular bundles.
Fig. 82.—Steles ofMedullosain Cross Section of the Stem
A, Primary solid wood;S, surrounding secondary wood.
Fig. 83.—Part of a Leaf ofMedullosa, known asAlethopteris, for long supposed to be a Fern
The connection between this plant and certain large three-ribbed seeds known asTrigonocarpusis strongly suspected, though actual continuity is not yet established in any of the specimens hitherto discovered. These seeds have been mentioned before (p. 76andp. 82). They were larger than the other fossil seeds which wehave mentioned, and, with their fleshy coat, were similar in general organization to the Cycads, though the fact that the seed coat stood free from the inner tissues right down to the base seems to mark them as being more primitive (cf.fig. 55,p. 76).
Of impressions of the Pteridosperms the most striking is, perhaps, the foliage known asNeuropteris(seefig. 6,p. 13), to which the large seeds are found actually attached (cf.fig. 85).
Fig. 84.—Diagrammatic Section of a Transverse Section of a Seed ofTrigonocarpus
S, Stone of coat with three main ridges and six minor ones.F, Flesh of coat:i f, inner flesh;n, nucellus, crushed and free from coat;s, spore wall.
Fig. 85.—Fragment of Foliage ofNeuropteriswith Seed attached, showing the manner in which the seeds grew on the normal foliage leaves in the Pteridosperms
Ever-increasing numbers of the “ferns” are being recognized as belonging to the Pteridosperms, butHeterangium,Lyginodendron, andMedullosaform the three principal genera, and are in themselves a series indicating the connection between the fernlike and Cycadean characters.
Before the fructifications were suspected of being seeds the anatomy of these plants was known, andtheir nature was partly recognized from it alone, though at that time they were supposed to have only fernlike spores.
The very numerous impressions of their fernlike foliage from the Palæozoic rocks indicate that the plants which bore such leaves must have existed at that time in great quantity. They must have been, in fact, one of the dominant types of the vegetation of the period. The recent discovery that so large a proportion of them were not ferns, but were seed-bearing plants, alters the long-established belief that the ferns reached their high-water mark of prosperity in the Coal Measure period. Indeed, the fossils of this age which remain undoubtedly true ferns are far from numerous. It is the seed-bearing Pteridosperms which had their day in Palæozoic times. Whether they led directly on to the Cycads is as yet uncertain, the probability being rather that they and the Cycads sprang from a common stock which had in some measure the tendencies of both groups.
That the Pteridosperms in themselves combined many of the most important features of both Ferns and Gymnosperms is illustrated in the account of them given above, which may be summarized as follows:—
Salient Characters of the PteridospermsG=GymnospermicF=Fernlike
FPrimary structure of root.GSecondary thickening of root.FInHeterangiumandMedullosatheFsolid centripetal primary wood of stele.GPits on tracheæ of primary wood.GSecondary thickening of stem.GDouble leaf trace.FFernlike stele in petiole.FFernlike leaves.FSporangia pollen-sac-like.FReproductive organs borne directly on ordinary foliage leaves.GGeneral organization of the seed.
FPrimary structure of root.
GSecondary thickening of root.
FInHeterangiumandMedullosathe
Fsolid centripetal primary wood of stele.
GPits on tracheæ of primary wood.
GSecondary thickening of stem.
GDouble leaf trace.
FFernlike stele in petiole.
FFernlike leaves.
FSporangia pollen-sac-like.
FReproductive organs borne directly on ordinary foliage leaves.
GGeneral organization of the seed.
Thus it can be seen at a glance, without entering into minutiæ, that the characters are divided between the two groups with approximate equality. The connection with Ferns is clear, and the connection with Gymnosperms is clear. The point which is not yet determined, and about which discussion will probably long rage, is the position of this group in the whole scheme of the plant world. Do they stand as a connecting link between the ferns on one hand and the whole train of higher plants on the other, or do they lead so far as the Cycads and there stop?
Unfortunately the records in the rocks do not go back so far as to touch what must have been the most interesting period in the history of the ferns, namely, the point where they diverged from some simple ancestral type, or at least were sufficiently primitive to give indications of their origin from some lower group.
Before the Devonian period all plant impressions are of little value, and by that early pre-Carboniferous time there are preserved complex leaves, which are to all appearance highly organized ferns.
