Chapter IV: The Origin of Plant Life on the Land

If the graphite of the Laurentian rocks was derived from vegetable matter, the further question arises, Was this vegetation of the land, or of the sea? and something may be said on both sides of this question. If there were land plants in the Laurentian period, they must have grown either on rocks older than the Laurentian itself, or on such portions of the beds of the latter as had been raised out of the sea, forming perhaps swampy flats of newly-made soil. But we know no rocks older than the Laurentian, and there is no positive evidence that any of the beds of that formation were other than marine. Still it is not impossible that some of the beds which are now graphitic gneisses may originally have been similar to the bituminous shales, coals, or under-clays of the coal formation. The graphite occurring in veins, if of vegetable origin, must have been derived from liquid bitumen oozing into fissures; and veins of this kind occur in later formations, both in marine and fresh-water beds. The only other positive argument which has been adduced in favour of the existence of abundant land plants in the Laurentian is that of Dr. Sterry Hunt, derived from the great beds of iron ore, which it is difficult to account for chemically except on the hypothesis of the decay in the air of great quantities of vegetable matter. The question must remain in doubt till some one is fortunate enough to find portions of the Laurentian carbon retaining traces of organic structure. My own observations, though somewhat numerous, allow me only to say that the graphite sometimes presents fibrous forms, that it occasionally appears as vermicular threadsaEuro"which, however, I suppose to be fillings of canals of _Eozoon_aEuro"and that in the graphitic beds there are occasionally slender root-like bodies of a lighter colour than the mass; but none of these indications are sufficient to determine anything as to its vegetable origin, or the nature of the plants from which it may have been derived.

In any case, the quantity of carbon which has been accumulated in the Laurentian rocks is very great. I have measured one bed at Buckingham, on the Ottawa, estimated to contain 20 per cent. of carbon, and which is at least eight feet in thickness. Sir William Logan has described another similar bed from ten to twelve feet thick, and more recent reports of the Geological Survey of Canada mention a bed supposed to be twenty-five feet thick, in which Mr. Hoffman finds 30 per cent. of carbon. On the whole the quantity of carbon in the graphitic zone of the Laurentian is comparable with that in certain productive coal-fields, and we certainly have in the subsequent geological history no examples of such accumulations except from remains of the luxuriant vegetation of swampy flats.

The Upper Laurentian and Huronian have as yet afforded no evidence of land vegetation. The Cambrian, as already stated, abounds in remains of sea-weeds; but though the forms which have been named _Eophyton_ have been regarded as land plants, this claim is, to say the least, very doubtful; and I have as yet seen nothing of this kind which did not appear to me to be merely markings made by objects drifted over the bottom or remains of marine plants. Yet in the Upper Cambrian there are wide surfaces of littoral sandstone often containing minute carbonised fragments, and which might be expected to afford indications of land vegetation, had such existed. I have myself devoted many days of fruitless labour to the examination of the large areas of Potsdam sandstone exposed in some parts of Canada. But as these rocks were evidently formed along the borders of a Laurentian continent capable of supporting vegetation, we may still hope for some discovery of this kind, more especially if we could find the point where some fresh-water stream ran into the Cambrian sea.

The oldest plants, probably higher than AlgA|, known to me by their external forms, are those described by Nicholson[25] from the Siluro-Cambrian Skiddaw slates of the north of England (Fig. 83). Their discoverer has named them _Buthotrephis Harknessii_ and _B. radiata_,[26] stating, however, that these two species are not improbably portions of the same plant, and that its form is rather that of a land plant than of an Alga. The specimens of these plants which I have seen appear to me to support the conclusion that they represent one species, and this allied to the _AnnulariA|_ of the Devonian and Carboniferous periods, which probably grew in shallow water with only their upper parts in the air, and bore whorls or verticles of narrow leaves. They were either relatives of the MareaEuro(TM)s-tails, or of the Rhosocarps, of our modern swamps and ponds.

