4. Older Pliocene. } }
}
5. Upper Miocene. } Miocene. } Tertiary }
6. Lower Miocene. } } or }
} Cainozoic. }
7. Upper Eocene. } } }
8. Middle Eocene. } Eocene. } }
9. Lower Eocene. } } }
} Neozoic.
10. Maestricht Beds. } } }
11. White Chalk. } } }
12 Chloritic Series. } } Secondary }
13. Gault } Cretaceous. } or }
14. Neocomian. } } Mesozoic. }
15. Wealden. } } }
16. Purbeck Beds. } } }
17. Portland Stone. } } }
18. Kimmeridge Clay. } } }
19. Coral Rag. } Jurassic. } }
20. Oxford Clay. } } Secondary }
21. Great or Bath Oolite. } } or } Neozoic.
22. Inferior Oolite. } } Mesozoic. }
23. Lias. } } }
} }
24. Upper Trias. } } }
25. Middle Trias. } Triassic. } }
26. Lower Trias. } } }
27. Permian. Permian. }
28. Coal Measures. } }
29. Carboniferous } Carboniferous. }
limestone. } }
}
30. Upper } { Devonian or }
31. Middle } Devonian. { Old Red } }
32. Lower } { Sandstone. } Primary }
} or } Palæozoic.
33. Upper } } Palæozoic. }
34. Lower } Silurian. Silurian. }
}
35. Upper } }
36. Lower } Cambrian. Cambrian. }
}
37. Upper } }
38. Lower } Laurentian. Laurentian. }
NATURAL ORDERS TO WHICH FOSSIL PLANTS BELONG.
The plants found in different strata are either terrestrial or
aquatic, and the latter exhibit species allied to the salt and fresh
water vegetables of the present day. Their state of preservation
depends much on their structure. Cellular plants have probably in a
great measure been destroyed, and hence their rarity; while those
having a woody structure have been preserved. The following is the
number of fossil genera and species, as compiled from Unger's work on
Palæophytology--(Unger, Genera et Species Plantarum Fossilium, 1850).
DICOTYLEDONES. Genera. Species.
Thalamifloræ. 24 84
Calycifloræ 56 182
Corollifloræ 23 60
Monochlamydeæ Angiospermæ 48 221
------------- Gymnospermæ 56 363
MONOCOTYLEDONES.
Petaloideæ 38 130
Glumiferæ 5 12
ACOTYLEDONES.
Thallogenæ 31 203
Acrogenæ 121 969
Doubtful 35 197
---- ----
437 2421
These plants are arranged in the different strata as follows:--
{Cambrian, Silurian, and Devonian 73
Palæozoic {Carboniferous 683
{Permian 97
{Triassic 115
Mesozoic {Jurassic 294
{Cretaceous 183
{Eocene 414
Cainozoic {Miocene 496
{Pliocene 35
Recent Post-Pliocene 31
----
Fossil Species. 2421
During the twenty years that have elapsed since this enumeration was
made, the number of fossil species has been very greatly increased.
The proportion exhibited in this table is likewise greatly altered
from the enormous additions made to the Tertiary Flora by Unger,
Ettingshausen, and Heer, and from the important contributions by
Principal Dawson to the Devonian Flora.
Among the fossil Thalamifloral Dicotyledons, Unger mentions species
belonging to the orders--
Magnoliaceæ.
Anonaceæ.
Nymphæaceæ.
Capparidaceæ.
Malvaceæ.
Byttneriaceæ.
Tiliaceæ.
Aurantiaceæ.
Malpighiaceæ.
Aceraceæ.
Sapindaceæ.
Cedrelaceæ.
Zygophyllaceæ.
Xanthoxylaceæ.
Coriariaceæ.
Among Calycifloral Dicotyledons--
Celastraceæ.
Rhamnaceæ.
Anacardiaceæ.
Amyridaceæ.
Leguminosæ.
Rosaceæ.
Calycanthaceæ.
Combretaceæ.
