PRESERVATION OF PLANTS AS FOSSILS
'Some whim of Nature locked them fast in stone for us afterthoughts
of Creation.'
Lowell.
The failure of the earlier naturalists to grasp the true significance
of fossils or even to appreciate their nature is an extraordinary fact
when we consider the pioneer work which they accomplished in biological
and geological science. The following extract from the writings of so
enlightened a man as John Ray serves to illustrate an almost incredible
disinclination to admit what seems to us the obvious. He wrote:--'Yet
I must not dissemble that there is a Phenomenon in Nature, which doth
somewhat puzzle us to reconcile with the prudence observable in all its
work, and seems strongly to prove, that Nature doth sometimes _ludere_,
and delineates figures, for no other end, but for the ornament of some
stone, and to entertain or gratify our curiosity, and exercise our
wits. This is, those elegant impressions of leaves and plants upon
cole-slate, the knowledge of which, I must confess myself to leave to
my learned and ingenious friend Mr Edward Lhwyd of Oxford.... He told
me that Mr Woodward, a Londoner, shewed him very good draughts of the
common female fern, naturally formed in cole.... But these figures are
more diligently to be observed and considered... Dr Woodward will have
them to be the impressions of the leaves of plants which were there
lodged at the time of the Deluge'(31).
The Mr Woodward alluded to by Ray thus expressed his views on fossils
in an _Essay towards the Natural History of the Earth_:--'The whole
terrestrial globe was taken all to pieces and dissolved at the Deluge,
the particles of stone, marble, and all solid fossils dissevered, taken
up into the water, and then sustained together with sea shells and
other animal and vegetable bodies; the present earth consists and was
formed out of that promiscuous mass of sand, earth, shells, and the
rest falling down again, and subsiding from the water'(32).
In the later part of the seventeenth-century Steno, a Dane by birth and
Professor of Anatomy at Padua, by his recognition of the identity of
the teeth in a shark's head, which he had dissected, with some fossils
from Malta known as Glossopetrae, established the true nature of
fossils. He also recognised a certain orderly sequence in fossiliferous
strata, and in the opinion of Professor Sollas he is entitled to be
considered the 'Father and Founder' of Geology(33).
It was by slow degrees that the early observers freed themselves from
the obsession that the remains of animals and plants in the earth's
crust bear witness to a Universal Deluge and are all identical with
existing species. The possibility that some of the fossil plants in
English strata might be more clearly related to forms now met with in
warmer regions was gradually realised. The publication of the _Origin
of Species_ stimulated palaeontological research, and botanists as well
as zoologists turned to the investigation of extinct genera in search
of proofs of the doctrine of evolution.
The common occurrence of petrified wood in rocks of different ages
is well known. Fossil stems are occasionally found in their natural
position of growth, the structural details being rendered permanent by
the deposition of siliceous or calcareous material from water drawn by
capillarity into the dead but still sound tissues. Petrified wood from
Upper Jurassic beds is abundant in the Island of Purbeck; an unusually
long piece of stem may be seen in the small town of Portland fixed
to the wall of a house. Some of these stems have been referred by an
American author to the Araucarian family of Conifers, but the structure
is as a rule hardly well enough preserved to afford satisfactory
evidence for identification. In his _Testimony of the Rocks_, Hugh
Miller speaks of fossil wood from the upper beds of the Jurassic system
in sufficient abundance on the beach at Helmsdale in Sutherlandshire to
be collected in cart-loads; it is still easy to pick up good specimens
on the shingle beach a short distance north of Helmsdale, and a recent
microscopical examination showed that some specimens are pieces of an
Araucarian tree.
Impressive examples of petrified trees on a large scale are to be
seen in the United States, in Arizona and the Yellowstone Park.
(Frontispiece.) In the northern part of Arizona the country for over
an area of 10 square miles is covered with tree trunks, some reaching
200 feet in length and a diameter of 10 feet. The nature of the
mineralising substance has given rise to the name Chalcedony Park for
this Triassic forest(34). A striking example of one of the Arizona
trees is exhibited in the British Museum and in a neighbouring case is
a splendid petrified stem, 9 ft. in height, of a conifer discovered in
Tertiary lavas in Tasmania(35).
[Illustration: Fig. 6. Section of the north face of Amethyst Mountain,
Yellowstone Park, including upwards of 2000 ft. of strata. The
steepness of the slope is exaggerated. (After W. H. Holmes.)]
Figure 6 illustrates the preservation of a series of forests of
Tertiary age in the mass of volcanic sediments, 2000 feet in thickness,
known as Amethyst mountain, in the Yellowstone Park district. By the
weathering away of the surrounding volcanic material the tall stems of
the trees are exposed in places on the mountain sides like the 'columns
of a ruined temple.' The height of the river at the foot of the cliff
is 6700 ft. above sea-level and the mountain rises to a height of
9400 ft. above the sea. In the lower part of the section the volcanic
strata are seen to rest on a foundation of older rocks _A_, and these
in turn were laid down on the eroded surface of a still more ancient
foundation, _B_(36).
