(coal-formation. Nova Scotia.) A, _Antholithes squamosus_ (two thirds).
B, _A. rhabdocarpi_. (two thirds). B^1, Carpel restored. C, _A. spinosus_
(natural size). D, _Trigonocarpum intermedium_. E, _T. Noeggerathii_.
F, _T. avellanum_. G, _Rhabdocarpus insignis_, reduced. H, _Antholithes
pygmæus_. I, _Cardiocarpum fluitans_. K, _Cardiocarpum bisectum_. L,
_Sporangites papillata_, lycopodiaceous macrospores (natural size and
magnified).]
On the stems so constructed were placed long and often broad many-nerved
leaves, with rows of stomata or breathing-pores, and attached by somewhat
broad bases to the stem and branches. The fruit consisted of racemes, or
clusters of nutlets, which seem to have been provided with broad lateral
wings for flotation in the air, or in some cases with a pulpy envelope,
which flattens into a film. There seem to have been structures of both
these kinds, though in the state of preservation of these curious seeds
it is extremely difficult to distinguish them. In the first case they
must have been intended for dissemination by the wind, like the seeds
of spruces. In the latter case they may have been disseminated like the
fruits of taxine trees by the agency of animals, though what these were
it would be difficult to guess. These trees had very great reproductive
power, since they produced numerous seeds, not singly or a few together,
as in modern yews, but in long spikes or catkins bearing many seeds (Fig.
59).
It is to be observed that the Cordaites, or the _Cordaitinæ_, as
they have been called, as a family,[CF] constitute another of those
intermediate groups with which we have already become familiar. On
the one hand they approach closely to the broader-leaved yews like
Gingko, Phyllocladus, and Podocarpus, and, on the other hand, they
have affinities with Cycadaceæ, and even with Sigillariæ. They were
beautiful and symmetrical trees, adding something to the variety of the
rather monotonous Palæozoic forests. They contributed also somewhat to
the accumulation of coal. I have found that some thin beds are almost
entirely composed of their leaves, and the tissues of their wood are not
infrequent in the mineral charcoal of the larger coal-seams. There is
no evidence that their roots were of the stigmaroid type, though they
evidently grew in the same swampy flats with the Sigillariæ and Calamites.
[CF] Engler; Cordaitées of Renault.
It may, perhaps, be well to say here that I believe there was a
considerably wide range of organisation in the Cordaitinæ as well as in
the Calamites and Sigillariæ, and that it will eventually be found that
there were three lines of connection between the higher cryptogams and
the phænogams, one leading from the lycopods by the Sigillariæ, another
leading by the Cordaites, and the third leading from the Equisetums by
the Calamites. Still further back the characters afterward separated in
the club-mosses, mare's-tails, and ferns, were united in the Rhizocarps,
or, as some now, but I think somewhat unreasonably, prefer to call
them, the "heterosporous Filicinæ." In the more modern world, all the
connecting links have become extinct and the phænogams stand widely
separated from the higher cryptogams. I do not make these remarks in
a Darwinian sense, but merely to state what appear to be the lines of
natural affinity and the links wanting to give unity to the system of
nature.
Of all the trees of the modern world, none are perhaps so widely
distributed as the pines and their allies. On mountain-tops and
within the Arctic zone, the last trees that can struggle against the
unfavourable conditions of existence are the spruces and firs, and in
the warm and moist islands of the tropics they seem equally at home
with the tree-ferns and the palms. We have already seen that they are
a very ancient family, and in the sandstones of the coal-formation
their great trunks are frequently found, infiltrated with calcareous or
silicious matter, and still retaining their structure in the greatest
perfection (Fig. 60). So far as we know, the foliage of some of them
which constitutes the genera _Walchia_ and _Araucarites_ of some authors
(Figs. 60, 63) was not dissimilar from that of modern yews and spruces,
though there is reason to believe that some others had broad, fern-like
leaves like those of the gingko. None of them, so far as yet certainly
known, were cone-bearing trees, their fruit having probably been similar
to that of the yews (Fig. 61). The minute structures of their stems are
nearer to those of the conifers of the islands of the southern hemisphere
than to that of those in our northern climes--a correlation, no doubt, to
the equable climate of the period. There is not much evidence that they
grew with the Sigillariæ in the true coal-swamps, though some specimens
have been found in this association. It is more likely that they were in
the main inland and upland trees, and that in consequence they are mostly
known to us by drifted trunks borne by river inundations into the seas
and estuaries.
[Illustration: Fig. 60.--Coniferous wood and foliage (Carboniferous). A,
_Araucarites gracilis_, reduced, b, _Dadoxylon Acadianum_ (radial), 90
diams.; B^1 (tangential), 90 diams; B^2, cell showing areolation, 250
diams. C, _Dadoxylon materiarium_ (radial), 90 diams.; C^1 (tangential),
90 diams. C^2, cell showing areolation, 250 diams. D, _Dadoxylon
antiquius_ (radial), 90 diams.; D^1 (tangential), 90 diams.; D^2, cell
showing areolation, 250 diams.]
[Illustration: Fig. 61.--__Trigonocarpum Hookeri_, Dawson from the
coal-measures of Cape Breton. Probably the fruit of a Taxine tree. A,
Broken specimen magnified twice natural size, B, Section magnified: _a_,
the testa; _b_, the tegmen; _c_, the nucleus; _d_, the embryo, _c_,
Portion of the surface of the inner coat more highly magnified.]
A remarkable fact in connection with them, and showing also the manner in
which the most durable vegetable structures may perish by decay, is that,
like the Cordaites, they had large piths with transverse partitions, a
structure which, as I have already mentioned, appears on a minute scale
in the twigs of the fir-tree, and that sometimes casts of these piths in
sandstone appear in a separate form, constituting what have been named
_Sternbergiæ_ or _Artisiæ_. As Renault well remarks with reference to
Cordaites, the existence of this chambered form of pith implies rapid
elongation of the stem, so that the Cordaites and conifers of the
coal-formation were probably quickly growing trees (Fig. 62).
[Illustration: Fig. 62.--_Sternbergia_ pith of _Dadoxylon_. A, Specimen
(natural size), showing remains of wood at _a_, _a_. B, Junction of
wood and pith, magnified. C, Cells of the wood of do., _a_, _a_; _b_,
medullary ray; _c_, areolation.]
The same general statements may be made as to the coal-vegetation as in
relation to that of the Erian. In the coal period we have found none of
the higher exogens, and there are only obscure and uncertain indications
of the presence of endogens, which we may reserve for a future chapter;
but gymnosperms abound and are highly characteristic. On the other hand,
we have no mosses or lichens, and very few Algæ, but a great number of
ferns and Lycopodiaceæ or club-mosses (Fig. 63). Thus, the coal-formation
period is botanically a meeting-place of the lower phænogams and the
higher cryptogams, and presents many forms which, when imperfectly known,
have puzzled botanists in regard to their position in one or other
series. In the present world, the flora most akin to that of the coal
period is that of warm, temperate regions in the southern hemisphere. It
is not properly a tropical flora, nor is it the flora of a cold region,
but rather indicative of a moist and equable climate. Still, we must bear
in mind that we may often be mistaken in reasoning as to the temperature
required by extinct species of plants, differing from those now in
existence. Further, we must not assume that the climatal conditions of
the northern hemisphere were in the coal period at all similar to those
which now prevail. As Sir Charles Lyell has shown, a less amount of land
in the higher latitudes would greatly modify climates, and there is
every reason to believe that in the coal period there was less land than
now. Further, it has been shown by Tyndall that a very small additional
amount of carbonic acid in the atmosphere would, by obstructing the
radiation of heat from the earth, produce almost the effect of a glass
roof or conservatory, extending over the whole world. Again, there is
much in the structure of the leaves of the coal-plants, as well as in
the vast amount of carbon which they accumulated in the form of coal,
and the characteristics of the animal life of the period, to indicate,
on independent grounds, that the carboniferous atmosphere differed
from that of the present world in this way, or in the presence of more
carbonic acid--a substance now existing in the very minute proportion
of one thousandth of the whole--a quantity adapted to the present
requirements of vegetable and animal life, but probably not to those of
the coal period.
[Illustration: Fig. 63.--_Walchia imbricatula_, S. N., Permian, Prince
Edward Island.]
Thus, if we inquire as to any analogous distribution of plants in the
modern world, we find this only in the warmer insular climates of the
southern hemisphere, where ferns, lycopods, and pines appear under forms
somewhat akin to those of the Carboniferous, but mixed with other types,
some of which are modern, others allied to those of the next succeeding
geological ages of the Mesozoic and Tertiary; and under these periods it
will be more convenient to make comparisons.
The readers of recent English popular works on geology will have observed
the statement reiterated that a large proportion of the material of
the great beds of bituminous coal is composed of the spore-cases of
lycopodiaceous plants--a statement quite contrary to that resulting
from my microscopical examinations of the coal of more than eighty
coal-beds in Nova Scotia and Cape Breton, as stated in "Acadian Geology"
(page 463), and more fully in my memoir of 1858 on the "Structures
in Coal,"[CG] and that of 1866, on the "Conditions of Accumulation
of Coal."[CH] The reason of this mistake is, that an eminent English
naturalist, happening to find in certain specimens of English coal a
great quantity of remains of spores and spore-cases, though even in his
specimens they constitute only a small portion of the mass, and being
apparently unacquainted with what others had done in this field, wrote a
popular article for the "Contemporary Review," in which he extended an
isolated and exceptional fact to all coals, and placed this supposed
origin of coal in a light so brilliant and attractive that he has been
followed by many recent writers. The fact is, as stated in "Acadian
Geology," that trunks of _Sigillariæ_ and similar trees constitute a
great part of the denser portion of the coal, and that the cortical
tissues of these rather than the wood remain as coal. But cortical or
epidermal tissues in general, whether those of spore-cases or other
parts of plants, are those which from their resistance to water-soakage
and to decay, and from their highly carbonaceous character, are best
suited to the production of coal. In point of fact, spore-cases, though
often abundantly present, constitute only an infinitesimal part of the
matter of the great coal-beds. In an article in "The American Journal
of Science," which appeared shortly after that above referred to, I
endeavoured to correct this error, though apparently without effect in
so far as the majority of British geological writers are concerned. From
this article I have taken with little change the following passages, as
it is of importance in theoretical geology that such mistakes, involving
as they do the whole theory of coal accumulation, should not continue to
pass current. The early part of the paper is occupied with facts as to
the occurrence of spores and spore-cases as partial ingredients in coal.
Its conclusions are as follows: It is not improbable that sporangites,
or bodies resembling them, may be found in most coals; but it is most
likely that their occurrence is accidental rather than essential to coal
accumulation, and that they are more likely to have been abundant in
shales and cannel coals, deposited in ponds or in shallow waters in the
vicinity of lycopodiaceous forests, than in the swampy or peaty deposits
which constitute the ordinary coals. It is to be observed, however,
that the conspicuous appearance which these bodies, and also the strips
and fragments of epidermal tissue, which resemble them in texture,
present in slices of coal, may incline an observer, not having large
experience in the examination of coals, to overrate their importance; and
this I think has been done by most microscopists, especially those who
have confined their attention to slices prepared by the lapidary. One
must also bear in mind the danger arising from mistaking concretionary
accumulations of bituminous matter for sporangia. In sections of the
bituminous shales accompanying the Devonian coal above mentioned, there
are many rounded yellow spots, which on examination prove to be the
spaces in the epidermis of _Psilophyton_ through which the vessels
passing to the leaves were emitted. To these considerations I would add
the following, condensed from the paper above referred to (p. 139), in
which the whole question of the origin of coal is fully discussed:[CI]
[CG] "Journal of the Geological Society," vol. xv.
[CH] _Ibid._, vol. xxii.
[CI] See also "Acadian Geology," 2d ed., pp. 138, 461, 493.
1. The mineral charcoal or 'mother coal' is obviously woody tissue
and fibres of bark, the structure of the varieties of which, and the
plants to which it probably belongs, I have discussed in the paper above
mentioned.