_a_, _Sphenophyllum primA|vum_. _b_, _Protostigma sigillarioides_.]

Somewhat higher up in the Lower Silurian, in the Cincinnati group of America, Lesquereux finds objects which he refers to the genus _Sphenophyllum_, which is closely allied to _Annularia_ (Fig. 84, _a_), and also a plant which he terms _Protostigma_ (Fig. 84, _b_), and believes to be the stem of a tree allied to the club-mosses.[27] He also finds minute branching stems, which he refers to the genus _Psilophyton_, to be mentioned in the sequel; but as to these I have some doubts whether they may not be Zoophytes allied to the Graptolites, rather than plants of that genus. These discoveries tend to show the probable existence in the Siluro-Cambrian of plants representing two of the three leading families of the higher cryptogams or flowerless plants, namely, the Club-mosses and the MareaEuro(TM)s-tails. Thus land vegetation begins with the highest members of the lower of the two great series into which botanists divide the vegetable kingdom.

If we now turn to the Silurian, further evidence of land vegetation presents itself. Near the base of this great series, the club-moss family is represented by a plant discovered by Claypole in the Clinton group, and referred to a new genus (_Glyptodendron_, Fig. 86). Plants of this family have also been noticed by Barrande in Bohemia, and by page in Scotland; and a humble but interesting member of the family, connecting it with the pillworts, _Psilophyton_ (Fig. 87), though more characteristic of the Devonian, has been found in the Upper Silurian both in Canada and the United States. No Ferns or Equiseta have as yet been found in the Silurian; but in 1870 I recognised in some fragments of wood from the Ludlow bone-bed, in the Museum of the Geological Survey of Great Britain, the structure of that curious prototypal tree, to which I have given the name _Nematophyton_, and which was first recognised in the Devonian of GaspA(C). Since that time I have found in the Upper Silurian beds of Cape Bon Ami, in New Brunswick, similar fragments of fossil wood, associated with round seed-like bodies, having a central nucleus and a thick wall or test of radiating fibres. These bodies show a structure similar to that of those found in the Upper Ludlow of England, and described by Hooker under the name _Pachytheca_. In my judgment they are certainly true seeds.[28] Seeds of this kind have also been found by Hicks in the still older Denbighshire grits of North Wales, along with fragments of the wood of _Nematophyton_, and with remains of branching stems which have been described under the name _Berwynia_, though it is not unlikely that they represent the branches of _Nematophyton_. It is proper to add that these ancient vegetable fossils are regarded by some English botanists as gigantic algA| or sea-weeds, but I confess I am unable to adopt this view of their nature. The supposed fern of the Upper Silurian, figured in the first edition of this work, has proved on further examination to be merely an imitative form produced by crystallisation. On the other hand, the recent discovery of a cockroach and two species of Scorpion in the Silurian, proves the existence of land animals as well as plants at this period.

_a_, Fruit, natural size. _b_, Stem, natural size. _c_, Scalariform tissue of the axis, highly magnified. In the restoration one side is represented in vernation, and the other in fruit.]

It is probable that these discoveries represent merely a small proportion of the plants actually existing in the Silurian period. All the deposits of this age at present known to us are marine; and most of them were probably formed at a distance from land, so that it is little likely that land plants could find their way into them. At any time the discovery of an estuarine or lacustrine deposit of Silurian age might wonderfully extend our knowledge of this ancient flora.

The Devonian or Erian age, that of the classic Old Red Sandstone of Scotland, is that in which we find the first great and complete land flora; and though this is inferior in number of species to that of the succeeding Carboniferous, and greatly less important with reference to its practical bearing on our welfare, it is in some respects superior in that variety which depends on diversity of soil and of station. To appreciate this, it will be necessary to glance at the range and subdivisions of the modern flora.

