Fig. 47.—Germinating Fern Spores
AandB, from carboniferous fossils;C, living fern. (AandBafter Scott.)
While these few cases illustrate points of likeness between the fructifications of the Coal Measures and of to-day, the large size and successful character of the primitive Coal Measure plants was accompanied by many developments on the part of their reproductive organs which are no longer seen in living forms, and the greaternumber of palæozoic fructifications must be considered in the next chapter.
We have seen in the last chapter that the main morphological divisions, roots, stems, leaves, and fructifications, were as distinct in the Coal Measure period as they are now. There is one structure, however, found in the Coal Measure fossils, which is hardly paralleled by anything similar in the living plants, and that is the fossil known asStigmaria.Stigmariais the name given, not to a distinct species of plant, but to the large rootlike organs which we know to have belonged to all the species ofLepidodendronand ofSigillaria. In thefrontispiecethese organs are well seen, and branch away at the foot of the trunk, spreading horizontally, to all appearance merely large roots. They are especially regularly developed, however, the main trunk giving rise always to four primary branches, these each dividing into two equal branches, and so on—in this they are unlike the usual roots of trees. They bore numerous rootlets, of which we know the structure very well, as they are the commonest of all fossils, but in their internal anatomy the main “roots” had not the structure which is characteristic of roots, but were likestems. In living plants there are many examples of stems which run underground, but they always have at least the rudiments of leaves in the form of scales, while the fossil structures have apparently no trace of even the smallest scales, but bear only rootlets, thus resembling true roots. The questions of morphology these structures raise are too complex to be discussed here, andStigmaria is only introduced as an example, one of the very few available, of a palæozoic structure which seems to be of a nature not clearly determinable as either root, stem, leaf, or fructification. Among living plants the fine rootlike rhizophores of Selaginella bear some resemblance to Stigmaria in essentials, though so widely different from them in many ways, and they are probably the closest analogy to be found among the plants of to-day.
The individual cells, we have already seen, are strikingly similar in the case of fossil and living plants. There are, of course, specific varieties peculiar to the fossils, of which perhaps the most striking seem to be some forms ofhaircells. For example, in a species of fern from the French rocks there were multicellular hairs which looked like little stems of Equisetum owing to regular bands of teeth at the junctions of the cells. These hairs were quite characteristic of the species—but hairs of all sorts have always abounded in variety, so that such distinction has but minor significance.
Fig. 48.—Stele ofLepidodendronW, surrounded by a small ring of secondary woodS
As was noted in the table (p. 58) the only cell types of prime importance which were not evolved by the Palæozoic plants were the wood vessels, phloem and accompanying cells which are characteristic of the flowering plants.
Among the fossils the vascular arrangements are most interesting, and, as well as all the types of stele development noted in the previous chapter as common to both living and fossil plants, there are further varieties found only among the fossils (seefig. 50).
The simple protostele described (onp. 61) is still found, particularly in the very young stages of livingferns, but it is a type of vascular arrangement which is not common in the mature plants of the present day. In the Coal Measure period, however, the protostele was characteristic of one of the two main groups of ferns. In different species of these ferns, the protostele assumed a large variety of shapes and forms as well as the simple cylindrical type. The central mass of wood became five-rayed in some, star-shaped, and even very deeply lobed, with slightly irregular arms, but in all these cases it remained fundamentally monostelic. Frequently secondary tissue developed round the protosteles of plants whose living relatives have no such tissue. A case of this kind is illustrated infig. 48, which shows a simple circular stele surrounded by a zone of secondary woody tissue in a species ofLepidodendron.
Fig. 49.—Lepidodendron, showing Part of the Hollow Ring of Primary WoodW, with a relatively large amount of Secondary TissueS, surrounding it
In many species ofLepidodendronthe quantity ofsecondary wood formed round the primary stele was very great, so that (as is the case in higher plants) the primary wood became relatively insignificant compared with it. In most species ofLepidodendronthe primary stele is a hollow ring of wood (cf.fig. 38,p. 62) round which the secondary wood developed, as is seen infig. 49. These two cases illustrate a peculiarity of fossil plants. Among living ones the solid and the simple ring stele are almost confined to the Pteridophytes, where secondary wood does not develop, but the palæozoic Pteridophytes, while having the simple primary types of steles, had quantities of secondary tissue, which was correlated with their large size and dominant position.