2. The coarser layers of coal show under the microscope a confused mass
of fragments of vegetable matter belonging to various descriptions of
plants, and including, but not usually in large quantities, sporangites.
3. The more brilliant layers of the coal are seen, when separated by thin
laminæ of clay, to have on their surfaces the markings of _Sigillariæ_
and other trees, of which they evidently represent flattened specimens,
or rather the bark of such specimens. Under the microscope, when their
structures are preserved, these layers show cortical tissues more
abundantly than any others.
4. Some thin layers of coal consist mainly of flattened layers of leaves
of _Cordaites_ or _Pychnophyllum_.
5. The _Stigmaria_ under-clays and the stumps of _Sigillaria_ in the
coal-roofs equally testify to the accumulation of coal by the growth of
successive forests, more especially of _Sigillariæ_. There is, on the
other hand, no necessary connection of sporangite-beds with Stigmarian
soils. Such beds are more likely to be accumulated in water, and
consequently to constitute bituminous shales and cannels.
6. _Lepidodendron_ and its allies, to which the spore-cases in question
appear to belong, are evidently much less important to coal accumulation
than _Sigillaria_, which cannot be affirmed to have produced spore-cases
similar to those in question, even though the observation of Goldenberg
as to their fruit can be relied on; the accuracy of which, however, I am
inclined to doubt.
On the whole, then, while giving due credit to those who have advocated
the spore-theory of coal, for directing attention to this curious and no
doubt important constituent of mineral fuel, and admitting that I may
possibly have given too little attention to it, I must maintain that
sporangite-beds are exceptional among coals, and that cortical and woody
matters are the most abundant ingredients in all the ordinary kinds; and
to this I cannot think that the coals of England constitute an exception.
It is to be observed, in conclusion, that the spore-cases of plants,
in their indestructibility and richly carbonaceous character, only
partake of qualities common to most suberous and epidermal matters, as I
have explained in the publications already referred to. Such epidermal
and cortical substances are extremely rich in carbon and hydrogen,
in this resembling bituminous coal. They are also very little liable
to decay, and they resist more than other vegetable matters aqueous
infiltration--properties which have caused them to remain unchanged,
and to continue free from mineral additions more than other vegetable
tissues. These qualities are well seen in the bark of our American white
birch. It is no wonder that materials of this kind should constitute
considerable portions of such vegetable accumulations as the beds of
coal, and that when present in large proportion they should afford richly
bituminous beds. All this agrees with the fact, apparent on examination
of the common coal, that the greater number of its purest layers consist
of the flattened bark of _Sigillariæ_ and similar trees, just as any
single flattened trunk embedded in shale becomes a layer of pure coal.
It also agrees with the fact that other layers of coal, and also the
cannels and earthy bitumens, appear under the microscope to consist of
finely comminuted particles, principally of epidermal tissues, not only
from the fruits and spore-cases of plants, but also from their leaves and
stems. These considerations impress us, just as much as the abundance of
spore-cases, with the immense amount of the vegetable matter which has
perished during the accumulation of coal, in comparison with that which
has been preserved.
I am indebted to Dr. T. Sterry Hunt for the following very valuable
information, which at once places in a clear and precise light the
chemical relations of epidermal tissue and spores with coal. Dr. Hunt
says: "The outer bark of the cork-tree, and the cuticle of many if not
all other plants, consists of a highly carbonaceous matter, to which
the name of _suberin_ has been given. The spores of _Lycopodium_ also
approach to this substance in composition, as will be seen by the
following, one of two analyses by Duconi,[CJ] along with which I give
the theoretical composition of pure cellulose or woody fibre, according
to Payen and Mitscherlich, and an analysis of the suberin of cork, from
_Quercus suber_, from which the ash and 2·5 per cent of cellulose have
been deducted.[CK]
[CJ] Liebig and Kopp, "Jahresbuch," 1847-'48.
[CK] Gmelin, "Handbook," xv., 145.
---------+-----------+--------+------------
| Cellulose.| Cork. | Lycopodium.
---------+-----------+--------+------------
Carbon | 44·44 | 65·73 | 64·80
Hydrogen | 6·17 | 8·33 | 8·73
Nitrogen | .... | 1·50 | 6·18
Oxygen | 49·39 | 24·44 | 20·29
+-----------+--------+------------
Total | 100·00 | 100·00 | 100·00
---------+-----------+--------+------------
"This difference is not less striking when we reduce the above centesimal
analyses to correspond with the formula of cellulose, C{24}H{20}O{20},
and represent cork and _Lycopodium_ as containing twenty-four equivalents
of carbon. For comparison I give the composition of specimens of peat,
brown coal, lignite, and bituminous coal:[CL]
[CL] "Canadian Naturalist," vi., 253.
Cellulose C{24}H{20}O{20}
Cork C{24}H{18-2/10}O{6-7/10}
Lycopodium C{24}H{19-4/10}NO{5-6/10}
Peat (Vaux) C{24}H{14-4/10}O{10}
Brown coal (Schröther) C{24}H{14-3/10}O{10-6/10}
Lignite (Vaux) C{24}H{11-3/10}O{6-4/10}
Bituminous coal (Regnault) C{24}H{10}O{3-3/10}
"It will be seen from this comparison that, in ultimate composition, cork
and _Lycopodium_ are nearer to lignite than to woody fibre, and may be
converted into coal with far less loss of carbon and hydrogen than the
latter. They in fact approach closer in composition to resins and fats
than to wood, and, moreover, like those substances repel water, with
which they are not easily moistened, and thus are able to resist those
atmospheric influences which effect the decay of woody tissue."
I would add to this only one further consideration. The nitrogen
present in the _Lycopodium_ spores, no doubt, belongs to the protoplasm
contained in them, a substance which would soon perish by decay; and
subtracting this, the cell-walls of the spores and the walls of the
spore-cases would be most suitable material for the production of
bituminous coal. But this suitableness they share with the epidermal
tissue of the scales of strobiles, and of the stems and leaves of ferns
and lycopods, and, above all, with the thick, corky envelope of the
stems of _Sigillariæ_ and similar trees, which, as I have elsewhere
shown,[CM] from its condition in the prostrate and erect trunks contained
in the beds associated with coal, must have been highly carbonaceous
and extremely enduring and impermeable to water. In short, if, instead
of "spore-cases," we read "epidermal tissues in general, including
spore-cases," all that has been affirmed regarding the latter will
be strictly and literally true, and in accordance with the chemical
composition, microscopical characters, and mode of occurrence of coal. It
will also be in accordance with the following statement, from my paper on
the "Structures in Coal," published in 1859:
[CM] "Vegetable Structures in Coal," "Journal of Geological Society,"
xv., 626. "Conditions of Accumulation of Coal," _ibid._, xxii., 95.
"Acadian Geology," 197, 464.
"A single trunk of _Sigillaria_ in an erect forest presents an epitome of
a coal-seam. Its roots represent the _Stigmaria_ under-clay; its bark the
compact coal; its woody axis the mineral charcoal; its fallen leaves (and
fruits), with remains of herbaceous plants growing in its shade, mixed
with a little earthy matter, the layers of coarse coal. The condition of
the durable outer bark of erect trees concurs with the chemical theory
of coal, in showing the especial suitableness of this kind of tissue for
the production of the purer compact coals. It is also probable that the
comparative impermeability of the bark to mineral infiltration is of
importance in this respect, enabling this material to remain unaffected
by causes which have filled those layers, consisting of herbaceous
materials and decayed wood, with pyrites and other mineral substances."
We need not go far in search of the uses of the coal vegetation, when .we
consider the fact that the greatest civilised nations are dependent on
it for their fuel. Without the coal of the Carboniferous period and the
iron-ore which is one of the secondary consequences of coal accumulation,
just as bog-ores of iron occur in the subsoils of modern peats, it would
have been impossible either to sustain great nations in comfort in the
colder climates of the northern hemisphere or to carry on our arts and
manufactures. The coal-formation yields to Great Britain alone about one
hundred and sixty million tons of coal annually, and the miners of the
United States extract mainly from the same formation nearly a hundred
million tons, while the British colonies and dependencies produce about
five million tons; and it is a remarkable fact that it is to the English
race that the greatest supply of this buried power and heat and light has
been given.
The great forests of the coal period, while purifying the atmosphere of
its excess of unwholesome carbonic acid, were storing up the light and
heat of Palæozoic summers in a form in which they could be recovered in
our human age, so that, independently of their uses to the animals which
were their contemporaries, they are indispensable to the existence of
civilised man.
Nor can we hope soon to be able to dispense with the services of
this accumulated store of fuel. The forests of to-day are altogether
insufficient for the supply of our wants, and though we are beginning
to apply water-power to the production of electricity, and though some
promising plans have been devised for the utilisation of the direct heat
and light of the sun, we are still quite as dependent as any of our
predecessors on what has been done for us in the Palæozoic age.
In the previous pages I have said little respecting the physical
geography of the Carboniferous age; but, as may be inferred from the
vegetation, this in the northern hemisphere presented a greater expanse
of swampy flats little elevated above the sea than we find in any other
period. As to the southern hemisphere, less is known, but the conditions
of vegetation would seem to have been essentially the same.
Taking the southern hemisphere as a whole, I have not seen any evidence
of a Lower Devonian or Upper Silurian flora; but in South Africa and
Australia there are remains of Upper Devonian or Lower Carboniferous
plants. These were succeeded by a remarkable Upper Carboniferous or
Permian group, which spread itself all over India, Australia, and South
Africa,[CN] and contains some forms (_Vertebraria_, _Phyllotheca_,
_Glossopteris_, &c. ) not found in rocks of similar age in the northern
hemisphere, so that, if the age of these beds has been correctly
determined, the southern hemisphere was in advance in relation to some
genera of plants. This, however, is to be expected when we consider that
the Triassic and Jurassic flora of the north contains or consists of
intruders from more southern sites. These beds are succeeded in India by
others holding cycads, &c., of Upper Jurassic or Lower Cretaceous types
(Rajmahal and Jabalpur groups).
[CN] Wyley, "Journal Geol. Society," vol. xxiii., p. 172; Daintree,
_ibid._, vol. xxviii.; also Clarke and McCoy.
Blanford has shown that there is a very great similarity in this series
all over the Australian and Indian region.[CO] Hartt and Darby have in
like manner distinguished Devonian and Carboniferous forms in Brazil
akin to those of the northern hemisphere. Thus the southern hemisphere
would seem to have kept pace with the northern, and according to Blanford
there is evidence there of cold conditions in the Permian, separating
the Palæozoic flora from that of the Mesozoic, in the same manner that
Ramsay has supposed a similar period of cold to have done north of the
equator. This would imply a very great change of climate, since we have
evidence of the extension of the Lower Carboniferous flora at least as
far north as Spitzbergen. The upper coal-formation we cannot, however,
trace nearly so far north; so that a gradual refrigeration may have been
going on before the Permian. Thus in both hemispheres there was a general
similarity in the later Palæozoic flora, and perhaps similar conditions
leading to its extinction and to its replacement by that to be described
in the next chapter.
[CO] "Journal Geol. Society," vol. xxxi.
* * * * *
NOTES TO CHAPTER IV.
I. Characters and Classification of Palæozoic Plants.
In the space available in this work it would be impossible to enter fully
into the classification of Palæozoic plants; but it may be well to notice
some important points for the guidance of those who may desire to collect
specimens; more especially as much uncertainty exists as to affinities
and very contradictory statements are made. The statements below may
be regarded as the results of actual observation and of the study of
specimens _in situ_ in the rocks, as well as in the cabinet and under the
microscope.
Gymnospermeæ.
_Family_ Coniferæ; _Genus_ Dadoxylon, Endlicher; Araucarites, Goeppert;
Araucarioxylon, Kraus.
The trunks of this genus occur from the Middle Devonian to the Permian
inclusive, as drift-logs calcified, silicified, or pyritised. The
only foliage associated with them is of the type of _Walchia_ and
_Araucarites_--viz., slender branches with numerous small spiral acicular
leaves. Two of the coal-formation species, _D. materiarum_ and another,
had foliage of this type. That of the others is unknown. They are all
distinct from the wood of _Cordaites_, for which see under that genus.