In the modern world we divide all vegetation into two great series, that of the Flowering Plants (_PhA|nogams_), which also produce true fruits and seeds, and that of the Flowerless Plants (_Cryptogams_), which produce minute spores instead of seeds. The latter is in every respect the lower group. This lower series is again divisible into three classesaEuro"first and lowest, that of the Seaweeds, Moulds, and Lichens (Thallophytes). Secondly, that of the Mosses and their allies (Anophytes). Thirdly, that of the Ferns, Equisetums and Club-mosses (Acrogens). In like manner the second, or higher series is divisible into three classes: that of the Pines and Cycads (Gymnosperms), having naked seeds not covered by true fruits, and woody tissue of simple structure; that of the Palms and Grasses and their allies (Endogens); and last and highest, that of the ordinary timber trees and other plants allied to them, with exogenous stems, netted-veined leaves, and a two-leaved embryo (Exogens). These last are in every respect the dominant plants on our present continents. Carrying with us this twofold division of the vegetable kingdom and its subdivisions, we shall be prepared to understand the relation of the more ancient floras to that now living.

In the Devonian age we meet with no land plants of the two lower classes of the Cryptogams, and with scarcely any that can be referred to the two higher classes of PhA|nogams, so that the vegetation of this period presents a remarkable character of mediocrity, being composed almost entirely of the highest class of the flowerless plants and the lowest class of those that flower. Of the former there are Tree-ferns and vast numbers of herbaceous forms (Figs. 88, 89), great Lycopodiaceous plants, immensely better developed than those now existing (Fig. 90), and gigantic _Calamites_, allied to the MaresaEuro(TM)-tails (Fig. 91), along with humbler members of the same group (Fig. 95). Of the latter there were Pines of great stature, known to us at present only by their wood (Fig. 92); and that other allied trees existed we have evidence in numerous seeds which must have belonged to this class (Fig. 93), and in long flag-like leaves[29] which modern discoveries would refer to the same group. As yet we know no Devonian Palms or Grasses; and only a single specimen has been found indicating the existence of a plant of the highest vegetable class, that of the true exogens. This unique specimen, found by Hall in the Devonian of the shores of Lake Erie, is a fragment of mineralised wood, the structures of which are represented in Fig. 94. The large ducts seen in cross section in Nos. 1, 2, and 3, and in longitudinal section in Nos. 4 and 5, and the medullary rays, seen in Nos. 1, 4, and 6, testify to the fact that this chip of wood must have belonged to a tree of the same type which contains our oaks, maples, and poplars; a type which does not appear to have become dominant till near the close of the Mesozoic, but which already existed, though perhaps only in few species, and only in upland and inland positions, as far back as the Middle Devonian. Perhaps one of the most interesting discoveries in the Erian or Devonian rocks has been that of the immense abundance of spores of those humble plants the Rhizocarps, represented in modern times by the Pillworts and SalviniA|, &c. To these it is believed that _Sphenophyllum_ and _Psilophyton_ were allied; but in addition to this there are thick and vastly extended beds of bituminous shale which owe their inflammable properties to countless multitudes of Macrospores (_Sporangites_) of the genus _Protosalvinia_.[30] In Ohio there are beds of this kind 350 feet thick, and extending across the State. They occur also in Canada, where these forms were first recognised by the writer in the bituminous shale of Kettle Point, Lake Huron.

A, Fragment showing _Sternbergia_ pith and wood; _a_, Medullary sheath; _b_, Pith; _c_, Wood; _d_, Section of pith.

B, Wood cell _a_, and hexagonal areole and pore _b_.

C, Longitudinal section of wood, showing _a_, Areolation, and _b_, Medullary rays.

D, Transverse section showing _a_, Wood-cells, and _b_, Limit of layer of growth.]

A, _Cardiocarpum cornutum_.
B, _Cardiocarpum acutum_.
C, _Cardiocarpum Crampii_.
D, _Cardiocarpum Baileyi_.
E, _Trigonocarpum racemosum_.
EA╣, EA squared, Fruits enlarged.
F, _Antholithes Devonicus_.
FA╣, Fruit of the same.
G, _Annularia acuminata_.
H, _Asterophyllites acicularis_.
HA╣, Leaf.
K, _Cardiocarpum_. (? young of A.)
L, _Pinnularia dispalans_.