To-day the dominant family in this group is thePolypodiaceæ. It includes nearly all our British ferns, and the majority of species for the whole world. This family does not appear to be very old, however, and it cannot be recognized with certainty beyond Mesozoic times.
From the later Mesozoic we have only material in the form of impressions, from which it is impossible todraw accurate conclusions unless the specimens have sporangia attached to them, and this is not often the case. The cuticle of the epidermis or the spores can sometimes be studied under the microscope after special treatment, but on the whole we have very little information about the later Mesozoic ferns.
A couple of specimens from the older Mesozoic have been recently described, with well-preserved structure, and they belong to the family of the Osmundas (the so-called “flowering ferns”, because of the appearance of special leaves on which all the sporangia are crowded), and show in the anatomical characters of their stems indications that they may be related to an old group, theBotryopterideæ, in which are the most important of the Palæozoic ferns.
In the Palæozoic rocks there are numerous impressions as well as fern petrifactions, but in the majority of cases the connection between the two is not yet established. There were two main series of ferns, which may be classed as belonging to
I. Marattiaceæ.II. Botryopterideæ.
I. Marattiaceæ.
II. Botryopterideæ.
Of these the former has still living representatives, though the group is small and unimportant compared with what it once was; the latter is entirely extinct, and is chiefly developed in the Carboniferous and succeeding Permian periods.
The latter group is also the more interesting, for its members show great variety, and series may be made of them which seem to indicate the course taken in the advance towards the Pteridosperm type. For this reason the group will be considered first, while the structure of the Pteridosperms is still fresh in our minds.
The Botryopterideæ formed an extensive and elaborate family, with its numerous members of differentdegrees of complexity. There is, unfortunately, but little known as to their external appearance, and almost no definite information about their foliage. They are principally known by the anatomy of their stems and petioles. Some of them had upright trunks like small tree ferns (living tree ferns belong to quite a different family, however), others appear to have had underground stems, and many were slender climbers.
Fig. 86.—Stele ofAsterochlaena, showing its deeply lobed nature
In their anatomy all the members of the family have monostelic structure (seep. 62). This is noteworthy, for at the present time though a number of genera are monostelic, no family whose members reach any considerable size or steady growth is exclusively monostelic. In the shape of the single stele, there is much variety in the different genera, some having it so deeply lobed that only a careful examination enables one to recognize its essentially monostelic nature. Infig. 86a radiating star-shaped type is illustrated. Between this elaborate type of protostele inAsterochlaena, and the simple solid circular mass seen inBotryopterisitself (fig. 88) are all possible gradations of structure.
Fig. 87.—The Stele of a Botryopteridean Stem, showing soft tissue in the centre of the solid wood of the protostele. (Microphoto.)
In several of the genera the centre of the wood is not entirely solid, but has cells of soft tissue, an incipient pith, mixed with scattered tracheids, as infig. 87.
In most of the genera numerous petioles are given off from the main axis, and these are often of a large size compared with it, and may sometimes be thickerthan the axis itself. Together with the petiole usually come off adventitious roots, as is seen infig. 88, which shows the main axis of aBotryopteris. The petioles of the group show much variety in their structure, and some are extremely complex. A few of the shapes assumed by the steles of the petioles are seen infig. 89; they are not divided into separate bundles in any of the known forms, as are many of the petiole steles of other families.
Fig. 88.—Main Axis ofBotryopteriswith simple solid Protostelex. A petiole about to detach itselfpand the strand going out to an adventitious rootrare also seen. (Micro-photograph.)
Fig. 89.—Diagrams showing the Shapes of the Steles in some of the Petioles of different Genera of Botryopterideæ
A,Zygopteris;B,Botryopteris;C,Tubicaulis;D,Asterochlaena.
In one genus of the family secondary wood has been observed. This is highly suggestive of the condition of the stele inHeterangium, where the large mass of the primary wood is surrounded by a relatively small quantity of secondary thickening, developed in normal radial rows from a cambium.
Another noteworthy point in the wood of these plants is the thickening of the walls of the wood cells. Many of them have several rows of bordered pits, and are, individually, practically indistinguishable from those ofthe Pteridosperms, cf.fig. 81andfig. 90. These are unlike the characteristic wood cells of modern ferns and of the other family of Palæozoic ferns.
The foliage of most members of the family is unknown, or at least, of the many impressions which possibly belong to the different genera, the most part have not yet been connected with their corresponding structural material. There are indications, however, that the leaves were large and complexly divided.