The following are North American species:
_Trunks._
_Dadoxylon Ouangondianum_, Dn. M. Erian Report, 1871.[CP]
_D. Halli_, Dn. " "
_D. Newberryi_, Dn. " "
_D. Clarkii_, Dn. (Cordæoxylon ?) " Report, 1882.
_D. Acadianum_, Dn. Coal-formation Acadian Geology.
and millstone
grit.
_D. Materiarum_, Dn. Do. and Permo- "
Carb.
_D._ (_Palæoxylon_) _antiquius_, Dn. L. Carboniferous. "
_D. annulatum_, Dn. Coal-formation. "
_Ormoxylon Erianum_, Dn. Erian Report, 1871.
_Foliage._
_Araucarites gracilis_, Dn. N. Coal-formation "
and Permian.
{ Report on
_Walchia robusta_, Dn. Permian. { Prince Edward
_W. imbricatula_, Dn. " { Island.
[CP] "Geological Survey of Canada: Fossil Plants of Erian and Upper
Silurian Formations," by J. W. Dawson.
All of the above can be vouched for as good species based upon
microscopic examination of a very large number of trunks from different
parts of North America. The three Erian species of _Dadoxylon_ and _D.
antiquius_ from the Lower Carboniferous have two or more rows of cells
in the medullary rays. The last named has several rows, and is a true
_Palæoxylon_ allied to _D. Withami_ of Great Britain. _D. materiarium_
is specially characteristic of the upper coal-formation and Permian,
and to it must belong one or both of the species of foliage indicated
above. _D. Clarkii_ has very short, simple medullary rays of only a few
cells superimposed, and has an inner cylinder of scalariform vessels,
approaching in these points to _Cordaites_. _Ormoxylon_ has a very
peculiar articulated pith and simple medullary rays.
Witham in 1833 described several Carboniferous species of pine-wood,
under the generic name Pinites, separating under the name _Pitus_ species
which appeared to have the discs on the cell-walls separate and in
transverse lines. Witham's name was changed by Goeppert to _Araucarites_,
to indicate the similarity of these woods to Araucaria, _Pinites_ being
reserved for trees more closely allied to the ordinary pines. Endlicher,
restricting Araucarites to foliage, etc., of Araucaria-like trees, gave
the name _Dadoxylon_ to the wood; and this, through Unger's "Genera and
Species," has gained somewhat general acceptance. Endlicher also gave the
name _Pissadendron_ to the species which Witham had called _Pitus_; but
Brongniart proposed the name _Palæoxylon_ to include all the species with
thick and complex medullary rays, whatever the arrangement of the discs.
In Schimper's new work Kraus substitutes _Araucarioxylon_ for Endlicher's
_Dadoxylon_, and includes under _Pissadendron_ all the species placed by
Brongniart in _Palæoxylon_.
To understand all this confusion, it may be observed that the characters
available in the determination of Palæozoic coniferous wood are chiefly
the form and arrangement of the wood-cells, the character of the bordered
pores or discs of their walls, and the form and composition of the
medullary rays.
The character on which Witham separated his genus _Pitus_ from _Pinites_
is, as I have ascertained by examination of slices of one of his original
specimens kindly presented to me by Mr. Sanderson, of Edinburgh,
dependent on state of preservation, the imperfectly preserved discs
or areolations of the walls of the fibre presenting the appearance of
separate and distinct circles, while in other parts of the same specimens
these discs are seen to be contiguous and to assume hexagonal forms, so
that in this respect they do not really differ from the ordinary species
of _Dadoxylon_. The true character for subdividing those species which
are especially characteristic of the Carboniferous, is the composite
structure of the medullary rays, which are thick and composed of several
radial piles of cells placed side by side. This was the character
employed by Brongniart in separating the genus _Palæoxylon_, though he
might with convenience have retained Witham's name, merely transferring
to the genus the species of Witham's _Pinites_ which have complex
medullary rays. The Erian rocks present the greatest variety of types,
and _Palæoxylon_ is especially characteristic of the Lower Carboniferous,
while species of _Dadoxylon_ with two rows of bordered pores and simple
medullary rays are especially plentiful in the upper coal-formation and
Permo-Carboniferous.
The following table will clearly show the distinctive characters and
relations of the genera in question, as held by the several authors above
referred to:
_Wood of Palæozoic Conifers._
Woody | Medullary rays | Generic | Geological
fibres. | and pith. | names. | age.
-------------+--------------------+---------------------+---------------
No discs. | One or two series | _Aporoxylon_, | Devonian
| of cells. | Unger. | (Erian).
-------------+--------------------+---------------------+---------------
| Complex, or of two | { _Pitus_, |
| or more series of | { Witham. | Middle and
| or more series of | { _Palæoxylon_, | Lower
| cells. | { Brongniart. | Carboniferous
| Pith Sternbergian. | { _Pissadendron_, | and Devonian.
| | { Endlicher. |
+--------------------+---------------------+---------------
Discs in one | Simple, or of one | { _Araucarites_, |
series | row of cells. | { Goeppert | Upper
contiguous, | Pith Sternbergian. | { _Dadoxylon_, | Carboniferous
or in | | { Endlicher. | and Permian.
several | | { _Araucarioxylon_, |
series | | { Schimper. |
spirally +--------------------+---------------------+---------------
arranged. | Pith in spherical | _Ormoxylon_,[CQ] Dn. | Devonian.
| chambers. | |
+--------------------+---------------------+---------------
| Medullary sheath | | Devonian.
| scalariform. | _Dadoxylon_ |
| Medullary rays | (Cordæoxylon),[CR] |
| frequent, simple, | Dn. |
| short. | |
-------------+--------------------+---------------------+---------------
[CQ] Type _O. Erianum_, Dn., "Report on Canadian Plants," 1871.
[CR] Type _D. Clarkii_, Dn., "Report on Canadian Plants," 1882. This may
be wood of Cordaites, to which it approaches very closely.
_Family_ Cordaites, _Genus_ Cordaites, Brongniart.
Trunks marked by transverse scars of attachment of bases of leaves;
leaves broad, with many parallel veins, and attached by a broad
base; pistillate and staminate catkins of the nature of Antholithes.
Fruit winged or pulpy, of the kind known as _Cardiocarpum_. Stem
with a Sternbergia pith, usually large, surrounded by a ring of
pseudo-scalariform vessels, and with a cylinder usually narrow, of woody
wedges, with bordered pores in one or more series, and with simple
medullary rays.
From specimens kindly presented to me by Prof. Renault, I have been
able to ascertain that the stems of some at least of these plants
(Eucordaites) are distinct in structure from all the species of
_Dadoxylon_, above mentioned, except _D. Clarkii_, of the Erian. They may
be regarded as intermediate between those of conifers and cycads, which
is indeed the probable position of these remarkable plants.
Grand d'Eury has divided the _Cordaites_ into sub-genera, as follows:
1. _Eucordaites._--Leaves spatulate, obovate, elliptical, or lanceolate,
sessile, entire, with rounded apices and of leathery consistency. The
leaves are from twenty to ninety centimetres in length. The nerves are
either equally or unequally strong.
2. _Dorycordaites._--Leaves lanceolate, with sharp points; nerves
numerous, fine, and equal in strength. The leaves attain a length of from
forty to fifty centimetres.
3. _Poacordaites._--Leaves narrow, linear, entire, blunt at the point,
with nerves nearly equally strong. The leaves are as much as forty
centimetres in length.
To these Renault and Zeiller have added a fourth group, _Scutocordaites_.
_Genus_ Sternbergia.
This is merely a provisional genus intended to receive casts of the pith
cylinders of various fossil trees. Their special peculiarity is that,
as in the modern _Cecropia peltata_, and some species of _Ficus_, the
pith consists of transverse dense partitions which, on the elongation
of the internodes, become separated from each other, so as to produce a
chambered pith cavity, the cast of which shows transverse furrows. The
young twigs of the modern _Abies balsamifera_ present a similar structure
on a minute scale. I have ascertained and described such pith-cylinders
in large stems of _Dadoxylon Ouangondianum_, and _D. materiarium_. They
occur also in the stems of _Cordaites_ and probably of _Sigillariæ_. I
have discussed these curious fossils at length in "Acadian Geology" and
in the "Journal of the Geological Society of London," 1860. The following
summary is from the last-mentioned paper:
_a._ As Prof. Williamson and the writer have shown, many of the
_Sternbergia_ piths belong to coniferous trees of the genus _Dadoxylon_.
_b._ A few specimens present multiporous tissue, of the type of
_Dictyoxylon_, a plant of unknown affinities, and which, according to
Williamson, has a _Sternbergia_ pith.
_c._ Other examples show a true scalariform tissue, comparable with that
of _Lepidodendron_ or _Sigillaria_, but of finer texture. Corda has shown
that plants of the type of the former genus (his _Lomatophloios_) had
_Sternbergia_ piths. Some plants of this group are by external characters
loosely reckoned by botanists as ribless _Sigillariæ_ (_Clathraria_); but
I believe that they are not related even ordinally to that genus.
_d._ Many Carboniferous _Sternbergiæ_ show structures identical with
those described above as occurring in _Cordaites_, and also in some of
the trees ordinarily reckoned as _Sigillariæ_.
_Genus_ Cardiocarpum.
I have found at least eight species of these fruits in the Erian and
Carboniferous of New Brunswick and Nova Scotia, all of which are
evidently fruits of gymnospermous trees. They agree in having a dense
coaly nucleus of appreciable thickness, even in the flattened specimens,
and surrounded by a thin and veinless wing or margin. They have thus
precisely the appearance of samaras of many existing forest-trees, some
of which they also resemble in the outline of the margin, except that the
wings of samaras are usually veiny. The character of the nucleus, and the
occasional appearance in it of marks possibly representing cotyledons or
embryos, forbids the supposition that they are spore-cases. They must
have been fruits of phænogams. Whether they were winged fruits or seeds,
or fruits with a pulpy envelope like those of cycads and some conifers,
may be considered less certain. The not infrequent distortion of the
margin is an argument in favour of the latter view, though this may also
be supposed to have occurred in samaras partially decayed. On the other
hand, their being always apparently flattened in one plane, and the
nucleus being seldom, if ever, found denuded of its margin, are arguments
in favour of their having been winged nutlets or seeds. Until recently I
had regarded the latter view as more probable, and so stated the matter
in the second edition of "Acadian Geology." I have, however, lately
arrived at the conclusion that the _Cardiocarpa_ of the type of _C.
cornutum_ were gymnospermous seeds, having two cotyledons embedded in an
albumen and covered with a strong membranous or woody tegmen surrounded
by a fleshy outer coat, and that the notch at the apex represents the
foramen or micropyle of the ovule. The structure was indeed very similar
to that of the seeds of _Taxus_ and of _Salisburia_. With respect to some
of the other species, however, especially those with very broad margins,
it still appears likely that they were winged.
The _Cardiocarpa_ were borne in racemes or groups, and it seems certain
that some of them at least are the seeds of _Cordaites_. The association
of some of them and of those of the next genus with _Sigillariæ_ is so
constant that I cannot doubt that some of them belong to plants of that
genus, or possibly to taxine conifers. The great number of distinct
species of these seeds, as compared with that of known trees which could
have produced them, is very remarkable.
_Genus_ Trigonocarpum.
These are large angled nuts contained in a thick envelope, and showing
internal structures resembling those of the seeds of modern _Taxineæ_.
There are numerous species, as well as allied seeds referred to the
provisional genera _Rhabdocarpus_ and _Carpolithes_. In _Trigonocarpum
Hookeri_ I have described the internal structure of one of those seeds,
and many fine examples from the coal-field of St. Etienne, in France,
have been described by Brongniart, so that their internal structure is
very well known.
_Genus_ Antholithes.