Fig. 50.—Diagram of Steles of the EnglishMedullosa, showing three irregular, solid, stelesA, with secondary thickeningsS, all round each.a, Small accessory steles
Amongpolystelictypes (seep. 63) we find interesting examples in the fossil group of theMedulloseæ, which are much more complex than any known at present, both owing to their primary structure and also to the peculiar fact that all the steles developed secondary tissue towards the inner as well as the outer side. One of the simpler members of this family found in the English Coal Measures is illustrated infig. 50. Here there are three principal protosteles (and several irregular minor ones) each of which has a considerable quantity of secondary tissue all round it, so that a portion of the secondary wood is growing in towards the actual centre of the stem as a whole—a very anomalous state of affairs.
In the more complex Continental type ofMedullosathere areverylarge numbers of steles. In the one figured from the Continent infig. 51but a few are represented. There is a large outer double-ring stele, withsecondary wood on both sides of it, and within these a number of small steles, all scattered through the ground tissue, and each surrounded by secondary wood. In actual specimens the number of these central steles is much greater than that indicated in the diagram.
No plant exists to-day which has such an arrangement of its vascular cylinder. It almost appears as though at the early period, when the Medulloseæ flourished, steles were experimenting in various directions. Such types as are illustrated in figs.50and51are obviously wasteful (for secondary wood developing towards the centre of a stem is bound to finally meet), and complex, but apparently inefficient, which may partly account for the fact that this type of structure has not survived to the present, though simpler and equally ancient types have done so.
Fig. 51.—ContinentalMedullosa, showingR, outer double-ring stele with secondary wood all round it;S, inner stellate steles, also surrounded in each case by secondary tissue
Further details of the anatomy of fossils will be mentioned when we come to consider the individual families; those now illustrated suffice to show that in the Coal Measures very different arrangements of steles were to be found, as well as those which were similar to those existing now. The significance of these differences will become apparent when their relation to the other characters of the plants is considered.
The fructifications, always the most important parts of the plant, offer a wide field, and the divergence between the commoner palæozoic and recent types seems at first to be very great. Indeed, when palæozoic reproductive bodies have to be described, it is often necessary to use the common descriptive terms in an altered and wider sense.
Among the plants of to-day there are many varieties of the simple single-celled reproductive masses which are calledspores, and which are usually formed in large numbers inside a spore case or sporangium. Among the higher plantsseedsare also known in endless variety, all of which, compared with spores, are very complex, for they are many-celled structures, consisting essentially of an embryo or young plant enclosed in various protective coats. The distinction between the two is sharp and well defined, and for the student of living plants there exists no difficulty in separating and describing seeds and spores.
But when we look back through the past eras to palæozoic plants the subject is not so easy, and the two main types of potentially reproductive masses are not sharply distinct. The seed, as we know it among recent plants, and as it is generally defined, had not fully evolved; while the spores were of great variety and had evolved in several directions, some of which seem to have been intermediate stages between simple spores and true seeds. These seedlike spores served to reproduce the plants of the period, but their type has since died out and left but two main methods among living plants, namely the essentially simple spores, the very simplicity of whose organization gives them a secure position, and the complex seeds with their infinite variety of methods for protecting and scattering the young embryos they contain.
Among the Coal Measure fossils we can pick up some of the early stages in the evolution of the seed from the spore, or at least we can examine intermediate stages between them which give some idea of the possible course of events. Hence, though the differences from our modern reproductive structures are so noticeable a feature of the palæozoic ones, it will be seen that they are really such differences as exist between the members at the two ends of a series, not such as exist between unrelated objects.
Very few types can be mentioned here, and to make their relations clear a short series of diagrams with explanations will be found more helpful than a detailed account of the structures.
Fig. 52.—Spores
Each spore a single cell which develops with three others in tetrads (groups of four). Very numerous tetrads enclosed in a spore case or sporangium which develops on a leaflike segment called the sporophyll. Each spore germinates independently of the others after being scattered, all being of the same size. Common in fossils and living Pteridophytes.