From _Acadian Geology_. ]

1, Transverse section x 100.
2 and 3, Portions of the same x 300.
4, Longitudinal section x 300.
5, Fragment of duct from the same x 600.
6, Wood cells and medullary ray x 600.]

The Devonian flora seems to have been introduced in the northern parts of the American continent at a time of warm and equable climate, and of elevation of new land out of the Silurian sea. It spread itself to the southward, and was finally destroyed in the great subsidences and disturbances which closed the Devonian age, and which were probably accompanied with refrigeration of climate. It was succeeded by the more massive and richer, but more monotonous flora of the Carboniferous, a period in which large areas of our continents were in the state of swampy and often submerged flats, and in which the climate was again warm and uniform.

The Carboniferous age was, even more emphatically than the Devonian, an age of Acrogens and Conifers. A few Carboniferous Fungi have recently been discovered, but there are no known Lichens or Mosses. There seem to be a few Endogens, but no true Exogens. The great bulk of the plants consists of Acrogens and Gymnosperms, as in the previous period. As this flora is so very important and so much better known than any other of those belonging to the infancy of the vegetable kingdom, we may notice a little in detail some of its leading forms.

A, _C. Suckovii_. B, _C. Cistii_ (Bt.). C, Base of _Calamites_. D, E, Structures.

From _Acadian Geology_.]

A, _Odontopteris subcuneata_ (Bunbury).
B, _Neuropteris cordata_ (Brongniart).
C, _Alethopteris tonchitica_ (Brongniart).]

Beginning with the MaresaEuro(TM)-tails, we find these represented in the Carboniferous by many gigantic species, attaining to almost tree-like dimensions (Fig. 96). These are the _Calamites_, which formed dense brakes and jungles on the margins of the great swampy flats of this period. Their tall stems, ribbed and jointed, bore whorls of leaves or branchlets. Sending out horizontal root-stocks and budding out from the base, they grew in great clumps, and had the capacity to resist the effects of accumulating sediment by constantly sending out new stems at higher and higher levels. The larger species assumed a complexity in the structure of their stems unknown in their modern congeners, and enabling them to grow to a great height;[31] but their foliage and fructification were not correspondingly advanced. Thus the family of the EquisetaceA| culminated in the Carboniferous, and thenceforth descended gradually in the succeeding ages, leaving the comparatively humble MaresaEuro(TM)-tails and Scouring Rushes as its present representatives.

The Ferns of the Carboniferous, like those of the Devonian, presented both gigantic forms like those of the tree-ferns of the modern tropics, and delicate herbaceous species, and these in great profusion. On the whole, they do not strike the observer as very dissimilar from those of modern times. A more critical examination, however, shows that the bulk of the tree-ferns of the Devonian and Carboniferous are allied not to the Polypod type, which is the most common at present, but to certain comparatively rare southern ferns, the _Marattias_ and their allies, characterised by a peculiar style of fructification, perhaps adapting them to a moist and warm atmosphere (Fig. 97).[32] Thus the ferns, while a wonderfully persistent type, were in their grander forms far more widely distributed in the Carboniferous than at present; and genera now comparatively rare, and limited to warm and moist climates, were then abundant, and ranged over those temperate and boreal regions of the Northern Hemisphere where only a few humble and hardy species can now subsist. There were also some remarkable and anomalous tree-ferns, of which that represented in Fig. 98 is an example.

A, _Megaphyton magnificum_ (Dn.).
C, _PalA|opteris Hartii_ (Dn.).
D, _P. Acadica_ (Dn.).]