This is also a provisional genus, to include spikes of floral organs,
some of which are known to have belonged to _Cordaites_, others probably
to _Sigillariæ_.
Of Uncertain Affinities.
_Family_ Sigillariaceæ.
Under this name palæobotanists have included a great number of trees
of the Carboniferous system, all of which are characterised by broad
leaf-sears, with three vascular scars, and usually arranged in
vertical rows, and by elongated three-nerved leaves, and roots of the
stigmaria type--that is, with rounded pits, marking the attachment of
rootlets spirally arranged. These trees, however, collected in the
genus Sigillaria by arbitrary characters, which pass into those of
the Lepidodendroid trees, have been involved in almost inextricable
confusion, to disentangle which it will be necessary to consider: 1. The
external characters of _Sigillariæ_, and trees confounded with them. 2.
Subdivision of _Sigillariæ_ by external markings. 3. The microscopic
character of their stems. 4. What is known of their foliage and fruit.
1. _Characters of Sigillaroid and Lepidodendroid Trunks_.
It may be premised that the modes of determination in fossil botany
are necessarily different from those employed in recent botany. The
palæobotanist must have recourse to characters derived from the leaves,
the scars left by their fall, and the internal structures of the stem.
These parts, held in little esteem by botanists in describing modern
plants, and much neglected by them, must hold the first place in the
regard of the fossil botanist, whereas the fructification, seldom
preserved, and generally obscure, is of comparatively little service. It
is to be remarked also that in such generalised plants as those of the
Palæozoic, remarkable rather for the development of the vegetative than
of the reproductive organs, the former rise in importance as compared
with their value in the study of modern plants.
In _Sigillariæ_, _Lepidodendra_, &c., the following surfaces of the stem
may be presented to our inspection:
1. The outer surface of the epidermis without its leaves, but with
the leaf-bases and leaf-scars more or less perfectly preserved. On
this surface we may recognise: (1) Cellular swellings or projections
of the bark to which the leaves are attached. These may be called
leaf-bases, and they are sometimes very prominent. (2) The actual mark
of the attachment of the leaf situated in the most prominent part of
the leaf-base. This is the _leaf-scar_. (3) In the leaf-scar when well
preserved we can see one or more minute punctures or prominences which
are the points where the vascular bundles passing to the leaf found exit.
These are the vascular scars.
When the leaves are attached, the leaf-scars and vascular scars cannot
be seen, but the leaf-bases can be made out. Hence it is important, if
possible, to secure specimens with and without the leaves. In flattened
specimens the leaf-bases are often distorted by pressure and marked with
furrows which must not be mistaken for true structural characters. The
leaf-bases, which are in relief on the outer surface of the stem, of
course appear as depressions on the mould in the containing rock, in
which the markings often appear much more distinctly than on the plant
itself.
2. The outer surface of the epidermis may have been removed or may be
destroyed by the coarseness of the containing rock. In this case the
leaf-bases are usually preserved on the surface of the outer or corky
bark, but the leaf-scars and vascular scars have disappeared. This gives
that condition of Lepidodendroid trees to which the name _Knorria_ has
been applied. When plants are in this state careful inspection may
sometimes discover traces of the leaf-scars on portions of the stem, and
thus enable the _Knorria_ to be connected with the species to which it
belongs.
3. The outer or corky bark may be removed, exposing the surface of the
inner or fibrous and cellular bark, which in the plants in question is
usually of great thickness. In this case neither the leaf-bases nor the
scars are seen, but punctures or little furrows or ridges appear where
the vascular bundles entered the inner bark. Specimens in this state are
usually said to be decorticated, though only the outer bark is removed.
It is often difficult to determine plants in this condition, unless some
portion of the stem can be found still retaining the bark; but when care
is taken in collecting, it will not infrequently be found that the true
outer surface can be recovered from the containing rock, especially if
a coaly layer representing the outer bark intervenes between this and
the inner impression. Specimens of this kind, taken alone, have been
referred to the genera _Knorria_, _Bothrodendron_, and _Halonia_.
4. In some cases, though not frequently, the outer surface of the
ligneous cylinder is preserved. It almost invariably presents a regularly
striated or irregularly wrinkled appearance, depending upon the vertical
woody wedges, or the positions of the medullary rays or vascular bundles.
Specimens of this kind constituted some of the _Endogenites_ of the
older botanists, and the genus _Schizodendron_ of Eichwald appears to
include some of them. Many of them have also been incorrectly referred to
Calamites.
5. In some cases the cast of the medullary cylinder or pith may alone
be preserved. This may be nearly smooch or slightly marked by vertical
striæ, but more usually presents a transverse striation, and not
infrequently the transverse constrictions and septa characteristic of the
genus Sternbergia. Loose _Sternbergiæ_ afford little means of connecting
them with the species to which they belong, except by the microscopic
examination of the shreds of the ligneous cylinder which often cling to
them.[CS]
[CS] See my paper, "Journal of Geological Society," vol. xxvii.
These facts being premised, the following general statements may be made
respecting some of the more common Palæozoic genera, referring, however,
principally to the perfect markings as seen on the epidermis:
_Sigillaria._--Leaf-bases hexagonal or elongated, or confluent on a
vertical ridge. Leaf-scars hexagonal or shield-shaped. Vascular scars
three, the two lateral larger than the central. This last character
is constant, depending on the fact that the leaves of Sigillaria have
two or more vascular bundles. All so-called _Sigillariæ_ having the
central vascular scar largest, or only one vascular bundle, should be
rejected from this genus. In young branches of branching _Sigillariæ_ the
leaf-scars sometimes appear to be spiral, but in the older stems they
form vertical rows; interrupted, however, by transverse rows or bands
of _fruit-scars_, each with a single large central vascular scar, and
which have borne the organs of fructification. _Arthrocaulis_ of McCoy is
founded on this peculiarity.
_Syringodendron._--Differs from Sigillaria in the leaf-scars, which are
circular and with a single vascular bundle. It is a matter of doubt
whether these plants were of higher rank than Sigillaria tending toward
the pines, or of lower rank tending toward Cyclostigma. Their leaf-bases
form vertical ridges.
_Lepidodendron._--Leaf-bases rhombic, oval, or lanceolate, moderately
prominent. Leaf-scars rhombic or sometimes shield-shaped or heart-shaped,
in the middle or upper part of the leaf-base. Vascular scars three--the
middle one always largest and corresponding to the single nerve of the
leaf; the lateral ones sometimes obsolete.
In older stems three modes of growth are observed. In some species
the expansion of the bark obliterates the leaf-bases and causes the
leaf-scars to appear separated by wide spaces of more or less wrinkled
bark, which at length becomes longitudinally furrowed and simulates the
ribbed character of Sigillaria. In others the leaf-bases grow in size
as the trunk expands, so that even in large trunks they are contiguous
though much larger than those on the branches. In others the outer bark,
hardening at an early age, is incapable of either of the above changes,
and merely becomes cleft into deep furrows in the old trunks.
_Lepidophloios._--Leaf-bases transverse and prominent--often very much
so. Leaf-scars transversely rhombic or oval with three vascular scars,
the central largest. Leaves very long and one-nerved. Large strobiles
or branchlets borne in two ranks or spirally on the sides of the stem,
and leaving large, round scars (_cone-scars_), often with radiating
impressions of the basal row of scales.
Species with long or drooping leaf-bases have been included in
_Lepidophloios_ and _Lomatophloios_, Species with short leaf-bases and
cone-scars in two rows have been called _Ulodendron_, and some of them
have been included in Sigillaria (sub-genus _Clathraria_). Decorticated
stems are Bothrodendron and _Halonia_. Some of the species approach near
to the last genus, especially to the Lepidodendra with rhombic leaf-bases
like _L. tetragonum_.
_Cyclostigma._--Leaf-bases undeveloped. Leaf-scars circular or
horseshoe-shaped, small, with a central vascular scar. In old trunks of
Cyclostigma the leaf-scars become widely separated, and sometimes appear
in vertical rows. Young branches of Lepidodendron sometimes have the
leaf-scars similar to those of Cyclostigma.
_Leptophleum._--Leaf-bases flat, rhombic; leaf-scars obsolete; vascular
scar single, central. The last two genera are characteristically Devonian.
In contradistinction from the trees above mentioned, the following
general statements may be made respecting other groups:
In conifers the leaf-bases are usually elongated vertically, often scaly
in appearance, and with the leaf-scar terminal and round, oval, or
rhombic, and with a single well-marked vascular scar.
In Calamites, Calamodendron, and Asterophyllites the scars of the
branchlets or leaves are circular or oval, with only a single vascular
scar, and situated in verticils at the top of well-marked nodes of the
stem.
In tree-ferns the leaf-bases are large and usually without a distinct
articulating surface. The vascular bundles are numerous. Protopteris
has rounded leaf-scars with a large horseshoe-shaped bundle of vessels
above and small bundles below. Caulopteris has large elliptic or oval
leaf-scars with vascular scars disposed concentrically. Palæopteris,[CT]
of Geinitz, has the leaf-scars transversely oval and the vascular bundles
confluent in a transverse band with an appendage or outlying bundle
below. Stemmatopteris has leaf-scars similar to those of Caulopteris, but
the vascular bundles united into a horseshoe-shaped band.
[CT] This name, preoccupied by Geinitz, has been inadvertently misapplied
to the Devonian ferns of the genus _Archæopteris_.
2. _Subdivision of Sigillariæ in Accordance with their Markings_.
The following groups may be defined in this way; but, being based on one
character only, they are of course in all probability far from natural:
1. _Sigillaria_, Brongniart. Type, _Sigillaria reniformis_, Brongniart,
or _S. Brounii_, Dawson.--Stem with broad ribs, usually much broader than
the usually oval or elliptical tripunctate areoles, but disappearing at
base, owing to expansion of the stem. Leaves narrow, long, three-nerved.
2. _Rhytidolepis_, Sternberg. Type, _S. scutellata_, Brongniart.--Ribs
narrow, and often transversely striate. Areoles large, hexagonal or
shield-shaped, tripunctate. Leaves as in last group. Kings of rounded
scars on the stems and branches mark attachment of fruit. It is possible
that some of the smaller stems of this group may be branches of trees of
group first.
3. _Syringodendron_, Sternberg. Type, _S. organum_, L. and H., _S.
oculata_, Brongniart.--Stems ribbed; areoles small and round, and
apparently with a single scar, or three closely approximated. These are
rare, and liable to be confounded with decorticated examples of other
groups; but I have some specimens which unquestionably represent the
external surface.
4. _Favularia_, Sternberg. Type, _Sigillaria elegans_ of
Brongniart.--Leaf-bases hexagonal, or in young branches elliptical, in
vertical rows, but without distinct ribs, except in old or decorticated
stems. Fruit borne in verticils on the branches bearing transverse rows
of rounded scars. Leaves somewhat broad and longitudinally striate.
5. _Leioderma_, Goldenberg. Type, _S. Sydnensis_, Dawson.--Ribs obsolete.
Cortical and ligneous surfaces striate. Vascular scars double, elongate
longitudinally, and alike on cortical and inner surfaces. Areoles in rows
and distinct; stigmaria-roots striate, with small and distinct areoles.
6. _Clathraria_, Brongniart. Type, _S. Menardi_, Brongniart.--Areoles
hexagonal, not in distinct rows, but having a spiral appearance. Some
of the plants usually referred to this group are probably branches of
_Favularia_. Others are evidently fragments of plants of the genus
_Lepidophloios_.
3. _Internal Structures of Sigillaria-Stems_.
I long ago pointed out, on the evidence of the external markings and
mode of growth, that the stems of _Sigillariæ_ must have been exogenous,
and this conclusion has now been fully confirmed by the microscopic
researches of Williamson, not only in the case of _Sigillariæ_, but
of _Lepidodendra_ and _Calamodendra_ as well. Confining myself to my
own observations, three types of _Sigillariæ_ are known to me by their
internal structures, though I cannot certainly correlate all of these
with the external markings referred to above.