Melastomaceæ.
Myrtaceæ.
Halorageaceæ.
Cucurbitaceæ.
Cornaceæ.
Loranthaceæ.
Rubiaceæ.
Among Corollifloral Dicotyledons--
Ericaceæ.
Styracaceæ.
Ebenaceæ.
Aquifoliaceæ.
Sapotaceæ.
Oleaceæ.
Apocynaceæ.
Gentianaceæ.
Among Monochlamydeous Angiosperms--
Nyctaginaceæ.
Lauraceæ.
Proteaceæ.
Aquilariaceæ.
Samydaceæ.
Santalaceæ.
Euphorbiaceæ.
Urticaceæ.
Artocarpaceæ.
Ceratophyllaceæ.
Salicaceæ.
Myricaceæ.
Betulaceæ.
Altingiaceæ.
Platanaceæ.
Corylaceæ.
Juglandaceæ.
Rafflesiaceæ.
Among Monochlamydeous Gymnosperms--
Coniferæ.
Taxaceæ.
Gnetaceæ.
Cycadaceæ.
Among Petaloid Monocotyledons--
Smilaceæ.
Orchidaceæ.
Zingiberaceæ.
Musaceæ.
Liliaceæ.
Palmæ.
Pandanaceæ.
Araceæ.
Typhaceæ.
Naiadaceæ.
Restiaceæ.
Among Glumiferous Monocotyledons--
Cyperaceæ.
Gramineæ.
Among Acrogenous Acotyledons--
Filices.
Marsileaceæ.
Lycopodiaceæ.
Equisetaceæ.
Musci.
Hepaticæ.
Among Thallogenous Acotyledons--
Lichenes.
Characeæ.
Algæ.
Fungi.
PERIODS OF VEGETATION AMONG FOSSIL PLANTS.
On taking a general survey of the known fossil plants, Brongniart
thought that he could trace three periods of vegetation,
characterised by the predominance of certain marked forms of
plants. In the ancient period there is a predominance of Acrogenous
Cryptogamic plants; this is succeeded by a period in which there is
a preponderance of Gymnospermous Dicotyledons; while a third period
is marked by the predominance of Angiospermous Dicotyledons. There
is thus--1. The reign of Acrogens, which includes the plants of the
Devonian, Carboniferous, and Permian periods. During these periods
there seems to be a predominance of Ferns, and a great development of
arborescent Lycopodiaceæ, such as Lepidodendron and Sigillaria, and
with them are associated some Gynmosperms, allied to Araucaria, and
some anomalous plants, as Noeggerathia. 2. The reign of Gymnosperms,
comprehending the Triassic and Jurassic periods. Here we meet with
numerous Coniferæ and Cycadaceæ, while Ferns are less abundant. 3.
The reign of Angiosperms, embracing the Cretaceous and the Tertiary
periods. This is characterised by the predominance of Angiospermous
Dicotyledons, a class of plants which constitute more than
three-fourths of the present vegetable productions of the globe, and
which appear to have acquired a predominance from the commencement of
the Tertiary formations. These plants appear sparingly even at the
beginning of the chalk formation in Europe, but are more abundant in
this formation as developed in North America.
FLORA OF THE PRIMARY OR PALÆOZOIC PERIOD.
REIGN OF ACROGENS.
In the present day, acrogenous plants are represented by cellular
and vascular Cryptogams. In considering fossil plants our attention
is specially directed to the latter. In the recent Floras, vascular
Acrogens are represented by such plants as Ferns, Lycopods, and
Equisetums. Some of them have an arborescent habit, but the greater
number are shrubby and herbaceous. Many of them have creeping
rhizomes, which are either subterranean, or run along the surface
of the ground. One of these arborescent forms is seen in Tree-ferns
(Fig. 9). Another form with a rhizome is seen in Fig. 10. The trunks
of ferns are marked by scars, which indicate the parts where the
bases of the fronds were attached, and where the vascular tissue
passes out from the interior (Fig. 11, _a_ and _b_). A transverse
section of the stem (Fig. 12) shows a continuous cylinder of
scalariform vessels (Fig. 13), enclosing a large mass of cellular
tissue frequently penetrated by small scalariform bundles. The
cylinder is pierced by meshes, from the inner sides of which rise the
vascular bundles going to the leaves, while some of the free bundles
of the axis pass through the mesh, carrying with them a portion of
the cellular tissue into the petiole. The fructification consists
of spore-cases (sporangia), often with an elastic ring round them,
containing spores in their interior (Fig. 14).