Fig. 53.—Spores
Each a single cell like the preceding, but here only one tetrad in a sporangium ripens, so that each contains only four spores. Compared with the preceding types these spores are very large. Otherwise details similar to above. Some fossils have such sporangia with eight spores, or some other small number; living Selaginellas have four. In the same cone sporangia with small spores are developed and give rise to the male organs.
Fig. 54.—“Spores” of Seedlike Structure
Out of a tetrad in each sporangium only one spore ripens,Sin figure, the others,s, abort. The wall of the sporangium,w, is more massive than in the preceding cases, and from the sporophyll, flaps,sp f, grow up on each side and enclose and protect the sporangium. The one big spore appears to germinate inside these protective coats, and not to be scattered separately from them. Only found in fossils, one of the methods of reproduction inLepidodendron. Other sporangia with small spores were developed which gave rise to the male organs.
Fig. 55.—“Seed”
In appearance this is like a seed, but differs from a true seed in having no embryo, and is like the preceding structure in having a very large spore,S, though there is no trace of the three aborting ones. The spore develops in a special mass of tissue known as the nucellus,n, which partly corresponds to the sporangium wall of the previous types. In it a cavity,p c, the pollen chamber, receives the pollen grains which enter at the apex of the “seed”. There is a complex coat,C, which stands round the nucellus but is not joined to it, leaving the spacelbetween them. Only in fossils;Trigonocarpus(seep. 122) is similarly organized. Small spores in fern-like sporangia, called pollen grains.
Fig. 56.—“Seed”
Very similarly organized to the above, but the coat is joined to the nucellus about two-thirds of its extent, and up to the levell. In the pollen chamber,p c, a cone of nucellar tissue projects, and the upper part of the coat is fluted, but these complexities are not of primary importance. The large sporeSgerminated and was fertilized within the “seed”, but apparently produced no embryo before it ripened. Small “spores” in fern-like sporangia form the pollen grains. Only in fossils,e.g.Lagenostoma. (Seep. 119.)
Fig. 57.—Seed
Essentially similar to the preceding, except in the possession of an embryoe, which is, however, small in comparison with the endosperm which fills the sporeS. The whole organization is simpler than in the fossilLagenostoma, but the coat is fused to the nucellus further up (seel). Small “spores” form the pollen grains. Living and fossil type, Cycads and Ginkgo.
Fig. 58.—Seed
In the ripe seed the large embryoepractically fills up all the space within the two seed coatsc1andc2; endosperm, pollen chamber, &c., have been eliminated, and the young ovule is very simple and small as a result of the protection and active service of the carpels in which it is enclosed. Small “spores” form the pollen grains. Typical of living Dicotyledons.
These few illustrations represent only the main divisions of an army of structures with an almost unimaginable wealth of variety which must be left out of consideration.
For the structures illustrated in figs.54,55, and56we have no name, for their possible existence was not conceived of when our terminology was invented, and no one has yet christened them anew with distinct names. They are evidently too complex in organization and too similar to seeds in several ways to be called spores, yet they lack the essential element in a seed, namely, an embryo. The term “ovule” (usually given to the young seed which has not yet developed an embryo) does not fit them any better, for their tissues are ripened and hard, and they were of large size and apparently fully grown and mature.
For the present a name is not essential; the one thing that is important is to recognize their intermediate character and the light they throw on the possible evolution of modern seeds.
A further point of great interest is the manner in which these “seeds” were borne on the plant. To-day seeds are always developed (with the exception of Cycas) in cones or flowers, or at least special inflorescences. But the “seed” ofLagenostoma(fig. 56), as well as a number of others in the group it represents, were not borne on a special structure, but directly on the greenfoliage leaves. They were in this on a level with the simple sporangia of ferns which appear on the backs of the fronds, a fact which is of great significance both for our views on the evolution of seeds as such, and for the bearing it has on the relationships of the various groups of allied plants. This will be referred to subsequently (Chapter XI), and is mentioned now only as an example of the difference between some of the characters of early fossils and those of the present day.
It is true that botanists have long recognized the organ which bears seeds as a modified leaf. The carpels of all the higher plants are looked on ashomologouswith leaves, although they do not appear to be like them externally. Sometimes among living plants curious diseases cause the carpels to become foliar, and when this happens the diseased carpel reverts more or less to the supposed ancestral leaf-like condition. It is only among the ancient (but recently discovered) fossils, however, that seeds are known to be borne normally on foliage leaves.