1. _Diploxylon_, in which the stem consists of a small internal axis
surrounded by a very thick inner bark and a dense outer cortex. A fine
example from the South Joggins is thus described:[CU]
[CU] "Journal of the Geological Society of London," November, 1877.
"The axis of the stem is about six centimetres in its greatest diameter,
and consists of a central pith-cylinder and two concentric coats of
scalariform tissue. The pith-cylinder is replaced by sandstone, and is
about one centimetre in diameter. The inner cylinder of scalariform
tissue is perfectly continuous, not radiated, and about one millimetre
in thickness. Its vessels are somewhat crushed, but have been of large
diameter. Its outer surface, which readily separates from that of the
outer cylinder, is striated longitudinally. The outer cylinder, which
constitutes by much the largest part of the whole, is also composed of
scalariform tissue; but this is radially arranged, with the individual
cells quadrangular in cross-section. The cross-bars are similar on all
the sides and usually simple and straight, but sometimes branching or
slightly reticulated. The wall intervening between the bars has extremely
delicate longitudinal waving lines of ligneous lining, in the manner
first described by Williamson as occurring in the scalariform tissue
of certain _Lepidodendra_. A few small radiating spaces, partially
occupied with pyrites, obscurely represent the medullary rays, which must
have been very feebly developed. The radiating bundles passing to the
leaves run nearly horizontally; but their structure is very imperfectly
preserved. The stem being old and probably long deprived of its leaves,
they may have been partially disorganised before it was fossilised. The
outer surface of the axis is striated longitudinally, and in some places
marked with impressions of tortuous fibres, apparently those of the inner
bark. In the cross-section, where weathered, it shows concentric rings;
but under the microscope these appear rather as bands of compressed
tissue than as proper lines of growth. They are about twenty in number.
This tree has an erect, ribbed trunk, twelve feet in height and fifteen
inches in diameter, swelling to about two feet at the base."
2. _Favularia Type._--This has been well described by Brongniart and
by Renault,[CV] and differs from the above chiefly in the fact that
the outer exogenous woody zone is composed of reticulated instead of
scalariform tissue, and the inner zone is of the peculiar form which I
have characterised as pseudo-scalariform.
[CV] "Botanique Fossile," Paris, 1881.
3. _Sigillaria Proper._--This I have illustrated in my paper in the
"Journal of the Geological Society" for May, 1871, and it appears
to represent the highest and most perfect type of the larger ribbed
_Sigillaria_. This structure I have described as follows, basing my
description on a very fine axis found in an erect stem, and on the
fragments of the woody axis found in the bases of other erect stems:
_a._ A dense cellular outer bark, usually in the state of compact
coal--but when its structure is preserved, showing a tissue of thickened
parenchymatous cells.
_b._ A very thick inner bark, which has usually in great part perished,
or been converted into coal, but which, in old trunks, contained a large
quantity of prosenchymatous tissue, very tough and of great durability.
This "bast-tissue" is comparable with that of the inner bark of modern
conifers, and constitutes much of the mineral charcoal of the coal-seams.
_c._ An outer ligneous cylinder, composed of wood-cells, either with
a single row of large bordered pores,[CW] in the manner of pines
and cycads, or with two, three, or four rows of such pores sometimes
inscribed in hexagonal areoles in the manner of Dadoxylon. This woody
cylinder is traversed by medullary rays, which are short, and composed
of few rows of cells superimposed. It is also traversed by oblique
radiating bundles of pseudo-scalariform tissue proceeding to the leaves.
In some _Sigillariæ_ this outer cylinder was itself in part composed of
pseudo-scalariform tissue, as in Brongniart's specimen of _S. elegans_;
and in others its place may have been taken by multiporous tissue, as in
a case above referred to; but I have no reason to believe that either of
these variations occurred in the typical ribbed species now in question.
The woody fibres of the outer cylinder may be distinguished most readily
from those of conifers, as already mentioned, by the thinness of their
walls, and the more irregular distribution of the pores. Additional
characters are furnished by the medullary rays and the radiating bundles
of scalariform tissue when these can be observed.
[CW] These are the same with the wood-cells elsewhere called discigerous
tissue, and to which I have applied the terms uniporous and multiporous.
The markings on the walls are caused by an unlined portion of the
cell-wall placed in a disk or depression, and this often surrounded by an
hexagonal rim of thickened wall; but in all cases these structures are
less pronounced than in _Dadoxylon_, and less regular in the walls of the
same cell, as well as in different layers of the tissues of the axis.
_d._ An inner cylinder of pseudo-scalariform tissue. I have adopted the
term pseudo-scalariform for this tissue, from the conviction that it is
not homologous with the scalariform ducts of ferns and other acrogens,
but that it is merely a modification of the discigerous wood-cells, with
pores elongated transversely, and sometimes separated by thickened bars,
corresponding to the hexagonal areolation of the ordinary wood-cells.
A similar tissue exists in cycads, and is a substitute for the spiral
vessels existing in ordinary exogens.
_e._ A large medulla, or pith, consisting of a hollow cylinder of
cellular tissue, from which proceed numerous thin diaphragms towards the
centre of the stem.
These structures of the highest type of _Sigillaria_ are on the one
hand scarcely advanced beyond those of Calamopitus, as described by
Williamson, and on the other approach to those of _Cordaites_, as seen in
specimens presented to me by Renault.
Finally, as to the fruit of _Sigillariæ_, I have no new facts to offer.
The strobiles or spikes associated with these trees have been variously
described as gymnospermous (Renault) or cryptogamous (Groldenberg and
Williamson). 1 have never seen them in place. Two considerations,
however, have always weighed with me in reference to this subject. One
is the constant abundance of Trigonocarpa and Cardiocarpa in the soil
of the Sigiliaria forests, as I have studied this at the South Joggins.
The other is that the rings of fruit-scars on the branches of Sigiliaria
are homologous with leaf-scars, not with branches, and therefore should
have borne single carpels and not cones or spikes of inflorescence. These
are merely suggestions, but I have no doubt they will be vindicated
by future discoveries, which will, I have no doubt, show that in the
family _Sigillariaceæ_ we have really two families, one possibly of
gymnospermous rank, or at least approaching to this, the other allied to
the Lepidodendra.
Cryptogamia.
(_Acrogenes._)
_Family_ Lepidodendreæ; _Genus_ Lepidodendron, Sternberg.
These are arboreal Lycopods having linear one-nerved leaves, stems
branching dichotomously, and with ovate or rhombic leaf-bases bearing
rhombic leaf-scars, often very prominent. The fruit is in scaly
strobiles, terminal or lateral, and there are usually, if not always,
macrospores and microspores in each strobile. The young branches and
stems have a central pith, a cylinder of scalariform tubes sending out
ascending bundles to the leaves through a thick cellular and fibrous
inner bark, and externally a dense cortex confluent with or consisting of
the leaf-bases. Older stems have a second or outer layer of scalariform
fibres in wedges with medullary rays, and strengthening the stem by a
true exogenous growth, much as in the Diploxylon type of Sigiliaria. The
development of this exogenous cylinder is different in amount and rate
in different species.[CX] This different development of the exogenous
axis is accompanied with appropriate external appearances in the stems,
and the changes which take place in their markings. These are of three
kinds. In some species the areoles, at first close together, become, in
the process of the expansion of the stem, separated by intervening spaces
of bark in a perfectly regular manner; so that in old stems, while widely
separated, they still retain their arrangement, while in young stems they
are quite close to one another. This is the case in _L. corrugatum_. In
other species the leaf-scars or bases increase in size in the old stems,
still retaining their forms and their contiguity to each other. This
is the case in _L. undulatum_, and generally in those _Lepidodendra_
which have large leaf-bases. In these species the continued vitality
of the bark is shown by the occasional production of lateral strobiles
on large branches, in the manner of the modern red pine of America. In
other species the areoles neither increase in size nor become regularly
separated by growth of the intervening bark; but in old stems the bark
splits into deep furrows, between which may be seen portions of bark
still retaining the areoles in their original dimensions and arrangement.
This is the case with _L. Pictoense_. This cracking of the bark no doubt
occurs in very old trunks of the first two types, but not at all to the
same extent.
[CX] See "Memoirs of Dr. Williamson," in "Philosophical Transactions,"
for ample details.
As a type of Lepidodendron, I may describe one of the oldest
Carboniferous species characteristic of the Lower Carboniferous in
America, and corresponding to _L. Veltheimianum_ of Europe.
Lepidodendron Corrugatum, Dawson.--(See Fig. 43, _supra_.) "Quarterly
Journal of Geological Society," vol. xv.; "Acadian Geology," page 451.
_Habit of Growth._--Somewhat slender, with long branches and long,
slender leaves having a tendency to become horizontal or drooping.
_Markings of Stem._--Leaf-bases disposed in quincunx or spirally,
elongate, ovate, acute at both ends, but more acute and slightly
oblique at the lower end; most prominent in the upper third, and with
a slight vertical ridge. Leaf-scars small, rounded, and showing only
a single punctiform vascular scar. The leaf-scar on the outer surface
is in the upper third of the base; but the obliquity of the vascular
bundle causes it to be nearly central on the inside of the epidermis.
In young succulent shoots the leaf-scars are contiguous and round as
in Cyclostigma, without distinct leaf-bases. In this state it closely
resembles _L. Olivieri_, Eichwald.[CY]
[CY] Lethæa Rossica, Plate Y, Figs. 12, 13.
In the ordinary young branches the leaf-scars are contiguous, and closely
resemble those of _L. elegans_, Brongt. (Fig. 43 C). As the branches
increase in diameter the leaf-scars slightly enlarge and sometimes assume
a verticillate appearance (Fig. 43 D). As they still further enlarge they
become separated by gradually increasing spaces of bark, marked with many
waving striæ or wrinkles (Fig. 43 I, N). At the base of old stems the
bark assumes a generally wrinkled appearance without distinct scars.
_Knorria or Decorticated States._--Of these there is a great variety,
depending on the state of preservation, and the particular longitudinal
ridges. Fig. 43 D shows a form in which the vascular bundles appear
as cylindrical truncate projections. Other forms show the leaf-bases
prominent, or have an appearance of longitudinal ribbing produced by the
expansion of the bark.
_Structure of Stem._--This is not perfectly preserved in any of my
specimens, but one flattened specimen shows a central medulla with a
narrow ring of scalariform vessels surrounding it, and constituting the
woody axis. The structure is thus similar to that of _L. Harcourtii_,
which I regard as probably the same with the closely allied European
species L. Veltheimianum.
_Leaves._--These are narrow, one-nerved, curving somewhat rapidly outward
(Figs. 43, B, C, D). They vary from one to two inches in length.
_Roots._--I have not seen these actually attached, but they occur very
abundantly in the under-clays of some erect forests of these plants at
Horton Bluff, and are of the character of Stigmariæ (Figs. 30, 31). In
some of the under-clays the long, flattened rootlets are excessively
abundant, and show the mark of a central vascular bundle.
_Fructification._--Cones terminal, short, with many small, acute
imbricate scales. Spore-cases globular, smooth (Fig. 43 C). On the
surface of some shales and sandstones at Horton there are innumerable
round spore-cases of this tree about the size of mustard-seed (Fig. 43
F). Large slabs are sometimes covered with these, and thin layers of
shale are filled with flattened specimens.
This is the characteristic species of the Lower Carboniferous
coal-measures, occurring in great profusion at Horton Bluff and its
vicinity, also at Sneid's Mills near Windsor, Noel and Five-Mile River,
at Norton Creek and elsewhere in New Brunswick (Matthew's collection),
and at Antigonish (Honeyman's collection).
I have received from the lowest Carboniferous beds of Ohio specimens
of this species.[CZ] According to Rogers and Lesquereux similar forms
occur in the Vespertine of Pennsylvania and in the Lower Carboniferous
of Illinois. _L. Veltheimianum_ of western Europe and _L. glincanum_ of
Russia are closely allied Lower Carboniferous species.[DA]
[CZ] "Journal of Geological Society," November, 1862, p. 313.
[DA] For comparisons of these see "Report on Plants of Lower
Carboniferous of Canada," p. 21.