[Illustration: Fig. 9.]
[Sidenote: Fig. 9. Tree-fern, with a slender cylindrical trunk and a
crown of drooping fronds. It is a vascular acrogen.]
[Illustration: Fig. 10.]
[Sidenote: Fig. 10. _Asplenium_; a species of Spleenwort. A. Rhizome,
_r_, covered with the bases (stalks or stipes) of the fronds; _f_,
fronds in bud, rolled up in a circinate manner (this is very rarely
seen in fossil ferns); _g_, fronds bearing fructification on their
backs. B. Portion of a frond separated to show the linear sori or
clusters of sporangia (spore-cases).]
Among Acrogens of the present day there are also plants belonging
to the natural order Lycopodiaceæ or Club-mosses (Fig. 15), having
creeping stems, which give rise to leafy branches. The leaves are
small, sessile, and moss-like, and the fructification consists of two
kinds of cellular bodies, small spores or microspores (Fig. 16),
and large spores or macrospores (Fig. 17). They consist of cellular
and vascular tissues, the latter occurring in the form of woody,
annular, and scalariform vessels, which occupy the axis or central
part of the stem. They differ from ferns in the distribution of
their vascular bundles. The order is represented also by such plants
as Selaginella, Psilotum, Phylloglossum, and Isoetes. In the plant
called Isoetes (Quillwort) there is a peculiar short stem which does
not increase in height. It produces additions laterally, so that the
stem increases in thickness. The leaves continue to multiply, and
bear fructification at their bases. They have both large and small
spores.
[Illustration: Fig. 11, _a_. Fig. 11, _b_. Fig. 12.]
[Sidenote: Fig. 11, _a_. Bifurcating (forked or dichotomous) trunk
(caudex) of a Tree-fern (_Alsophila Perrottetiana_), showing the
scars (cicatrices) left by the fallen fronds. These scars exhibit
the arrangement of the vascular bundles. Fig. 11, _b_. Rhizome of
_Lastrea Filix-mas_ (male fern), showing scars of the leaves, _c_,
with markings of the vascular bundles.]
[Sidenote: Fig. 12. Transverse section of the stem (caudex) of a
Tree-fern (_Cyathea_), showing the arrangement of the cellular and
vascular tissue. The cellular tissue of the centre, _m_; that of
the circumference, _p_; vascular cylinder, _f v_, consisting of
dark-coloured pleurenchyma or ligneous tubes, _f_, and paler vessels,
_v_, chiefly scalariform and closed spiral, and pierced by the meshes
for the leaf-bundles at _m_; the outer cortical portion connected
with the bases of the leaves, _e_.]
[Illustration: Fig. 13-17.]
[Sidenote: Fig. 13. Scalariform vessels taken from a Tree-fern. They
are marked with bars like the steps of a ladder, hence their name.
The membrane occasionally disappears, so that the walls are made up
of fibres only at some parts.
Fig. 14. Sporangia of a Fern, supported on stalks, _p_, each of which
ends in an elastic cellular ring, _s_, partially surrounding the
spore-case, and opening it when mature.
Fig. 15. _Lycopodium clavatum_, a common Club-moss. The leafy branch,
_l_, ends in a stalk bearing two spikes of fructification, _f_.
Fig. 16. A kidney-shaped 2-valved case, containing small spores
(microspores) of Lycopodium.
Fig. 17. Two-valved case, containing large spores (macrospores) of
Selaginella.]