The fructifications were presumably fern sporangia of normal but rather massive type. Of most genera they are not known, though in a few they have been found in connection with recognizable parts of their tissue. The best known of the sporangia are large, in comparison with living sporangia (actually about 2.5 millimetres long), oval sacs clustered together on little pedicels. The spores within them seem in no way essentially different from normal fern spores.
The coexistence of the Botryopterideæ and Pteridosperms, and the several points in the structure of the former which seem to lead up to the characters of the latter group, are significant. The Botryopterideæ, even were they an entirely isolated group, would be interesting from the variety of structures and the variations of the monostele in their anatomy; and the prominent place they held in the Palæozoic flora, as the greatest family of ferns of that period, gives them an important position in fossil botany.
Fig. 90.—Tracheæ of Wood of Botryopteridean Fern in Longitudinal Section, showing the rows of pits on the walls. (Microphoto.)
The other family of importance in Palæozoic times,theMarattiaceæ, has descendants living at the present day, though the family is now represented by a small number of species belonging to but five genera which are confined to the tropics. Perhaps the best known of these is the giant “Elephant Fern”, which sends up from its underground stock huge complex fronds ten or a dozen feet high. Other species are of the more usual size and appearance of ferns, while some have sturdy trunks above-ground supporting a crown of leaves. The members of this family have a very complex anatomy, with several series of steles of large size and irregular shape. Their fructifications are characteristic, the sporangia being placed in groups of about five to a dozen, and fused together instead of ripening as separate sacs as in the other fern families.
Impressions of leaves with this type of sorus (group of fern sporangia) are found in the Mesozoic rocks, and these bridge over the interval between the living members of the family and those which lived in Palæozoic times.
In the Coal Measure and Permian periods these plants flourished greatly, and there are remains of very numerous species from that time. The family was much more extensive then than it is now, and the individual members also seem to have reached much greater dimensions, for many of them had the habit of large tree ferns with massive trunks. Up till Triassic times half of the ferns appear to have belonged to this family; since then, however, they seem to have dwindled gradually down to the few genera now existing.
On the Continent fossils of this type with well-preserved structure have long been known to the general public, as their anatomy gave the stones a very beautiful appearance when polished, so that they were used for decorative purposes by lapidaries before their scientific interest was recognized.
The members of the Palæozoic Marattiaceæ which have structure preserved generally go by the genericnamePsaronius, in which there is a great number of species. They show considerable uniformity in their essential structure (in which they differ noticeably from the group of ferns just described), so that but one type will be considered.
In external appearance they probably resembled the “tree ferns” of the present day (though these belong to an entirely different family), with massive stumps, some of which reached a height of 60 ft. The large spreading leaves were arranged in various ways on the stem, some in a double row along it, as is seen by the impressions of the leaf scars, and others in complex spirals. On the leaves were the spore sacs, which were in groups, some completely fused like those of the modern members of the family, and others with independent sporangia massed in well-defined groups. In their microscopic structure also they appear to have been closely similar to those of the living Marattiaceæ.
The transverse section of a stem shows the most characteristic and best-known view of the plant. This is shown infig. 91, in somewhat diagrammatic form.
The mass of rootlets which entirely permeate and surround the outer tissues of the stem is a very striking and characteristic feature of all the species ofPsaronius. Though such a mass of roots is not found in the living species, yet the microscopic structure of an individual fossil root is almost identical with that of a livingMarattia.
Though these plants were so successful and so important in Palæozoic times, the group even then seems to have possessed little variety and little potentiality for advance in new directions. They stand apart from the other fossils, and the few forms which now compose the living Marattiaceæ are isolated from the present successful types of modern ferns. From thePsaronieæwe can trace no development towards a modern series of plants, no connection with another important group in the past. They appear to have culminated in the later Palæozoic and to have slowly dwindled ever since. It has beensuggested that the male fructifications of the Bennettiteæ and the Pteridosperms show some likeness to the Marattiaceæ, but there does not seem much to support any view of phylogenetic connection between them.
Fig. 91.—Transverse Section of Stem ofPsaronius
v, Numerous irregularly-shaped steles;s, irregular patches of sclerenchyma;l, leaf trace going out as a horseshoe-shaped stele;c, zone of cortex with numerous adventitious rootsrrunning through it;sc, sclerized cortical zone of roots;w, vascular strand of roots.