A very different type is furnished by a new species from the middle
coal-formation of Clifton, New Brunswick.
Lepidodendron Cliftonense, Dawson.--Habit of Growth.--Robust, with thick
branches, and leaves several inches in length. Terminal branches becoming
slender, with shorter leaves.
_Markings of Stem._--Leaf-bases long oval, pointed at ends, enlarging
with growth of stem. Leaf-scars central, rhombic, transverse.
_Leaves._--One-nerved, acutely pointed, from four inches in length on the
larger branches to one inch or less on the branchlets.
_Fructification._--Cones large, cylindrical or long oval, with large
scales of trigonal form, and not elongated but lying close to the
surface. Borne on lateral, slender branchlets, with short leaves.
_Genus_ Lepidophloios, Sternberg; Ulodendron, L. and H.; Lomatophloios,
Corda.
_Lepidophloios._--Under this generic name, established by Sternberg,
I include those lycopodiaceous trees of the coal-measures which have
thick branches, transversely elongated leaf-scars, each with three
vascular points and placed on elevated or scale-like protuberances,
long one-nerved leaves, and large lateral strobiles in vertical rows or
spirally disposed. Their structure resembles that of _Lepidodendron_,
consisting of a _Sternbergia_ pith, a slender axis of large scalariform
vessels, giving off from its surface bundles of smaller vessels to the
leaves, a very thick cellular bark, and a thin dense outer bark, having
some elongated cells or bast-tissue on its inner side. In these trees the
exogenous outer cylinder is less developed than in the Lepidodendra, and
is sometimes wanting in stems or branches of some thickness.
Regarding _L. laricinum_ of Sternberg as the type of the genus, and
taking in connection with this the species described by Goldenberg,
and my own observations on numerous specimens found in Nova Scotia, 1
have no doubt that _Lomatophloios crassicaulis_ of Corda, and other
species of that genus described by Goldenberg, _Ulodendron_ and
_Bothrodendron_ of Lindley, _Lepidodendron ornatissimum_ of Brongniart,
and _Halonia punctata_ of Geinitz, all belong to this genus, and differ
from each other only in conditions of growth and preservation. Several
of the species of _Lepidostrobus_ and _Lepidophyllum_ also belong to
_Lepidophloios_.
The species of _Lepidophloios_ are readily distinguished from
_Lepidodendron_ by the form of the areoles, and by the round scars on the
stem, which usually mark the insertion of the large strobiles, though
in barren stems they may also have produced branches; still, the fact
of my finding the strobiles _in situ_ in one instance, the accurate
resemblance which the scars bear to those left by the cones of the red
pine when borne on thick branches, and the actual impressions of the
radiating scales in some specimens, leave no doubt in my mind that
they are usually the marks of cones; and the great size of the cones of
_Lepidophloios_ accords with this conclusion.
The species of _Lepidophloios_ are numerous, and individuals are quite
abundant in the coal formation, especially toward its upper part. Their
flattened bark is frequent in the coal-beds and their roofs, affording
a thin layer of pure coal, which sometimes shows the peculiar laminated
or scaly character of the bark when other characters are almost entirely
obliterated. The leaves also are nearly as abundant as those of
Sigillaria in the coal-shales. They can readily be distinguished by their
strong, angular midrib.
The markings of _Lepidophloios_ may easily be mistaken for those of the
_Clathraria_ type of _Sigillaria_. When the stem only is seen, they can
be distinguished by the length of the leaf-bases in _Lepidophloios_, and
by the dominant central vascular scar; also by the one-nerved and ribbed
leaves. Where the large, round marks of the cones are present, these are
an infallible guide, never being present in _Sigillaria_. As the cones
grew on the upper sides of the branches, the impression of the lower side
often shows no cone-scars, or only two lateral rows, whereas on the upper
side of the same branch they appear spirally arranged. I may describe as
an example--
_Lepidophloios Acadianus_, Dawson. Leaf-bases broadly rhombic, or in old
stems regularly rhombic, prominent, ascending, terminated by very broad
rhombic scars having a central point and two lateral obscure points.
Outer bark laminated or scaly. Surface of inner bark with single points
or depressions. Leaves long, linear, with a strong keel on one side,
five inches or more in length. Cone-scars sparsely scattered on thick
branches, either in two rows or spirally, both modes being sometimes seen
on the same branch. Scalariform axis scarcely an inch in diameter in a
stem five inches thick. Fruit, an ovate strobile with numerous acute
scales covering small globular spore-cases. This species is closely
allied to _Ulodendron majus_ and _Lepidophloios laricinus_, and presents
numerous varieties of marking. Coal-formation, Nova Scotia.
_Family_ Calamiteæ; _Genus_ Calamites, Suckow.
The plants of this genus are unquestionably allied to the modern
_Equisetaceæ_, but excel these so much in variety of form and structure,
and are so capricious in their states of preservation, and so liable to
be mistaken for parts of plants generically different, that they have
given rise to much controversy. The following considerations will enable
us to arrive at some certainty.
The genus _Calamites_ was originally founded in the longitudinally
ribbed and jointed stems so frequent in the coal-formation, and of which
the common _C. Suckovii_ is a typical form. The most perfect of these
stems represent the outer surface immediately within the epidermis, in
which case transverse lines or constrictions mark the nodes, and at the
nodes there are rounded spots, sometimes indicating radial processes of
the pith, first described by Williamson; in other cases, the attachment
of branchlets, or in some specimens both. But some specimens show the
outer surface of the epidermis, in which case the transverse nodal
lines are usually invisible, though the scars of branchlets may appear.
In still other examples the whole of the outer tissues have perished,
and the so-called Calamite is a cast of the interior of the stem,
showing merely longitudinal ribbing and transverse nodal constrictions.
In studying these plants _in situ_ in the erect Calamite brakes of
the coal-formation of Nova Scotia, one soon becomes familiar with
these appearances, but they are evidently unknown to the majority of
palæobotanists, though described in detail more than twenty years ago.
When the outer surface is preserved it is sometimes seen to bear
verticils of long needle-like leaves (_C. Cistii_), or of branchlets with
secondary whorls of similar leaves (_C. Suckovii_ and _C. undulatus_).
No Calamite known to me bears broad one-nerved leaves like those of
_Asterophyllites_ and _Annularia_, though the larger stems of these
plants have been described as Calamites, and the term _Calamocladus_ has
been used to include both groups. The base of the Calamite stem usually
terminates in a blunt point, and may be attached to a rhizome, or several
stems may bud out from each other in a group or stool. The roots are long
and cylindrical, sometimes branching. The fruit consists of spikes of
spore-cases, borne in whorls and subtended by linear floral leaves. To
these strobiles the name Calamostachys has been given.
Williamson has shown that the stem of Calamites consists of a central
pith or cavity of large size surrounded by a cylinder consisting of
alternate wedges of woody and cellular matter, with vertical canals at
the inner sides of the wedges, and slender medullary rays. The thick
cellular wedges intervening between the woody wedges he calls primary
medullary rays; the smaller medullary rays in the wedges, secondary
medullary rays. There is thus a highly complex exogenous stem based on
the same principle with the stem of a common _Equisetum_, but with much
greater strength and complexity.
Williamson has also shown that there are different sub-types of these
stems. More especially he refers to the three following:
(_a_) _Calamites_ proper, which has the woody wedges of scalariform or
barred tissue with thin medullary rays, and the thick primary medullary
rays are cellular.
(_b_) _Calamopitus_ has reticulated or multiporous tissue in the woody
wedges with medullary rays, and the primary medullary wedges are composed
of elongated cells.
(_c_) _Calamodendron_ has the woody wedges of barred tissue as in _a_,
with medullary rays, but has the intervening medullary wedges of an
elongated tissue approaching to woody fibre, and also with medullary rays.
To these I would add a fourth type, which I have described, from the
coal-formation of Nova Scotia.[DB]
[DB] "Quarterly Journal of the Geological Society," 1871.
(_d_) _Eucalamodendron_ differs from _Calamodendron_ in having true
bordered pores or pseudo-scalariform slit-pored tissue, and corresponds
to the highest type of calamitean stem.
I would also add that under _a_ and _b_ there are some species in which
the woody cylinder is very thin in comparison to the size of the stem. In
_c_ and _d_ the woody cylinder is thick and massive, and the stems are
often large and nodose.
As an example of an ordinary Calamite in which the external surface and
foliage are preserved, I may quote the following from my report on the
"Flora of the Lower Carboniferous and Millstone Grit," 1873:
Calamites Undulatus, Brongniart.--This species is stated by Brongniart
to be distinguished from the _C. Suckovii_, the characteristic Calamite
of the middle coal-formation, by its undulated ribs marked with peculiar
cellular reticulation. He suggests that it may be merely a variety of
_C. Suckovii_, an opinion in which Schimper coincides; but since I have
received large additional collections from Mr. Elder, containing not
only the stems and branches, but also the leaves and rhizomes, I am
constrained to regard it as a distinct though closely allied species.
The rhizomata are slender, being from one to two inches in diameter,
and perfectly flattened. They are beautifully covered with a cellular
reticulation on the thin bark, and show occasional round areoles marking
the points of exit of the rootlets. I have long been familiar with
irregular flattened stems thus reticulate, but have only recently been
able to connect them with this species of Calamite.
The main stems present a very thin carbonaceous bark reticulated like
the rhizomes. They have flat, broad ribs separated by deep and narrow
furrows, and undulated in a remarkable manner even when the stems are
flattened. This undulation is, however, perhaps an indication of vertical
pressure while the plant was living, as it seems to have had an unusually
thin and feeble cortical layer, and the undulations are apparently best
developed in the lower part of the stem. At the nodes the ribs are often
narrowed and gathered together, especially in the vicinity of the rounded
radiating marks which appear to indicate the points of insertion of
the branches. At the top of each rib we have the usual rounded areole,
probably marking the insertion of a primary branchlet.
The branches have slender ribs and distant nodes, from which spring
secondary branchlet s in whorls, these bearing in turn small whorls of
acicular leaflets much curved upward, and which are apparently round in
cross section and delicately striate. They are much shorter than the
leaves of _Calamites Suckovii_, and are less dense and less curved than
those of _C. nodosus_, which I believe to be the two most closely allied
species.
Lesquereux notices this species as characteristic of the lower part of
the Carboniferous in Arkansas.
It will be observed that I regard the striated and ribbed stems not as
internal axes, but as representing the outer surface of the plants. This
was certainly the case with the present species and with _C. Suckovii_
and _C. nodosus_. Other species, and especially those which belonged to
Calamodendron, no doubt had a smooth or irregularly wrinkled external
bark; but this gives no good ground for the manner in which some writers
on this subject confound Calamites with Calamodendra, and both with
Asterophyllites and Sphenophyllum. With this no one who has studied these
plants, rooted in their native soils, and with their appendages still
attached, can for a moment sympathise. One of the earliest geological
studies of the writer was a bed of these erect Calamites, which he
showed to Sir C. Lyell in 1844, and described in the "Proceedings of
the Geological Society" in 1851, illustrating the habit of growth as
actually seen well exposed in a sandstone cliff. Abundant opportunities
of verifying the conclusions formed at that time have since occurred,
the results of which have been summed up in the figures in Acadian
Geology, which, though they have been treated by some botanists as merely
restorations, are in reality representations of facts actually observed.
On these subjects, without entering into details, and referring for these
to the elaborate discussions of Schimper, Williamson, and McNab, and to
my paper on the subject, "Journal of the Geological Society," vol. xxvii,
p. 54, I may remark:
1. That the aërial stems of ordinary Calamites had a thin cortical layer,
with lacunæ and fibrous bundles and multiporous vessels--the whole not
differing much from the structure of modern Equiseta.
2. Certain arborescent forms, perhaps allied to the true Calamites,
as well as possibly the old underground stems of ordinary species[DC]
assumed a thick-walled character in which the tissues resembled the
wedges of an exogen, and abundance of pseudo-scalariform fibres were
developed, while the ribbing of the external surface became obsolete or
was replaced by a mere irregular wrinkling.