From Mesozoic plants we shall learn new conceptions about flowers and reproductive inflorescences in general, but these must be deferred to the consideration of the family as a whole (Chapter XIII).
Enough has been illustrated to show that though the individual cells, the bricks, so to speak, of plant construction, were so similar in the past and present, yet the organs built up by them have been continually varying, as a child builds increasingly ambitious palaces with the same set of bricks.
In comparison with the other groups of plants the flowering families are of recent origin, yet in the sense in which the word is usually used they are ancient indeed, and the earliest records of them must date at least to periods hundreds of thousands of years ago.
Through all the Tertiary period (seep. 34) there were numerous flowering plants, and there is evidence that many families of both Monocotyledons and Dicotyledons existed in the Upper Cretaceous times. Further back than this we have little reliable testimony, for the few specimens of so-called flowering plants from the Lower Mesozoic are for the most part of a doubtful nature.
The flowering plants seem to stand much isolated from the rest of the plant world; there is nodirectevidence of connection between their oldest representatives and any of the more primitive families. So far as our actual knowledge goes, they might have sprung into being at the middle of the Mesozoic period quite independently of the other plants then living; though there are not wanting elaborate and almost convincing theories of their connection with more than one group of their predecessors (seep. 108).
It is a peculiarly unfortunate fact that although the rocks of the Cretaceous and Tertiary are so much less ancient than those of the Coal Measures, they have preserved for us far less well the plants which were living when they were formed. Hitherto no one has found in Mesozoic strata masses of exquisitely mineralized Angiosperm fragments[8]like those found in the Coal Measures,which tell us so much about the more ancient plants. Cases are known of more or less isolated fragments with their microscopical tissues mineralized. For example, there are some palms and ferns from South America which show their anatomical structure very clearly preserved in silica, and which seem to resemble closely the living species of their genera. The bulk of the plants preserved from these periods are found in the form of casts or impressions (seep. 10), which, as has been pointed out already, are much less satisfactory to deal with, and give much less reliable results than specimens which have also their internal structure petrified. The quantity of material, however, is great, and impressions of single leaves innumerable, and of specimens of leaves attached to stems, and even of flowers and fruits, are to be found in the later beds of rock. These are generally clearly recognizable as belonging to one or other of the living families of flowering plants. Leaf impressions are by far the most frequent, and our knowledge of the Tertiary flora is principally derived from a study of them. Their outline and their veins are generally preserved, often also their petioles and some indication of the thickness and character of the fleshy part of the leaf. From the outline and veins alone an expert is generally able to determine the species to which the plant belongs, though it is not always quite safe to trust to these determinations or to draw wide-reaching conclusions from them.
Infig. 59is shown a photograph of the impression of a Tertiary leaf, which illustrates the condition of an average good specimen from rocks of the period. Its shape and the character of the veins are sufficient to mark it out immediately as belonging to the Dicotyledonous group of the flowering plants.
Seeds and fruits are also to be found; and in some very finely preserved specimens from Japan stamens from a flower and delicate seeds are seen clearly impressed on the light stone. Infig. 60is illustrated acouple of such seeds, which show not only their wings but also the small antennæ-like stigmas. Specimens so perfectly preserved are practically as good as herbarium material of recent plants, and in this way the externals of the Tertiary plants are pretty well known to us.
Fig. 59.—Dicotyledonous Leaf Impression from Tertiary Rocks
Fig. 60.—Seeds from Japanese Tertiary Rocks; ataare seen the two stigmas still preserved
A problem which has long been discussed, and which has aroused much interest, is the relative antiquity of the Monocotyledonous and the Dicotyledonous branches of the flowering plants. A peculiar fascination seems to hang over this still unsolved riddle, and a battle of flowers may be said to rage between thelily and the rose for priority. Recent work has thrown no decisive light on the question, but it has undoubtedly demolished the old view which supposed that the Monocotyledons (the lily group) appeared at a far earlier date upon this earth than the Dicotyledons. The old writers based their contention on incorrectly determined fossils. For instance, seeds from the Palæozoic rocks were described as Monocotyledons because of the three or six ribs which were so characteristic of their shell; we know now that these seeds (Trigonocarpus) belong to a family already mentioned in another connection (p. 72), the Medulloseæ (seep. 122), the affinity of which lies between the cycads and the ferns. Leaves ofCordaites, again, which are broad and long with well-marked parallel veins, were described as those of a Monocotyledonous plant like the Yucca of to-day; but we now know them to belong to a family of true Gymnosperms possibly distantly related toTaxus(the Yew tree).