The section as a whole affords a striking demonstration of the
magnitude of earth-movements since the last of these forests was
buried below the surface of a sea in which the volcanic material
was deposited. The account of the Yellowstone Park section recalls
Darwin's description in the _Naturalist's Voyage_(37) of snow-white
columns projecting from a bare slope, at an altitude of 7000 ft. in the
Cordillera.
The abundance of drift-wood on the coasts of some countries at the
present day helps us to picture the conditions under which the remains
of former forests have been preserved. In his _Letters from High
Latitudes_, Lord Dufferin gives the following description of drift-wood
on the shores of Spitzbergen:--'A little to the northward, I observed,
lying on the sea-shore innumerable logs of drift-wood. This wood is
floated all the way from America by the Gulf Stream, and as I walked
from one huge bole to another, I could not help wondering in what
primeval forest each had grown, what chance had originally cast them on
the waters, and piloted them to this desert shore'(38). A photograph
reproduced in Amundsen's book on _The North West Passage_ shows the
beach on the Alaskan coast strewn with drifted timber(39). For the
accompanying photograph (Fig. 7) of the flood-plain of the Colorado
River(40), I am indebted to Professor MacDougal of the Desert Research
Laboratory at Tucson, Arizona, who in a recent letter writes, 'During
times of high-water a thin sheet of flood covers the flat for many
miles and bears drift-wood so thickly that it is difficult to push a
boat through it.' The drift-wood consists of poplar, willow, pine, and
juniper, 'the last two have been brought from the upper river, from
as far away as a thousand miles.' A picture such as this affords an
admirable example of the wealth of material available for preservation
in a fossil state.
It is only in the minority of cases that the accidents of preservation
of fragments of ancient floras have given us the means of investigating
the internal structure of the plant organs. It is far more frequently
the case that fossil plants are represented only by a carbonised film
on the surface of a piece of shale or other rock: the actual substance
of the plant has been converted into a thin layer of coal, and though
the venation and other surface-features may be clearly revealed, the
internal tissues have been destroyed. If a lump of clay containing a
piece of fern frond is heated, the result is an impression of the leaf
on the hardened matrix and a coaly substance in place of the plant
substance. It is occasionally possible by detaching a piece of the
black film from a fossil, and heating it with nitric acid and chlorate
of potash and then dipping it in ammonia, to obtain a transparent
preparation suitable for microscopical examination of the cell-outlines
of the superficial layer of the leaf or other plant-fragment. This
method of examination, used by several students of fossil plants and
with conspicuous success by Professor Nathorst of Stockholm, often
affords valuable aids to identification.
[Illustration: Fig. 7. Flood-plain of the delta of Rio Colorado. The
hills in the background are 25 miles distant. (From a photograph by
Prof. MacDougal.)]
Pieces of plants embedded in sandy sediment, if not preserved by
petrifaction, that is by the introduction into the tissues of some
siliceous or calcareous solution, gradually decay and their fragmentary
remains may be washed away by percolating water, leaving a hollow mould
in the gradually hardening sediment, which is afterwards filled with
sand or other material. The plant itself is destroyed, but a cast is
taken which in the case of fine-grained sediments reproduces the form
and surface-pattern of the original specimen. The incrustation of
plants by the falsely named petrifying springs of Knaresborough and
other places illustrate another method of fossilisation.
[Illustration: Fig. 8. Flower of _Cinnamomum prototypum_ Conw.
preserved in amber. × 10. (After Conwentz.)]
Plants which owe their preservation to amber occur both as
incrustations and petrifactions. This fossil resin occurs in Tertiary,
Cretaceous, and Jurassic rocks; the amber found in abundance on the
Baltic coast near Danzig and occasionally picked up on the beach
in Norfolk and Suffolk comes from beds of Tertiary age. Pieces of
Pine-wood have been described from the Baltic beds in which the tissues
are perfectly preserved as the result of the conversion into amber
of the resinous secretion which permeates their cells: in this case
the amber is a petrifying agent. More frequently the preservation is
due to incrustation; as resin trickled down the stems of the Tertiary
pines from an open wound, flowers and leaves, blown by the wind on to
the sticky surface, were eventually sealed up in a translucent case
of amber. Though the actual substance may have gone, the mould which
remains exhibits in wonderful perfection each separate organ of a
flower or the delicate hair-clusters on the surface of a leaf. The
flower represented in Fig. 8, a species of Cinnamon, is one of several
specimens described by the authors of a monograph of Tertiary plants in
the Baltic amber(41).