[DC] Williamson, "Transactions of the Royal Society." McNab, in
"Proceedings of the Edinburgh Botanical Society."
3. Sufficient discrimination has not been exercised in separating casts
of the internal cavities of Calamites and Calamodendron from those
representing other surfaces and the proper external surface.
4. There is no excuse for attributing to Calamites the foliage of
Annularia, Asterophyllites, and Sphenophyllum, since these leaves have
not been found attached to true Calamite stems, and since the structure
of the stems of Asterophyllites as described by Williamson, and that of
Sphenophyllum as described by the writer,[DD] are essentially different
from those of Calamites.
[DD] "Journal of the Geological Society," 1866.
5. As the species above described indicates, good external characters can
be found for establishing species of this genus, and these species are of
value as marks of geological age.
_Genus_ Archæocalamites, Sternberg.
This genus has been established to include certain Calamites of the
Devonian and Lower Carboniferous, in which the furrows on the stem do not
alternate at the nodes or joints, and the leaves in one species at least
bifurcate. _C. radiatus_, Brongniart, is the typical species. In North
America it occurs in the Erian, probably as low as the Middle Erian. In
Europe it has so far been recognised in the Lower Carboniferous only. I
have, however, seen stems from alleged Devonian beds in Devonshire which
may have belonged to this species.
_Family_ Asterophylliteæ; _Genus_ Asterophyllites, Brongniart.
Stems ribbed and jointed like the _Calamites_, but with inflated nodes
and a stout internal woody cylinder, which has been described by
Williamson. From the joints proceeded whorls of leaves or of branchlets,
bearing leaves which differed from those of _Calamites_ in their having
a distinct middle rib or vein. The fructification consisted of long
slender cones or spikes, having whorls of scales bearing the spore-cases.
Some authors speak of _Asterophyllites_ as only branches and leaves of
_Calamites_; but though at first sight the resemblance is great, a close
inspection shows that the leaves of Asterophyllites have a true midrib,
which is wanting in _Calamites_.
_Genus_ Annularia.--It is perhaps questionable whether these plants
should be separated from _Asterophyllites_, The distinction is that
they produce branches in pairs, and that their whorls of leaves are
one-sided and usually broader than those of _Asterophyllites_, and united
into a ring at their insertion on the stem. One little species, _A.
sphenophylloides_, is very widely distributed.
Pinnularia--a provisional genus---includes slender roots or stems
branching in a pinnate manner, and somewhat irregularly. They are very
abundant in the coal shales, and were probably not independent plants,
but aquatic roots belonging to some of the plants last mentioned.
The probability of this is farther increased by their resemblance in
miniature to the roots of _Calamites_. They are always flattened, but
seem originally to have been round, with a slender thread-like axis of
scalariform vessels, enclosed in a soft, smooth, cellular bark.
_Family_ Rhizocarpeæ; _Genus_ Sphenophyllum.
Leaves in whorls, wedge-shaped, with forking veins. Fructification on
spikes, with verticils of sporocarps. These plants are by some regarded
as allied to the _Calamiteæ_ and _Asterophylliteæ_, by others as a high
grade of Rhizocarps of the type of Marsilia. The stem had a star-shaped
central bundle of scalariform or reticulato-scalariform vessels.
_Genus_ Sporangites. (_Sporocarpon_, Williamson.)
Under this name we may provisionally include those rounded spherical
bodies found in the coal and its accompanying beds, and also in
the Erian, which may be regarded as Macrospores or Sporocarps of
Protosalvinia, or other Rhizocarpean plants akin to those described above
in Chapter III, which see for description.
_Genus_ Protosalvinia.--Under this we include sporocarps allied to those
of _Salvinia_, as described in Chapter III.
_Family_ Filices.
Under this head I shall merely refer to a few groups of special interest,
and to the provisional arrangement adopted for the fronds of ferns when
destitute of fructification.
The external appearances of trunks of tree-ferns have been already
referred to.
With respect to tree ferns, the oldest known examples are those from
the Middle Devonian of New York and Ohio, which I have described in the
"Journal of the Geological Society," 1871 and 1881. As these are of some
interest, I have reproduced their descriptions in a note appended to
Chapter III, which see.
The other forms most frequently occurring in the Carboniferous are
_Caulopteris_, _Palæopteris_, and _Megaphyton_[DE] Stems showing merely
masses of aërial roots are known by the name _Psaronius_.
[DE] See my "Acadian Geology," also below.
With reference to the classification of Palæozoic ferns, this has
hitherto been quite arbitrary, being based on mere form and venation of
fronds, but much advance has recently been made in the knowledge of their
fructification, warranting a more definite attempt at classification. The
following are provisional genera usually adopted:
1. _Cyclopteris_, Brongniart.--Leaflets more or less rounded or
wedge-shaped, without midrib, the nerves spreading from the point of
attachment. This group includes a great variety of fronds evidently of
different genera, were their fructification known; and some of them
probably portions of fronds, the other parts of which may be in the next
genus.
2. _Neuropteris_, Brongniart.--Fronds pinnate, and with the leaflets
narrowed at the base; midrib often not distinct, and disappearing toward
the apex. Nervures equal, and rising at an acute angle. Ferns of this
type are among the most abundant in the coal-formation.
3. _Odontopteris_, Brongniart.--In these the frond is pinnate, and
the leaflets are attached by their whole base, with the nerves either
proceeding wholly from the base, or in part from an indistinct midrib,
which soon divides into nervures.
4. _Dictyopteris_, Gutbier.--This is a beautiful style of fern, with
leaflets resembling those of _Neuropteris_, but the veins arranged
in a network of oval spaces. Only a few species are known in the
coal-formation.
5. _Lonchopteris_, Brongniart.--Ferns with netted veins like the above,
but with a distinct midrib, and the leaflets attached by the whole base.
Of this, also, we can boast but few species.
6. _Sphenopteris_, Brongniart.--These are elegant ferns, very numerous
in species, and most difficult to discriminate. Their most distinctive
characters are leaflets narrowed at the base, often lobed, and with
nervures dividing in a pinnate manner from the base.
7. _Phyllopteris_, Brongniart.--These are pinnate, with long lanceolate
pinnules, having a strong and well-defined midrib, and nerves proceeding
from it very obliquely, and dividing as they proceed toward the margin.
The ferns of this genus are for the most part found in formations more
recent than the Carboniferous; but I have referred to it, with some
doubt, one of our species.
8. _Alethopteris_, Brongniart.--This genus includes many of the most
common coal-formation ferns, especially the ubiquitous _A. lonchitica_,
which seems to have been the common brake of the coal-formation,
corresponding to _Pteris aquilina_ in modern Europe and America. These
are brake-like ferns, pinnate, with leaflets often long and narrow,
decurrent on the petiole, adherent by their whole base, and united at
base to each other. The midrib is continuous to the point, and the
nervures run off from it nearly at right angles. In some of these ferns
the fructification is known to have been marginal, as in _Pteris_.
9. _Pecopteris_, Brongniart.--This genus is intermediate between the
last and _Neuropteris_. The leaflets are attached by the whole base,
but not usually attached to each other; the midrib, though slender,
attains to the summit; the nervures are given off less obliquely than
in _Neuropteris_. This genus includes a large number of our most common
fossil ferns.
10. _Beinertia_, Goeppert.--A genus established by Goeppert for a curious
Pecopteris-like fern, with flexuous branching oblique nervures becoming
parallel to the edge of the frond.
11. _Hymenophyllites_, Goeppert.--These are ferns similar to
Sphenopteris, but divided at the margin into one-nerved lobes, in the
manner of the modern genus _Hymenophyllum_.
12. _Palæopteris_, Geinitz.--This is a genus formed to include certain
trunks of tree-ferns with oval transverse scars of leaves.
13. _Caulopteris_, Lindley and Hutton.--Is another genus of fossil trunks
of tree-ferns, but with elongate scars of leaves.
14. _Psaronius_, Cotta.--Includes other trunks of tree-ferns with
alternate scars or thick scales, and ordinarily with many aërial roots
grouped round them, as in some modern tree-ferns.
15. _Megaphyton_, Artis.--Includes trunks of tree-ferns which bore their
fronds, which were of great size, in two rows, one on each side of the
stem. These were very peculiar trees, less like modern ferns than any
of the others. My reasons for regarding them as ferns are stated in the
following extract from a recent paper:
"Their thick stems, marked with linear scars and having two rows of
large depressed areoles on the sides, suggest no affinities to any known
plants. They are usually ranked with _Lepidodendron_ and _Ulodendron_,
but sometimes, and probably with greater reason, are regarded as allied
to tree-ferns. At the Joggins a very fine species (_M. magnificum_) has
been found, and at Sydney a smaller species (_M. humile_); but both
are rare and not well preserved. If the large scars bore cones and the
smaller bore leaves, then, as Brongniart remarks, the plant would much
resemble _Lepidophloios_, in which the cone-scars are thus sometimes
distichous. But the scars are not round and marked with radiating scales
as in _Lepidophloios_; they are reniform or oval, and resemble those
of tree-ferns, for which reason they may be regarded as more probably
leaf-scars; and in that case the smaller linear scars would indicate
ramenta, or small aërial roots. Further, the plant described by Corda
as _Zippea disticha_ is evidently a _Megaphyton_, and the structure
of that species is plainly that of a tree-fern of somewhat peculiar
type. On these grounds I incline to the opinion of Geinitz that these
curious trees were allied to ferns, and bore two rows of large fronds,
the trunks being covered with coarse hairs or small aërial roots. At
one time I was disposed to suspect that they may have crept along the
ground; but a specimen from Sydney shows the leaf-stalks proceeding from
the stem at an angle so acute that the stem must, I think, have been
erect. From the appearance of the scars it is probable that only a pair
of fronds were borne at one time at the top of the stem; and, if these
were broad and spreading, it would be a very graceful plant. To what
extent plants of this type contributed to the accumulation of coal I have
no means of ascertaining, their tissues in the state of coal not being
distinguishable from those of ferns and _Lycopodiaceæ_."
16. For descriptions of the genus _Archæopteris_ and other Erian ferns,
see Chapter III.
CHAPTER V.
THE FLORA OF THE EARLY MESOZOIC.
Great physical changes occurred at the close of the Carboniferous age.
The thick beds of sediment that had been accumulating in long lines
along the primitive continents had weighed down the earth's crust. Slow
subsidence had been proceeding from this cause in the coal-formation
period, and at its close vast wrinklings occurred, only surpassed by
those of the old Laurentian time. Hence in the Appalachian region of
America we have the Carboniferous beds thrown into abrupt folds, their
shales converted into hard slates, their sandstones into quartzite and
their coals into anthracite, and all this before the deposition of the
Triassic Red Sandstones which constitute the earliest deposit of the
great succeeding Mesozoic period. In like manner the coal-fields of Wales
and elsewhere in western Europe have suffered similar treatment, and
apparently at the same time.
This folding is, however, on both sides of the Atlantic limited to a
band on the margin of the continents, and to certain interior lines of
pressure, while in the middle, as in Ohio and Illinois in America, and
in the great interior plains of Europe, the coal-beds are undisturbed
and unaltered. In connection with this we have an entire change in the
physical character of the deposits, a great elevation of the borders
of the continents, and probably a considerable deepening of the seas,
leading to the establishment of general geographical conditions which
still remain, though they have been temporarily modified by subsequent
subsidences and re-elevations.
Along with this a great change was in progress in vegetable and animal
life. The flora and fauna of the Palæozoic gradually die out in the
Permian and are replaced in the succeeding Trias by those of the Mesozoic
time. Throughout the Permian, however, the remains of the coal-formation
flora continue to exist, and some forms, as the _Calamites_, even seem
to gain in importance, as do also certain types of coniferous trees. The
Triassic, as well as the Permian, was marked by physical disturbances,
more especially by great volcanic eruptions discharging vast beds and
dykes of lava and layers of volcanic ash and agglomerate. This was the
case more especially along the margins of the Atlantic, and probably
also on those of the Pacific. The volcanic sheets and dykes associated
with the Red Sandstones of Nova Scotia, Connecticut, and New Jersey are
evidences of this.