Recent work, which has carefully sifted the fossil evidence, can only say that no true Monocotyledons have yet been found below the Lower Cretaceous rocks, and that at that period we see also the sudden inrush of Dicotyledons. Hence, so far as palæontology can show, the two parallel groups of the flowering plants arose about the same time. It is of interest to note, however, that the only petrifaction of a flower known from any part of the world is an ovary which seems to be that of one of the Liliaceæ. In the same nodules, however, there are several specimens of Dicotyledonous woods, so that it does not throw any light on the question of priority.
With the evidence derived from the comparative study of the anatomy of recent flowering plants we cannot concern ourselves here, beyond noting that the results weigh in favour of the Dicotyledons as being the more primitive, though not necessarily developed much earlier in point of time. Until very much more is discovered than is yet known of the origin of theflowering plants as a whole, it is impossible to come to a more definite conclusion about this much-discussed subject.
Let us now attempt to picture the vegetable communities since the appearance of the flowering plants. The facts which form the bases of the following conceptions have been gathered from many lands by numerous workers in the field of fossil botany, from scattered plant remains such as have been described.
When the flowering plants were heralded in they appeared in large numbers, and already by the Cretaceous period there were very many different species. Of these a number seem to belong to genera which are still living, and many of them are extremely like living species. It would be wearisome and of little value to give a list of all the recorded species from this period, but a few of the commoner ones may be mentioned to illustrate the nature of the plants then flourishing.
Several species ofQuercus(the Oak) appeared early, particularlyQuercus Ilex; leaves of theJuglandaceæ(Walnut family) were very common, and among the Tertiary fossils appear its fruits. BothPopulus(the Poplar) andSalix(the Willow) date from the early rocks, whileFicus(the Fig) was very common, andCasuarina(the Switch Plant) seems to have been widely spread. Magnolias also were common, and it appears thatPlatanus(the Plane) andEucalyptuscoexisted with them.
It will be immediately recognized that the above plants have all living representatives, either wild or cultivated, growing in this country at the present day, so that they are more or less familiar objects, and there appears to have been no striking difference between the early flowering plants and those of the present day. Between the ancient Lycopods, for example, and those now living the differences are very noteworthy; but the earliest of the known flowering plants seem to have been essentially like those now flourishing. It must be rememberedin this connection that the existing flowering plants are immensely nearer in point of time to their origin than are the existing Lycopods, and that when such æons have passed as divide the present from the Palæozoic, the flowering plants of the future may have dwindled to a subordinate position corresponding to that held by the Lycopods now.
A noticeable character of the early flowering-plant flora, when taken as a whole, is the relatively large proportion of plants in it which belong to the familyAmentiferæ(oaks, willows, poplars, &c.). This is supposed by some to indicate that the family is one of the most primitive stocks of the Angiosperms. This view, however, hardly bears very close scrutiny, because it derives its main support from the large numbers of the Amentiferæ as compared with other groups. Now, the Amentiferæ were (and are) largely woody resistant plants, whose very nature would render them more liable to be preserved as impressions than delicate trees or herbs, which would more readily decay and leave no trace. Similarly based on uncertain evidence is the surmise that the group of flowers classed asGamopetalæ(flowers with petals joined up in a tube, like convolvulus) did not flourish in early times, but are the higher and later development of the flower type. Now,Viburnum(allied to the honeysuckle) belongs to this group, and it is found right down in the Cretaceous, andSambucus(Elder, of the same family) is known in the early Tertiary. These two plants are woody shrubs or small trees, while many others of the family are herbs, and it is noteworthy that it is just these woody, resistant forms which are preserved as fossils; their presence demonstrates the antiquity of the group as a whole, and the absence of other members of it may be reasonably attributed to accidents of preservation. In the Tertiary also we get a member of the heath family, viz.Andromeda, and another tube-flower,Bignonia, as well as several morewoodygamopetalous flowers.