The fragments of plants preserved in nodules of calcareous rock
occasionally met with in some of the Lancashire and Yorkshire
coal-seams are perhaps the most striking examples of the possibilities
of petrifaction. By cutting sections of these nodules and grinding them
to a transparent thinness, the most delicate tissues of Carboniferous
plants are rendered accessible to investigation under the high power of
a microscope. As our attention is absorbed by the examination of the
details of cell-structure it is easy to forget that the section has not
been cut from a living plant, but from the twig of a tree which grew in
the forests of the Coal age. The preservation is such as to enable us
not only to describe the anatomy of these extinct types of vegetation,
but, by the application of the knowledge of the relation between the
structure of the plant-machine and its functions gained by a study of
living species, it is possible in some degree to picture the plants
of the Coal period as living organisms and to see in the structural
framework a reflection of external environment. The recognition in the
general architectural plan of the Palaeozoic plants, as in many of the
finer anatomical features, of the closest resemblance to plants of
the modern world produces an almost overwhelming sense of continuity
between the past and the present.
The plants of the Palaeozoic period, though often differing
considerably from those of the same class in the floras of to-day,
exhibit a remarkably high type of organisation. Some of the most
abundant trees in the forest of the Coal age are decidedly superior in
the complexity of their structure, as also in size, to modern survivals
of the same stock. On the other hand, it must be remembered that
Monocotyledons and Dicotyledons which now occupy the highest place in
the hierarchy of plants have left no sign of their existence in any
of the Palaeozoic strata. The greater size of some of the Palaeozoic
plants, and in some respects the more advanced stage of evolution which
they represent as compared with their nearest relatives of the present
era, must be considered in relation to their more important and
relatively higher position in the plant-world than that which is now
held by their diminutive descendants. It is, however, impossible to get
away from the conclusion that the oldest Palaeozoic flora of which we
have an intimate knowledge must be the product of development of an age
which is represented by a chapter in the history of the plant kingdom
at least as far removed from the beginning as it is separated from the
chapter now being written. Examples might be quoted in illustration of
the risks attending the determination of fossils by means of external
features alone, but it may suffice to mention the case of a specimen
originally described as a fragment of a Cretaceous Dinosaur under the
name _Aachenosaurus multidens_. By the examination of thin sections
this supposed bone was shown to be a piece of Dicotyledonous wood(42).
The methods of preservation of plants as fossils are numerous and
varied and the few examples selected give but an incomplete idea of the
subject: for a fuller treatment of fossilisation the reader is referred
to more technical treatises (48 vol. I.).
The employment of fossil plants as 'Thermometers of the ages' is a
branch of Palaeobotany to which a passing allusion may be permitted
though it is only indirectly connected with the main question. As
one of the most interesting examples of changed climatic conditions
revealed by a study of fossil plants, reference may be made to the
wealth of material collected within the Arctic circle. The problems
suggested by the discovery of plants in rocks of various ages in
North Siberia, Spitzbergen, Franz Josef Land, Bear Island, Greenland,
and in many other localities in the far north are too difficult and
far-reaching to be discussed in these pages. In the Cretaceous and
Tertiary strata of the west coast of Greenland and Disco Island from
69° to 72° north latitude, to refer only to one case, a great number of
plants have been obtained by several of the earlier Arctic explorers
and more recently by members of one of the Peary Expeditions. At the
present day on the fringe of land on the western edge of Greenland
which is not permanently covered with ice, a considerable number of
herbaceous plants are able to exist and to produce seed during their
concentrated period of development; while trees are represented only
by a few low-growing shrubs such as the dwarf Juniper. In places
accessible to investigation beyond the ice-covered hills of northern
Greenland the rocks have been shown to consist of Cretaceous and
Tertiary sediments containing fossil plants associated with seams
of coal. From these beds numerous Dicotyledons have been obtained,
some of them almost identical with living species characteristic of
sub-tropical or tropical countries. In the lowest of the Cretaceous
series no Dicotyledons have been found, but flowering plants are
abundant in the higher Cretaceous rocks. Allowing for the fact that
closely allied species are often able to live under very different
climatic conditions, there can be no doubt that the Cretaceous
and Tertiary floras of Greenland indicate an average temperature
considerably higher than that which now prevails in the warmest parts
of the British Isles.
In the far south a fairly rich Jurassic flora has recently been
discovered by the members of a Swedish Antarctic expedition in Graham's
Land in latitude 63°·15 S. and longitude 57° W., which in its general
facies bears a close resemblance to the Jurassic flora of Yorkshire.
Although the great majority of the records of ancient plants are
difficult to interpret by reason of imperfect preservation and because
of the frequent separation of leaves, stems, and reproductive organs,
the student who tries to piece together the _disjecta membra_ of the
floras of the past shares the opinion expressed by the late Marquis of
Saporta,--'Si l'on s'attache à les déchiffrer, on oublie bien vite la
singularité des caractères, et le mauvais état des pages. La pensée se
lève, les ideés se développent, le manuscrit se déroule; c'est la tombe
qui parle et livre son secret.'