[Illustration: Fig. 18.]
[Sidenote: Fig. 18. Fructification of _Equisetum maximum_, Great
Water Horse-tail, showing the stalk surrounded by membranous sheaths,
_s s_, which are fringed by numerous processes called teeth. The
fructification, _f_, at the extremity, is in the form of a cone
bearing polygonal scales, under which are spore-cases containing
spores with filaments.]
Another important order of vascular Acrogens is the Equisetaceæ
or Horse-tails (Fig. 18). These are Cryptogams, having rhizomes,
bearing hollow, striated branches, which secrete in their epidermis
a considerable amount of silex. These branches are jointed and have
membranous sheaths at the articulations, which are whorls of leaves
reduced to a very rudimentary condition. The fructification consists
of cone-like bodies (Fig. 18, _f_) bearing peltate polygonal scales,
under which are spore-cases (Fig. 19), enclosing spores with four
hygrometric club-shaped filaments called elaters (Figs. 20 and 21).
At the present day some of these plants in tropical regions have
stems of 15 or 16 feet high.
[Illustration: Fig. 19-21.]
[Sidenote: Fig. 19. Polygonal scale, _s_, of a species of Horse-tail
(_Equisetum_), bearing membranous sacs, _t_, which open on their
inner surface to discharge spores.
Fig. 20. Spore of Equisetum, surrounded by two filaments
with club-shaped extremities. The filaments are represented as coiled
round the spore.
Fig. 21. Spore of Equisetum, with the filaments (elaters)
expanded.]
Among vascular Acrogens is included the natural order Marsileaceæ
or Rhizocarpeæ, the Pepperworts (Fig. 22). The order consists of
aquatic plants, with creeping stems, bearing leaves, which are either
linear, or divided into three or more wedge-shaped portions not
unlike clover. The fructification is at the base of the leaf-stalks,
and consists of sacs (sporocarps) containing spores of two kinds,
microspores and macrospores. The order contains Marsilea, Pilularia,
Azolla, and Salvinia.
For a fuller account of Acrogenous plants, see Balfour's Class Book
of Botany, p. 954.
These orders are represented in the Palæozoic flora. Many of the
fossil species assume a large size, and show a greater degree
of development than is seen in their recent congeners. The most
important coal plants belong to the Ferns, Lycopods, and Horse-tails.
The examination of the structure and conformation of the plants of
the present flora assists much in the determination of the fossil
carboniferous flora.
[Illustration: Fig. 22.]
[Sidenote: Fig. 22. _Marsilea Fabri_, a species of Pepperwort or
Rhizocarp, with a creeping stem, quadrifoliate stalked leaves on one
side, and roots on the other. The fructification, _s_, is at the base
of the leaves, and consists of sporangia, called sporocarps.]
In the lower Palæozoic strata the plants which have been detected
are few. In the Silurian and Cambrian systems, we meet with the
remains of ancient marine plants, as well as a few terrestrial
species. Even in the still older Laurentian rocks, if the remarkable
structure known as Eozoon canadense be considered, as it generally
is, an animal, the existence of contemporary plants may be inferred,
inasmuch as without vegetable life animals could not obtain food.
In the Lower Silurian or Grauwacke, near Girvan, Hugh Miller found
a species resembling Zostera in form and appearance. In the Lower
Old Red Sandstone of Scotland he detected Fucoids, a Lepidodendron,
and Lignite with a distinct Coniferous structure resembling that of
Araucaria,[1] besides a remarkable pinnate frond. In the middle Old
Red of Forfarshire, as seen in the Arbroath pavement, he found a
fern with reniform pinnæ and a Lepidodendron. In the Upper Old Red,
near Dunse, a Calamite and the well-known Irish fern Cyclopteris
Hibernica occur.[2] This fern, Palæopteris Hibernica of Schimper
(Plate I. Figs. 1 to 4), along with Sigillaria dichotoma, is very
abundant in beds of the same age in the south of Ireland, from
which the specimens described by Edward Forbes were obtained. The
fructification has recently been discovered. This shows that the fern
belongs to the Hymenophylleæ, and is consequently nearly related to
the equally famous Killarney fern, Trichomanes radicans.