Hence it is wise to be very cautious about drawing any important conclusions from the relative numbers of the different species, or the absence of any type of plant from the lists of those as yet known from the Cretaceous. When quantities of structurally preserved material can be examined containing the flowering plants in petrifactions, then it will be possible to speak with some security of the nature of the Mesozoic flora as a whole.
The positive evidence which is already accumulated, however, is of great value, and from it certain deductions may be safely made. Specimens of Cretaceous plants from various parts of the world seem to indicate that there was a very striking uniformity in the flora of that period all over the globe. In America and in Central Europe, for example, the same types of plants were growing. We shall see that, as time advanced, the various types became separated out, dying away in different places, until each great continent and division of land had a special set of plants of its own. At the commencement of the reign of flowering plants, however, they seem to have lived together in the way we are told the beasts first lived in the garden of Eden.
At the beginning of the Tertiary period there were still many tropical forms, such as Palms, Cycads,Nipa, variousArtocarpaceæ,Lauraceæ,Araliaceæ, and others, growing side by side with such temperate forms asQuercus,Alnus,Betula,Populus,Viburnum, and others of the same kind. Before the middle of the Tertiary was reached the last Cycads died in what is now known as Europe; and soon after the middle Tertiary all the tropical types died out of this zone.
At the same time those plants whose leaves appear to have fallen at the end of the warm season began to become common, which is taken as an indication of a climatic influence at work. Some writers consider that in the Cretaceous times there was no cold season, and therefore no regular period of leaf fall, but as the climate became temperate the deciduous trees increasedin numbers; yet the Gymnospermic and Angiospermic woods which are found with petrified structure show well-marked annual rings and seem to contradict this view.
Toward the end of the Tertiary times there were practically no more tropical forms in the European flora, though there still remained a number of plants which are now found either only in America or only in Asia.
The Glacial epoch at the close of the Tertiary appears to have driven all the plants before it, and afterwards, when its glaciers retreated, shrinking up to the North and up the sides of the high mountains, the plant species that returned to take possession of the land in the Quaternary or present period were those which are still inhabiting it, and the floras of the tropics, Asia, and America were no longer mixed with that of Europe.[9]
The more recent history of the higher Gymnosperms, in the Upper Cretaceous and Tertiary periods, much resembles that of the flowering plants as sketched in the previous chapter. Many of the genera appear to have been those still living, and some of the species even may have come very close to or have been identical with those of to-day. The forms now characteristic of the different continents were growing together, and appear to have been widely distributed over the globe. For example,SequoiaandTaxodium, two types now characteristic of America, andGlyptostrobus, at present foundin Asia, were still growing with the other European types in Europe so late as middle Tertiary times.
As in the case of the Angiosperms, the fossils we have of Cretaceous and Tertiary Gymnosperms are nearly all impressions and casts, though some more or less isolated stems have their structure preserved. Hence our knowledge of these later Gymnosperms is far from complete. From the older rocks, however, we have both impressions and microscopically preserved material, and are more fully acquainted with them than with those which lived nearer our own time. Hard, resistant leaves, which are so characteristic of most of the living genera of Gymnosperms, seem to have been also developed in the past members of the group, and these tend to leave clear impressions in the rocks, so that we have reliable data for reconstructing the external appearance of the fossil forms from the Palæozoic period.
The resinous character of Gymnosperm wood probably greatly assisted its preservation, and fragments of it are very common in rocks of all ages, generally preserved in silica so as to show microscopic structure. The isolated wood of Gymnosperms, however, is not very instructive, for from the wood alone (and usually it is just fragments of the secondary wood which are preserved) but little of either physiological or evolutional value can be learned. When twigs with primary tissues and bark and leaves attached are preserved, then the specimens are of importance, for their true character can be recognized. Fortunately among the coal balls there are many such fragments, some of which are accompanied by fruits and male cones, so that we know much of the Palæozoic Gymnosperms, and find that in some respects they differ widely from those now living.
There is, therefore, much more to be said about the fossil Gymnosperms than about the Angiosperms, both because of the better quality of their preservation and because their history dates back to a very much earlier period than does the Angiospermic record. Indeed, wedo not know when the Gymnosperms began; the well-developed and ancient group ofCordaiteæwas flourishing before the Carboniferous period, and must therefore date back to the rocks of which we have no reliable information from this point of view, and the origin of the Gymnosperms must lie in the pre-Carboniferous period.