The family of the Club-mosses, already, even in the Devonian, in advance of its modern development, experiences in the Carboniferous a remarkable and portentous extension into great trees of several genera and many species, constituting apparently extensive forests, and having the woody tissues of their stems developed to a degree unheard of in their present representatives (Fig. 99). Further, they become closely linked, in external form at least, with another and more advanced type, that of the _SigillariA|_. These remarkable trees were the most abundant of all in the swamps of the coal-formation, and probably those which most contributed to the accumulation of coal. They presented tall pillar-like trunks, often ribbed longitudinally, and with perpendicular rows of scars of fallen leaves. Dividing at top into a few thick branches, they were covered with long rigid grass-like foliage. Their fruit was borne in rings or whorls of spikes surrounding the branches at intervals (Fig. 100). Their roots were strangely symmetrical, spreading out like underground branches into the soft soil by a regular process of bifurcation, and were covered with rootlets diverging in every direction, and so jointed to the main root that when broken off they left round marks regularly arranged. These roots are the so-called _StigmariA|_, so abundant in every coal-field, and especially filling the aEurooeunder-claysaEuroŁ of the coal-beds, which are the soils on which the plants forming these beds were supported. The true botanical position of the _SigillariA|_ has been a matter of much controversy. Some of them undoubtedly have structures akin to those of the tree-like Club-mosses, as Williamson has well shown, and may have been cryptogamous. Others have structures of higher character, akin to those of the modern Cycads, and seem to have borne nutlets allied to those of these plants. Yet the external forms of these diverse sorts are so similar that no definite separation of them has yet been made. Either these anomalous trees constitute a link connecting the two great series of the vegetable kingdom, or we have been confounding two distinct groups, owing to imperfect information.

A, Restoration.
B, Leaf, natural size.
C, Cone. D, Leafy branch.
E, Forms of leaf-bases.
F, Sporangium.
I, L, M, N, O, Markings on stem and branches, in various states.]

A, _Sigillaria Brownii_ (Dn.).
B, _S. elegans_ (Brongniart).
BA╣, &c. Leaf and Leaf-scars.]

Another curious, and till recently little understood, group of Carboniferous trees is that known as _Cordaites_, which existed already in some of its species in the Devonian. Their leaves are long, and often broad as well, and with numerous delicate parallel veins, resembling in this the leaves of grasses. Corda long ago showed that one species at least has a stem allied to the Club-mosses. More recently GrandaEuro(TM) Eury has found in the South of France admirably preserved specimens, which show that others more resembled in their structure the Pines and Yews, and were probably Gymnosperms, approaching to the Pines, but with very peculiar and exceptional foliage, of which the only modern examples are the broad-leaved Pines of the genus _Dammara_ (Frontispiece to Chapter). Here again we have either two very distinct groups, combined through our ignorance, or a connecting link between the Lycopods and the Pines.

_a_, Testa. _b_, Tegmen. _c_, Nucleus. _d_, Embryo.]

The Yews and their allies among modern trees, while members of the great Cone-bearing order, bear nut-like seeds in fleshy envelopes, sometimes, as in the GA-nkgo of Japan, constituting edible fruits. Seeds of this type seem to have been extremely abundant in the Carboniferous age in all parts of the world, and were probably produced by trees of several genera (_Dadoxylon_, _Sigillaria_, _Cordaites_, etc.) (Fig. 101). Charles Brongniart has recently described no less than seventeen genera of these seeds from the coal-field of St. A%tienne alone, and it would be a low estimate to say that we probably know as many as sixty or seventy species in all, while the trunks of great coniferous trees allied to TaxineA|, and showing well-preserved structure, are by no means uncommon in the Devonian and Carboniferous. Had these great Yews appeared for the first time in the Coal-formation, we might have supposed that they had been developed from such Lycopods as _Lepidodendra_, and that the _Cordaites_ are the intermediate forms; but unfortunately the Pines go almost as far back in geological time as the Lycopods, and it does not help us, when in search of evidence of evolution, to find the link which is missing or imperfect in the Early Devonian supplied in the Coal-formation, where, for this purpose at least, it is no longer needed.