At the close of the Permian and beginning of the Trias, in the midst of
this transition time of physical disturbance, appear the great reptilian
forms characteristic of the age of reptiles, and the earliest precursors
of the mammals, and at this time the old Carboniferous forms of plants
finally pass away, to be replaced by a flora scarcely more advanced,
though different, and consisting of pines, cycads, and ferns, with
gigantic equiseta, which are the successors of the genus _Calamites_,
a genus which still survives in the early Trias. Of these groups the
conifers, the ferns, and the equiseta are already familiar to us, and, in
so far as they are concerned, a botanist who had studied the flora of the
Carboniferous would have found himself at home in the succeeding period.
The cycads are a new introduction. The whole, however, come within the
limits of the cryptogams and the gymnosperms, so that here we have no
advance.[DF]
[DF] Fontaine's "Early Mesozoic Flora of Virginia" gives a very good
summary of this flora in America.
[Illustration: Fig. 64.--Jurassic vegetation. Cycads and pines. (After
Saporta.)]
As we ascend, however, in the Mesozoic, we find new and higher types.
Even within the Jurassic epoch, the next in succession to the Trias,
there are clear indications of the presence of the endogens, in species
allied to the screw-pines and grasses; and the palms appear a little
later, while a few exogenous trees have left their remains in the Lower
Cretaceous, and in the Middle and Upper Cretaceous these higher plants
come in abundantly and in generic forms still extant, so that the dawn
of the modern flora belongs to the Middle and Upper Cretaceous. It will
thus be convenient to confine ourselves in this chapter to the flora of
the earlier Mesozoic.
Passing over for the present the cryptogamous plants already familiar
in older deposits, we may notice the new features of gymnospermous and
phænogamous life, as they present themselves in this earlier part of the
great reptilian age, and as they extended themselves with remarkable
uniformity in this period over all parts of the world. For it is a
remarkable fact that, if we place together in our collections fossil
plants of this period from Australia, India, China, Siberia, Europe,
or even from Greenland, we find wonderfully little difference in their
aspect. This uniformity we have already seen prevailed in the Palæozoic
flora; and it is perhaps equally marked in that of the Mesozoic. Still we
must bear in mind that some of the plants of these periods, as the ferns
and pines, for example, are still world-wide in their distribution; but
this does not apply to others, more especially the cycads (Fig. 65).
[Illustration: Fig. 65.--_Podozamites lanceolatus_, Sternb. L.
Cretaceous.]
The cycads constitute a singular and exceptional type in the modern
world, and are limited at present to the warmer climates, though very
generally distributed in these, as they occur in Africa, India, Japan,
Australia, Mexico, Florida, and the West Indies. In the Mesozoic age,
however, they were world-wide in their distribution, and are found as far
north as Greenland, though most of the species found in the Cretaceous
of that country are of small size/ and may have been of low growth, so
that they may have been protected by the snows of winter. The cycads
have usually simple or unbranching stems, pinnate leaves borne in a
crown at top, and fruits which, though somewhat various in structure and
arrangement, are all of the simpler form of gymnospermous type. The stems
are exogenous in structure, but with slender wood and thick bark, and
barred tissue, or properly as tissue intermediate between this and the
disc-bearing fibres of the pines.
Though the cycads have a considerable range of organisation and of
fructification, and though some points in reference to the latter might
assign them a higher place, on the whole they seem to occupy a lower
position than the conifers or the cordaiteæ of the Carboniferous.
In the Carboniferous some of the fern-like leaves assigned to the
genus _Noeggerathia_ have been shown by Stur and Weiss to have been
gymnosperms, probably allied to cycads, of which they may be regarded at
least as precursors. Thus the cycadean type does not really constitute an
advance in grade of organisation in the Mesozoic, any further than that,
in the period now in question, it becomes much more developed in number
and variety of forms. But the conifers would seem to have had precedence
of it for a long time in the Palæozoic, and it replaces in the Mesozoic
the _Cordaites_, which in many respects excelled it in complexity.
The greater part of the cycads of the Mesozoic age would seem to have had
short stems and to have constituted the undergrowth of woods in which
conifers attained to greater height. An interesting case of this is the
celebrated dirt-bed of the quarries of the Isle of Portland, long ago
described by Dean Buckland. In this fossil soil trunks of pines, which
must have attained to great height, are interspersed with the short,
thick stems of cycads, of the genus named _Cycadoidea_ by Buckland,
and which from their appearance are called "fossil birds' nests" by the
quarrymen. Some, however, must have attained a considerable height so as
to resemble palms.
The cycads, with their simple, thick trunks, usually marked with rhombic
scars, and bearing broad spreading crowns of large, elegantly formed
pinnate leaves, must have formed a prominent part of the vegetation
of the northern hemisphere during the whole of the Mesozoic period. A
botanist, had there been such a person at the time, would have found
this to be the case everywhere from the equator to Spitzbergen, and
probably in the southern hemisphere as well, and this throughout all
the long periods from the Early Trias to the Middle Cretaceous. In a
paper published in the "Linnæan Transactions" for 1868, Dr. Carruthers
enumerates twenty species of British Mesozoic cycads, and the number
might now be considerably increased.
[Illustration: Fig. 66.--_Salisburia_ (Gingko) _Sibirica_, Heer. L.
Cretaceous, Siberia and North America.]
The pines present some features of interest. We have already seen their
connection with the broad-leaved _Cordaites_, and in the Permian there
are some additional types of broad-leaved coniferæ. In the Mesozoic we
have great numbers of beautiful trees, with those elegant fan-shaped
leaves characteristic of but one living species, the Salisburia, or
gingko-tree of China. It is curious that this tree, though now limited
to eastern Asia, will grow, though it rarely fruits, in most parts of
temperate Europe, and in America as far north as Montreal, and that in
the Mesozoic period it occupied all these regions, and even Siberia and
Greenland, and with many and diversified species (Fig. 66).
_Salisburia_ belongs to the yews, but an equally curious fact applies to
the cypresses. The genus _Sequoia_, limited at present to two species,
both Californian, and one of them the so-called "big tree," celebrated
for the gigantic size to which it attains, is represented by species
found as far back at least as the Lower Cretaceous, and in every part
of the northern hemisphere.[DG] It seems to have thriven in all these
regions throughout the Mesozoic and early Kainozoic, and then to have
disappeared, leaving only a small remnant to represent it in modern days.
A number of species have been described from the Mesozoic and Tertiary,
all of them closely related to those now existing (Fig. 67).
[DG] In the Eocene of Australia.
[Illustration: Fig. 67.--_Sequoia Smithiana_, Heer. L. Cretaceous.]
The following notice of these trees is for the most part translated, with
some modifications and abridgment, from a paper read by the late Prof.
Heer before the Botanical Section of the Swiss Natural History Society:
The name itself deserves consideration. It is that of an Indian of the
Cherokee tribe, Sequo Yah, who invented an alphabet without any aid from
the outside world of culture, and taught it to his tribe by writing it
upon leaves. This came into general use among the Cherokees, before the
white man had any knowledge of it; and afterward, in 1828, a periodical
was published in this character by the missionaries. Sequo Yah was
banished from his home in Alabama, with the rest of his tribe, and
settled in New Mexico, where he died in 1843.
When Endlicher was preparing his synopsis of the conifers, in 1846, and
had established a number of new genera, Dr. Jacbon Tschudi, then living
with Endlicher, brought before his notice this remarkable man, and asked
him to dedicate this red-wooded tree to the memory of a literary genius
so conspicuous among the red men of America. Endlicher consented to do
so, and only endeavored to make the name pronounceable by changing two of
its letters.
Endlicher founded the genus on the redwood of the Americans, _Taxodium
sempervirens_ of Lamb; and named the species _Sequoia sempervirens_.
These trees form large forests in California, which extend along the
coast as far as Oregon. Trees are there met with of 300 feet in height
and 20 feet in diameter. The seeds have been brought to Europe a number
of years ago, and we already see in upper Italy and around the Lake of
Geneva, and in England, high trees; but, on the other hand, they have not
proved successful around Zurich.
In 1852, a second species of Sequoia was discovered in California, which,
under the name of big tree, soon attained a considerable celebrity.
Lindley described it, in 1853, as _Wellingtonia gigantea_; and, in the
following year, Decaisne and Torrey proved that it belonged to Sequoia,
and that it accordingly should be called _Sequoia gigantea_.
While the _Sequoia sempervirens_, in spite of the destructiveness of
the American lumbermen, still forms large forests along the coast, the
_Sequoia gigantea_ is confined to the isolated clumps which are met with
inland at a height of 5,000 to 7,000 feet above sea-level, and are much
sought after by tourists as one of the wonders of the country. Reports
came to Europe concerning the largest of them which were quite fabulous,
but we have received accurate accounts of them from Prof. Whitney. The
tallest tree measured by him has a height of 325 feet, and in the case of
one of the trees the number of the rings of growth indicated an age of
about 1,300 years. It had a girth of 50 to 60 feet.
We know only two living species of _Sequoia_, both of which are confined
to California. The one (_S. sempervirens_) is clothed with erect leaves,
arranged in two rows, very much like our yew-tree, and bears small, round
cones; the other (_S. gigantea_) has smaller leaves, set closely against
the branches, giving the tree more the appearance of the cypress. The
cones are egg-shaped, and much larger. These two types are therefore
sharply defined.
Both of these trees have an interesting history. If we go back into the
Tertiary, this same genus meets us with a long array of species. Two of
these species correspond to those living at present: the _S. Langsdorfii_
to the _S. sempervirens_, and the _S. Couttsiæ_ to the _S. gigantea_.[DH]
But, while the living species are confined to California, in the Tertiary
they are spread over several quarters of the globe.
[DH] _S. Couttsiæ_ has leaves like _S. gigantea_, and cones like those of
_8. sempervirens_.
Let us first consider the _Sequoia Langsdorfii_. This was first
discovered in the lignite of Wetterau, and was described as _Taxites
langsdorfii_. Heer found it in the upper Rhone district, and there lay
beside the twigs the remains of a cone, which showed that the _Taxites
Langsdorfii_ of Brongniart belonged to the Californian genus Sequoia
established by Endlicher. He afterward found much better preserved
cones, together with seeds, along with the plants of east Greenland,
which fully confirmed the determination. At Atanekerdluk in Greenland
(about 70° north latitude) this tree is very common. The leaves, and
also the flowers and numerous cones, leave no doubt that it stands very
near to the modern redwood. It differs from it, however, in having
a much larger number of scales in the cone. The tree is also found
in Spitzbergen at nearly 78° north latitude, where Nordenskiöld has
collected, at Cape Lyell, wonderfully preserved branches. From this high
latitude the species can be followed down through the whole of Europe
as far as the middle of Italy (at Senegaglia, Gulf of Spezia). In Asia,
also,, we can follow it to the steppes of Kirghisen, to Possiet, and to
the coast of the Sea of Japan, and across to Alaska and Sitka. It is
recognized by Mr. Starkie Gardner as one of the species found in the
Eocene of Mull in the Hebrides.[DI] It is thus known in Europe, Asia, and
America, from 43° to 78° north latitude, while its most nearly related
living species, perhaps even descended from it, is now confined to
California.
[DI] It is _Fareites Campbelli_ of Forbes.
With this _S. Langsdorfii_, three other Tertiary species are
nearly related (_S. brevifolia_, Hr., _S. disticha_, Hr., and _S.
Nordenskiöldi_, Hr.). These have been met with in Greenland and
Spitzbergen, and one of them has lately been found in the United States.
Three other species, in addition to these, have been described by
Lesquereux, which appear to belong to the group of the _S. Langsdorfii_,
viz., _S. longifolia_, Lesq., _S. angustifolia_, and _S. acuminata_,
Lesq. Several species also occur in the Cretaceous and Eocene of Canada.