The group of Gymnosperms includes a number of genera of different types, most of which may be arranged under seven principal families. In a sketch of this nature it is, of course, quite impossible to deal with all the less-important families and genera. Those that will be considered here are the following:—
Coniferales(seep. 90).Araucareæ,e.g.Monkey-puzzleGenera both living and fossil.Fossil forms undoubted so far back as the Jurassic, and presumably further.Abietineæ,e.g.Pine and LarchGenera both living and fossil.Fossils recognized as far back as the Lower Cretaceous.Cupresseæ,e.g.Juniper, CypressGenera both living and fossil.Fossils recognized as far back as the Jurassic.Taxeæ,e.g.YewGenera living and fossil.Fossils recognized as far back as the Cretaceous.Cordaitales(seep. 92).Cordaiteæ,e.g.CordaitesFossil only.Characteristic of Devonian, Carboniferous, and Permian periods.Poroxyleæ,e.g.PoroxylonFossil only.Characteristic of the Carboniferous and Permian.Ginkgoales(seep. 98).Ginkgoaceæ,e.g.GinkgoFossil and living, dating back, apparently with little change, to Palæozoic times.
Coniferales(seep. 90).
Araucareæ,e.g.Monkey-puzzle
Genera both living and fossil.
Fossil forms undoubted so far back as the Jurassic, and presumably further.
Abietineæ,e.g.Pine and Larch
Genera both living and fossil.
Fossils recognized as far back as the Lower Cretaceous.
Cupresseæ,e.g.Juniper, Cypress
Genera both living and fossil.
Fossils recognized as far back as the Jurassic.
Taxeæ,e.g.Yew
Genera living and fossil.
Fossils recognized as far back as the Cretaceous.
Cordaitales(seep. 92).
Cordaiteæ,e.g.Cordaites
Fossil only.
Characteristic of Devonian, Carboniferous, and Permian periods.
Poroxyleæ,e.g.Poroxylon
Fossil only.
Characteristic of the Carboniferous and Permian.
Ginkgoales(seep. 98).
Ginkgoaceæ,e.g.Ginkgo
Fossil and living, dating back, apparently with little change, to Palæozoic times.
We must pay the most attention to the two last groups, as they are so important as fossils, and theCordaiteæwere a very numerous family in Coal Measure times. They had their period of principal development so long ago that it is probable that no direct descendants remain to the present time, though some botanists consider that theTaxeæare allied to them.
Of the groups still living it is difficult, almost impossible,to say which is the highest, the most evolved type. In the consideration of the Gymnosperm family it is brought home with great emphasis how incomplete and partial our knowledge is as yet. Many hold that theAraucareæare the most primitive of the higher Gymnosperms. In support of this view the following facts are noted. They have a simple type of fructification, with a single seed on a simple scale, and many scales arranged round an axis to form a cone. In the microscopic structure of their wood they have double rows of bordered pits, a kind of wood cell which comes closer to the old fossil types than does the wood of any of the other living genera. Further than this, wood which is almost indistinguishable from the wood of recent Araucarias is found very far back in the rocks, while their leaves are broad and simple, and attached directly to the stem in a way similar to the leaves of the fossilCordaiteæ, and very different from the needle leaves on the secondary stems of the Pine family; so that there appears good ground for considering the group an ancient and probably a primitive one.[10]
On the other hand, there are not wanting scientists who consider theAbietineæthe living representatives of the most primitive and ancient stock, though on the whole the evidence seems to indicate more clearly that the Pine-tree group is specialized and highly modified. Their double series of foliage leaves, their complex cones (whose structures are not yet fully understood), and their wood all support the latter view.
Some, again, consider theTaxeæas a very primitive group, and would place them near the Cordaiteæ, with which they may be related. Their fleshy seeds, growing not in cones but on short special axes, support this view, and it is certainly true that in many ways the large seeds, with their succulent coats and big endosperm, are muchlike those of the lower Gymnosperms and of several fossil types. Those, however, who hold to the view that the Abietineæ are primitive, see in theTaxeæthe latest and most modified type of Gymnosperm.
It will be seen from this that there is no lack of variety regarding the interpretation of Gymnosperm structures.