Before leaving the palæozoic ferns, mention should be made of the very numerous leaf impressions which seem to show true fern characters, though they have not been connected with material showing their internal structure. Among them it is rare to get impressions with thesorior sporangia, but such are known and are in themselves enough to prove the contention that true ferns existed in the Palæozoic epoch. For it might be mentioned as a scientific curiosity, that after the discovery that so many of the leaf impressions which had always been supposed to be ferns, really belonged tothe seed-bearing Pteridosperms, there was a period of panic among some botanists, who brought forward the startling idea that there werenoferns at all in the Palæozoic periods, and that modern ferns were degenerated seed-bearing plants!
Fig. 92.—Impression of Palæozoic Fern, showingsorion the pinnules. (Photo.)
These two big groups from the Palæozoic include practically all the ferns that then flourished. They have been spoken of (together with a few other types of which little is known) as thePrimofilices, a name which emphasizes their primitive characters. As can be seen by the complex organization of the genera, however, they themselves had advanced far beyond their really primitive ancestors. There is clear indication that the Botryopterideæ were in a period of change, what might almost be termed a condition of flux, and that from their central types various families separated and specialized. Behind the Botryopterideæ, however, we have no specimens to show us the connection between them and the simpler groups from which they must have sprung. From a detailed comparative study of plant anatomy we can deduce some of the essential characters of such ancestral plants, but here the realm of fossil botany ceases, to give place to theoretical speculation. As a fact, there is a deep abyss between the ferns and the other families of the Pteridophytes,which is not yet bridged firmly enough for any but specialists, used to the hazardous footing on such structures, to attempt to cross it. Until the buttresses and pillars of the bridge are built of the strong stone of fossil structures we must beware of setting out on what would prove a perilous journey.
In the Coal Measures and previous periods we see the ferns already represented by two large families, differing greatly from each other, and from the main families of modern ferns which sprung at a later date from some stock which we have not yet recognized. But though their past is so obscure, the palæozoic ferns and their allies throw a brilliant light on the course of evolution of the higher groups of plants, and the gulf between ferns and seed-bearing types may be said to be securely bridged by the Botryopterideæ and the Pteridosperms.
The present-day members of this family are not at all impressive, and in their lowliness may well be overlooked by one who is not interested in unpretending plants. The fresh green mosslikeSelaginellagrown by florists as ornamental borders in greenhouses and the creeping “club moss” twining among the heather on a Highland moor are probably the best known of the living representatives of the Lycopods. In the past the group held a very different position, and in the distant era of the Coal Measures it held a dominant one. Many of the giants of the forest belonged to the family (seefrontispiece), and the number of species it contained was very great.
Let us turn at once to this halcyon period of the group. The history of the times intervening between it and the present is but the tale of the dying out of the large species, and the gradual shrinking of the family and dwarfing of its representative genera.
It is difficult to give the characters of a scientific family in a few simple words; but perhaps we may describe the living Lycopods as plants with creeping stems which divide and subdivide into two with great regularity, and which bear large numbers of very small pointed leaves closely arranged round the stem. The fruiting organs come at the tips of the branches, and sometimes themselves divide into two, and in these cone-like axes the spore cases are arranged, a single one on the upper side of each of the scales (seep. 67,fig. 46,A). In the Lycopods the spores are all alike, in the Selaginellas there are larger spores borne in a small number (four) in some sporangia (seefig. 53,p. 75), and others in large numbers and of smaller size on the scales above them. The stems are all very slender, and have no zones of secondary wood. They generally creep or climb, and from them are put out long structures something like roots in appearance, which are specially modified stem-like organs giving rise to roots.
From the fossils of the Coal MeasuresLepidodendronmust be chosen as the example for comparison. The different species of this genus are very numerous, and the various fossilized remains of it are among the commonest and best known of palæontological specimens. The huge stems are objects of public interest, and have been preserved in the Victoria Park in Glasgow in their original position in the rocks, apparently as they grew with their spreading rootlike organs running horizontally. A great stump is also preserved in the Manchester Museum, and is figured in thefrontispiece. While among the casts and impressions the leaf bases of the plant are among the best preserved and the most beautiful (seefig. 93). The cone has already been illustrated(seefig. 46andfig. 9), and is one of the best known of fossil fructifications.
Fig. 93.—Photo of Leaf Bases ofLepidodendron
C, Scar of leaf;S, leaf base. In the scar:v, mark of severed vascular bundle, andp, of parichnos.l, Ligule scar.