Mr. Carruthers states that the frond-stalk of this fern is thick,
of considerable length, and clothed with large scales, which form
a dense covering at the somewhat enlarged base. The well-defined
separation observed in several specimens probably indicates that
the frond-stalks were articulated to the stem or freely separated
from it, and some root-like structures which occur on the slabs with
the ferns may be their creeping rhizomes. The pinnæ are linear,
obtuse, and almost sessile. The pinnules are numerous, overlapping,
of an ovate or oblong-ovate form, somewhat cuneate below, and with
a decurrent base. The veins are very numerous, uniform, repeatedly
dichotomous, and run out to the margin, where they form a slight
serration. Single pinnules rather larger than those of the pinnæ
are placed over the free spaces of the rachis, as was pointed out
by Brongniart. Carruthers has not met with any recent fern in which
this occurs; but it has been observed in several fossil species, as
in the allied American Palæopteris Halliana (Sch.), in Sphenopteris
erosa (Morris), and others. The pinnules are sometimes entirely,
but only partially fertile. The ovate-oblong sori are generally
single and two-lipped, the slit passing one-third of the way down
the sorus. The vein is continued as a free receptacle in the
centre of the cup or cyst, as in existing Hymenophylleæ, in which
it is included, not reaching beyond its entire portion. In some
specimens the receptacle is broad or thick, indicating the presence
of something besides itself in the cup, and giving the appearance
that would be produced if it were covered with sporangia; there is
no indication on the outer surface which might have been expected
from the separate sporangia. The compression of the specimens in the
rock, which has made the free receptacle appear like a vein on the
wall of the cup, together with the highly altered condition of the
rock in which the fossils are contained, accounts for the imperfect
preservation of the minute structures. The interpretation here given
of the fructification of this interesting fossil exhibits so close
a resemblance to what we find in the living genus Hymenophyllum,
that, were it not for the vegetative portions, it would be placed
in that genus. Several ferns have been described by Bunbury from
Devonian rocks at Oporto. A still more extensive and varied land
flora of Devonian age (or Erian, as he calls it) has been described
and illustrated by Principal Dawson from the rocks of that period
occurring in Canada; and during a recent visit to Britain he has
correlated many of the fragments collected by Miller, Peach, and
others, with the American species he has described. The following
are some of the fossil plants from beds older than the Carboniferous
system:[3]--Prototaxites Logani, Dadoxylon Ouangondianum, Calamites
transitionis, Asterophyllites parvulus, Sphenophyllum antiquum,
Lepidodendron Gaspianum, Lepidostrobus Richardsoni, L. Matthewi,
Psilophyton princeps, P. robustius, Selaginites formosus, Cordaites
Robbii, C. angustifolius, Cyclopteris Jacksoni.
From the microscopic examination of the structure of specimens of
fossil trunks described under the name of Prototaxites Logani, and
which Principal Dawson believes to be the oldest known instance of
Coniferous wood, Mr. Carruthers has come to the conclusion that
they are really the stems of huge Algæ, belonging to at least more
than one genus. They are very gigantic when contrasted with the
ordinary Algæ of our existing seas, nevertheless some approach to
them in size is made in the huge and tree-like Lessonias which Dr.
Hooker found in the Antarctic Seas, and which have stems about 20
feet high, with a diameter so great that they have been collected by
mariners in these regions for fuel, under the belief that they were
drift-wood. They are as thick as a man's thigh. Schimper regards the
Psilophyton of Dawson (Plate IV. Fig. 5) as allied to Pilularia, one
of the Rhizocarps (Fig. 22), and Carruthers places it among the true
Lycopodiaceæ.