We have said something of what was in the PalA|ozoic flora; but what of that which was not? We may answer:aEuro"Nearly all that is characteristic of our modern forests, whether in the ordinary Exogens, which predominate so greatly in the trees and shrubs of temperate climates, or in the Palms and their allies, which figure so conspicuously within the tropics. The few rare, and to some extent doubtful, representatives of these types scarcely deserve to be noted as exceptions. Had a botanist searched the PalA|ozoic forests for precursors of the future, he would probably have found only a few rare species, while he would have seen all around him the giant forms and peculiar and monotonous foliage of tribes now degraded in magnitude and structure, and of small account in the system of nature.

It must not be supposed that the PalA|ozoic flora remained in undisturbed possession of the continents during the whole of that long period. In the successive subsidences of the continental plateaux, in which the marine limestones were deposited, it was to a great extent swept away, or was restricted to limited insular areas, and these more especially in the far north, so that on re-elevation of the land it was always peopled with northern plants. Thus there were alternate restrictions and expansions of vegetation, and the latter were always signalised by the introduction of new species, for here, as elsewhere, it was not struggle, but opportunity, that favoured improvement.

In the Lower Silurian such plants as existed must have experienced great restriction at the age of the Niagara or Wenlock limestone. Those of the Upper Silurian suffered a similar reverse at the time of the Lower Helderberg or Ludlow limestones. This recurred at the close of the Devonian and in the time of the Lower Carboniferous limestone; and finally the PalA|ozoic flora disappeared altogether in the Permian, to be replaced by new types in the Mesozoic. While, therefore, there is a great general similarity in the successive PalA|ozoic floras, there are minor differences, so that the Devonian plants are for the most part distinct specifically from those of the Lower Carboniferous, those of the Lower Carboniferous from those of the Coal-formation, and those of the latter from those of the Permian.

With all these vicissitudes it is to be observed that there is no apparent elevation of type in all the long ages from the Devonian to the Permian, that the Acrogens and Gymnosperms of these periods are in some respects superior, in all respects equal, to their modern successors, and that their history shows a decadence toward the modern period; that intermediate forms arrive too late to form connecting links in time, that several distinct types appear together at the beginning, and that all utterly and apparently simultaneously perish at the end of the PalA|ozoic, to make way for the entirely new vegetation of the succeeding age. Theories of evolution receive no support from facts like these, though their practical significance, as parts of the one great uniform scheme of nature, is sufficiently manifest.

Of what use then were these old floras? To the naturalist, vegetable life, with regard to its modern uses, is the great accumulator of pabulum for the sustenance of the higher forms of vital energy manifested in the animal. In the PalA|ozoic this consideration sinks in importance. In the Coal period we know few land animals, and these not vegetable feeders, with the exception of some insects, millipedes, and snails. But the Carboniferous forests did not live in vain, if their only use was to store up the light and heat of those old summers in the form of coal, and to remove the excess of carbonic acid from the atmosphere. In the Devonian period even these utilities fail, for coal does not seem to have been accumulated to any great extent, and the petroleum of the Devonian appears to have been produced from aquatic vegetation. Even with reference to theories of evolution, there seems no necessity for the long continuance and frequent changes of species of acrogenous plants without any perceptible elevation. We may have much yet to learn of the life of the Devonian; but for the present the great plan of vegetable nature goes beyond our measures of utility; and there remains only what is perhaps the most wonderful and suggestive correlation of all, namely, that our minds, made in the image of the Creator, are able to trace in these perished organisms structures similar to those of modern plants, and thus to reproduce in imagination the forms and habits of growth of living things which so long preceded us on the earth. We may indeed proceed a step further, and hold that, independently of human appreciation, these primitive plants commended themselves to the approval of their Maker, and perhaps of higher intelligences unknown to us; and that in the last resort it was for His pleasure that they were created.