The Gymnosperms do not stand in such an isolated position as do the Angiosperms. Whatever the variety of views held about the details of the relative placing of the families within the group, all agree in recognizing the evidence which enables us to trace with confidence the connection between the lower Gymnosperms and the families of ferns. There are many indications of the intimate connection between higher and lower Gymnosperms. Between the series exist what might be described as different degrees of cousinship, and in the lower groups lie unmistakable clues to their connection with more ancient groups in the past which bridge over the gaps between them and the ferns.
For the present, however, let us confine ourselves to the history of the more important Gymnosperms, the discussion of their origin and the groups from which they may have arisen must be postponed until the necessary details about those groups have been mentioned.
To a consideration of the living families ofAraucareæ,Abietineæ,Cupresseæ, andTaxeæwe can allow but a short space; their general characters and appearance are likely to be known to the reader, and their details can be studied from living specimens if they are not. For purposes of comparison with the fossils, however, it will be necessary to mention a few of the principal features which are of special importance in discussing phylogeny.
TheAraucariaceæare woody trees which attain a considerable size, with broad-based, large leaves attached directly to the stem. In the leaves are a series of numerous parallel vascular bundles. The wood cells in microscopicsection show two rows or more of round bordered pits. The cones are very large, but the male and female are different in size and organization. The female cone is composed of series of simple scales arranged spirally round the axis, and each scale bears a single seed and a small ligule.
The pollen grains from the male cone are caught on the ligule and the pollen tubes enter the micropyle of the ovule, bringing in passive male cells which may develop in large numbers in each grain. The seeds when ripe are stony, and some are provided with a wing from part of the tissue of the scale. In the ripe cones the scales separate from the cone axis.
TheAbietineæare woody trees, some reaching a great height, all with a strong main stem. The leaves are of two kinds: primary ones borne directly attached to the stem (as in first-year shoots of the Larch), and secondary ones borne in tufts of two (in Pine) or a large number (in older branches of Larch) on special short branches, the primary leaves only developing as brown scales closely attached to the stems. Leaves generally very fine and needlelike, and with a central vascular bundle. The wood in microscopic section shows a single row of round bordered pits on the narrow tracheæ.
The female cones are large, male and female differing greatly in size and organization. The female cone, composed of a spiral series of pairs of scales, which often fuse together as the cone ripens. Each upper scale of the pair bears two seeds. The pollen grains from the male cone enter the micropyle of the seed and are caught in the tissue (apex of nucellus) there; the pollen tubes discharge passive male cells, only two of which develop in each grain. The seeds when ripe are stony and provided with a wing from the tissue of the scale on which they were borne.
TheCupresseæare woody trees reaching no great height, and of a bushy, branching growth. The leaves are attached directly to the main stem, and arrangethemselves in alternating pairs of very small leaves, closely pressed to the stem. The wood in microscopic section shows a single row of round bordered pits on the tracheæ.
The cones are small, and the scales forming them arranged in cycles. The female scales bear a varying number of seeds. The pollen grain has two passive male cells. The seeds when ripe are stony, with wings, though in some cases (species of Juniper) the cone scales close up and become fleshy, so that the whole fruit resembles a berry.
TheTaxeæare woody, though not great trees, bushily branched. The leaves are attached spirally all round the stem, but place themselves so as to appear to lie in pairs arranged in one horizontal direction. The wood in microscopic section shows a single row of round bordered pits on the tracheæ.
There are small male cones, but the seeds are not borne on cones, growing instead on special short axes, where there may be several young ovules, but on which usually two seeds ripen. The seeds are big, and have an inner stone and outer fleshy covering. Some have special outer fleshy structures known as “arils”,e.g.the red outer cup round the yew “berry” (which is not a berry at all, but a single unenclosed seed with a fleshy coat).
When we turn to theCordaiteæwe come to a group of plants which bears distinct relationship to the preceding, but which has a number of individual characters. It is a group of which we should know nothing were it not for the fossils preserved in the Palæozoic rocks; yet, notwithstanding the fact that it flourished so long ago, it is a family of which we know much. At the time of the Coal Measures and the succeeding Permo-carboniferous period, it was of great importance, and, indeed, in some of the French deposits it would seem as though whole layers of coal were composed entirely of its leaves.