From the abundant, though scattered material, fossil botanists have reconstructed the plants in all their detail. The trunks were lofty and of great thickness, bearing towards the apex a much-branched crown, the branches, even down to the finest twigs, all dividing into two equal parts. The leaves, as would be expected from the great size of the plants, were much bigger than those of the recent species (fig. 93shows the actual size of the leaf bases), but they were of the samerelativelysmall size as compared with the stems, and of the same simplepointed shape. A transverse section across the apex of a fertile branch shows these closely packed leaves arranged in series round the axis, those towards the outside show the central vascular strand which runs through each.
Fig. 94.—Section across an Axis surrounded by many Leaves, which shows their simple shape and single central vascular bundlev
The markings left on the well-preserved leaf-scars indicate the main features of the internal anatomy of the leaves. They had a single central vascular strand (v,fig. 93), on either side of which ran a strand of soft tissuepcalled the parichnos, which is characteristic of the plants of this group. While another similarly obscure structure associated with the leaf is the little scale-like ligulelon its upper surface.
The anatomy of the stems is interesting, for in thedifferent species different stages of advance are to be found, from the simple solid protostele with a uniform mass of wood to hollow ring steles with a pith. An interesting intermediate stage between these two is found inLepidodendron selaginoides(seefig. 95), where the central cells of the wood are not true water-conducting cells, but short irregular water-storage tracheides (seep. 56), which are mixed with parenchyma. All the genera of these fossils have a single central stele, round which it is usual to find a zone of secondary wood of greater or less extent according to the age of the plant.
Fig. 95.—Transverse Section ofLepidodendron selaginoides, showing the circular mass of primary wood, the central cells of which are irregular water-storage tracheides
s, Zone of secondary wood;c, inner cortical tissues;r, intrusive burrowing rootlet;oc, outer cortical tissues with corky external layersk. (Microphoto.)
Some stems instead of this compact central stele have a ring of wood with an extensive pith. Such a type is illustrated infig. 96, which shows but a part of the circle of wood, and the zone of the secondary wood outside it,which greatly exceeds the primary mass in thickness. This zone of secondary wood became very extensive in old stems, for, as will be imagined, the primary wood was not sufficient to supply the large trunks. The method of its development from a normal cambium in radiating rows of uniform tracheides is quite similar to that which is found in the pines to-day. This is the most important difference between the living and the fossil stems of the family, for no living plants of the family have such secondary wood. On the other hand, the individual elements of this wood are different from those of the higher families hitherto considered, and have narrow slit-like pits separated by bands of thickening on the longitudinal walls. Such tracheides are found commonly in the Pteridophytes, both living and fossil. Their type is seen infig. 96,B, which should be compared with that in figs.78,Aand62,Bto see the contrast with the higher groups.
Fig. 96.—A,LepidodendronStem with Hollow Ring of WoodWand Zone of Secondary WoodS.B, Longitudinal View of the Narrow Pits of the Wood Elements.
To supply the vascular tissues of the leaf traces, simple strands come off from the outer part of the primary wood, where groups of small-celled protoxylem project (seepxinfig. 97). The leaf strandsltmove out through the cortex in considerable numbers to supply the many leaves, into each of which a single one enters.
Fig. 97.—Transverse Section of Outer Part of Primary Wood ofLepidodendron, showingpx, projecting protoxylem groups;lt, leaf trace coming from the stele and passing (aslt1) through the cortex
As regards the fructifications ofLepidodendronmuch could be said were there space. The many genera ofLepidodendronbore several distinct types of cones of different degrees of complexity. In several of the genera the cones were simple in organization, directly comparable with those of the living Lycopods, though on a much larger scale (seep. 67). In some the spores were uniform, all developing equally in numerous tetrads. The sporophyll was radially extended, and along it thelarge sausage-shaped sporangia were attached (seefig. 98). The tips of the sporophylls overlapped and afforded protection to the sporangia. The axis of the cone had a central stele with wood elements like those in the stem. The appearance of a transverse section of an actual cone is shown infig. 99. Here the sporangia are irregular in shape, owing to their contraction after ripeness and during fossilization. Other cones had sporangia similar in size and shape, but which produced spores of two kinds, large ones resulting from the ripening of only two or three tetrads in the lower sporangia, and numerous small ones in the sporangia above.