_FLORA OF THE CARBONIFEROUS EPOCH._
The Carboniferous period is one of the most important as regards
fossil plants. The vegetable forms are numerous, and have a great
similarity throughout the whole system, whether exhibited in the Old
or the New World. The important substance called Coal owes its origin
to the plants of this epoch. It has been subjected to great pressure
and long-continued metamorphic action, and hence the appearance
of the plants has been much altered. It is difficult to give a
definition of Coal. The varieties of it are numerous. There is a
gradual transition from Anthracite to Household and Parrot Coal; and
the limit between Coal and what is called bituminous shale is by no
means distinct. Coal may be said to be chemically-altered vegetable
matter inter-stratified with the rocks, and capable of being used as
fuel. On examining thin sections of coal under the microscope, we can
detect vegetable tissues both of a cellular and vascular nature. In
Wigan cannel coal, vegetable structure is seen throughout the whole
mass. Such is likewise the case with other cannel, parrot, and gas
coals. In common household coal, also, evident traces of organic
tissue have been observed. In some kinds of coal punctated woody
tissue (Plate III. Fig. 5) has been detected, in others scalariform
tissue (Plate III. Fig. 6), as well as cells of different kinds.
Sporangia are also frequently found in the substance of coal, as
shown by Mr. Daw in that from Fordel (Plate III. Figs. 1 to 3); and
some beds, like the Better bed of Bradford, are composed almost
entirely of these sporangia imbedded in their shed microspores,
as has been recently shown by Huxley. The structure of coal in
different beds, and in different parts of the same bed, seems to vary
according to the nature of the plants by which it has been formed,
as well as to the metamorphic action which it has undergone. Hence
the different varieties of coal which are worked. The occurrence of
punctated tissue indicates the presence of Coniferæ in the coal-bed,
while scalariform vessels point to ferns, and their allies, such as
Sigillaria and Lepidodendron. The anatomical structure of the stems
of these plants may have some effect on the microscopic characters
of the coal produced from them. In some cannel coals structure
resembling that of Acrogens has been observed. A brownish-yellow
substance is occasionally present, which seems to yield abundance of
carburetted hydrogen gas when exposed to heat.
It appears that in general each bed of coal is accompanied by the
remains of a somewhat limited amount of species. Their number,
particularly in the most ancient beds, is scarcely more than eight
or ten. In other cases the number is more considerable, but rarely
more than thirty or forty. In the same coal-basin each layer often
contains several characteristic species which are not met with
either in the beds above or below. Thus, there are sometimes small
local or temporary floras, each of which has given birth to layers
of coal. The quantity of carbonaceous and other matter required to
form a bed of coal is immense. Maclaren has calculated that one acre
of coal three feet thick is equal to the produce of 1940 acres of
forest.[4] The proportion of carbon varies in different kinds of
coal. Along with it there is always more or less of earthy matter
which constitutes the ashes. When the earthy substances are in such
quantity that the coaly deposit will not burn as fuel, then we have
what is called a shale. The coal contains plants similar to those of
the shales and sandstones above and below it. Underneath a coal-seam
lies the Underclay, containing roots only, and representing the
ancient soil; then comes the Coal, composed of plants whose roots are
in the clay, with others which have grown along with and upon them,
in a manner precisely similar to the growth of peat at the present
day; while above the coal is the Shale, marking how mud was laid
down on the plants, and bearing evidences of vigorous vegetation
on neighbouring land, from which currents brought down the fine
sediment, mingled with broken pieces of plants.
The total thickness of coal in the English coal-fields is about 50
or 60 feet. In the Mid-Lothian field there are 108 feet of coal.
Coal-beds are worked at 1725 feet below the sea-level, and probably
extend down to upwards of 20,000 feet. They rise to 12,000 feet above
the sea-level, and at Huanuco, in Peru, to 14,700.[5] It is said that
the first coal-works were opened at Belgium in 1198, and soon after
in England and Scotland; it was not till the fifteenth century that
they were opened in France and Germany.
The following calculations have been made as to the extent of the
coal formation in different countries, and the amount of coal
raised:--[6]