THE PROBLEMS OF JURASSIC PALAEOBOTANY

THE PROBLEMS OF JURASSIC PALAEOBOTANY
BY
TOM M. HARRIS,
UNIVERSITY OF READING, ENGLAND
INTRODUCTION
Most of the « problems» of palaeobotany are those questions
which we all feel we would like to be able to answer but cannot
because our knowledge is incomplete. Such problems are often
ephermeral. ln time some are answered; some get left aside by
progress because they cease to be interesting, or are later realised
to be meaningless ; there is one - the fossil origin of the Angiosperms ---'as old as palaeobotany with still raises its unscathed head
provokingly, Such problems in the Jurassic are closely parallelIed
by problems in other periods; but there are certain problems which
are peculiar to the Jurassic because they are caused by progress
rather than by ignorance and -progress in the Jurassic has developed
in its own way.
The problems caused by incomplete knowledge direct attention to the material which creates the problems by providing some
facts and witholding others, Fossil plant material is the rubbish
of the plant world - the discarded debris of some riverside forest
swept by flood into deltaic inarshes. Very rarely do we meet the .
specimen we want - a leafy twig with reproductive organs in full
bloom; but instead masses of fallen leaves, ripe seeds, empty husks.
Fossil 'plants have been called the herbarium of the past, but that
is a poor complement to the· makers of herbaria,
The palaeobotanist's difficulties are increased by the long,
slow process of partial destruction calIed preservation. We do not
168
fully understand these processes, we might do better work if we
did; but we can roughly guage their effects by comparing fossils of
different ages. First the plant organ is shed at maturity and then
it must be immersed in water and covered with anaerobic mud,
or it will quickly disappear, Here it undergoes processes of partial
liquefaction (particularly cellulose tissues) and a drainíng away of
water; the lignine dwindles to a small quantity of brown peaty
material. The processes of physical dehydration and compression
continue gradually and also a chemical dehydration whereby material of carbohydrate composition becomes íncreasingly poor in
oxygen and hydrogen andcorrespondingly rich in carbono But
unless it is severely roasted it never becomes elemental carbon
but remains an organíc compound with some hydrogen and a very
little oxygen, The closer this compound approaches pure carbon
the less amenable it becomes to the chemical manipulations of .the
palaeobotanist.
Palaeobotanical technique aims at reversing these processes:
it tries to put the broken fragments together to make a coherent
plant; it tries to reverse the chemical and physical deterioration
suffered by the planto The palaeobotanist's ambition is to do with
ancient plants just what he would with good spirit material or
herbarium sheets from the recent plant world. On the whole his
task should be easier for 1VIesozoic than for the Palaeozoic; but
harder than for younger material.
There is another kind of preservation called petrefaction.
Here the plant, falling into water saturated with some mineral is
filled up with a concretion of the mineral, starting from the inside.
The mineral fills every cell and impregnates the walls, while the
wall substance varies from being a fairly strong coaly matter,
through a weaker peaty matter, to a mere brown stain in others.
Such preservation is exceptional in all formations and is particularly
rare in the Jurassic. The only two 10t8 of petrified material which
have played any considerable part in the advance of Jurassic
palaeobotany are infact not Jurassic at ali but Lower Cretaceous,
though their plants are of Jurassic type: I refer to the Lower
Cretaceous Cycadeoids and to the Ginkgoales of Franz Joseph's
Land, Petrified conifer wood is quite common in marine Jurassíc
rocks and sometimes excellently preserved. It has been well
169
investígated, but there is unfortunately no way to relateit to the
ordinary material of plant compressions.
However it now appears that in India (Rao 1943) there is a
petrified Jurassic flora comparable with the Coal ball flora of
England and work on it has begun. Wbat a .diííerence it would
have made had Williamson and Scott devoted themselves to Jurassic
petreíactions !
The excellent workdone in last century and in the early
years of this on petrefactions had one unfortunate effect. It caused
people to despise compressions, which are about 99 % of fossil
plant material. It is still troe that a good petrefaction shows more
than a good compression, but thanks to modem technique there is
no longe r the dífferençe- there was once. Some people, even today,
regard an equisitely preserved compressed plant as a mere « impression with carbon» as though it were a charcoal drawing on
a slate.
Since almost all the material available consists of compressions, the history of Jurassic palaeobotany has been the investigation of this material and the future lies with it also, This
history has two stages, At the old stage compressions were consídered as mere carbonaceous marks showing no more than form
and surface markings, but at the new one they are considered as
plants, altered but capable of being partly restored by chemical
treatment. The old stage began with palaeobotany and still persists, the new began in the early years of this century and is very
slowly replacing it.
. The credit for the effecti ve pioneer work which created the
modero development must go to Nathorst. His work is the more
remarkable because he worked for many years in describing
fossils on entirely conventional, old íashioned Iines. No doubt
many besides Nathorst felt frustrated because tbey could not look
at their plants with the high powers of a microscope like those who
were studying petrefactions: and then suddenly at the beginning
of this century Nathorst seems to have realised that a way was
open 'to him, Years before, Schenk had treated Jurassic fossils
with the corrosive fluids popular with anatomists of his day who
isolated cells by maceration where we would probably prefer to
section them. He prepared and figured cuticles showing cellular
170
details c1ear1y, but though his publication was very well known
neither he nor his comtemporaries had seen much promise in
these cutic1es.
The value now recognísed in such cutic1es is twoíold. First1y
they display the form of the under1ying layer which secreted the
cutic1e; and its value is naturally greatest with rather intricate
organs like seeds with several internal cuticles. Secondly it is
found that the details of the cells often give features which can be
recognised as of specific, generic or family vaIue: specific features
help to piece a plant together, the others to classify it. These
two aspects morphological and systematic are oí course used
together.
•
Nathorst's first great success was in elucidating the structure
of compressed Bennettitalean flowers: he could demonstrate the
pollen and the detaíls of the mícropyles of the seed, His work
won generous recognition; it deserved the even higher tribute of
being imitated; but it was not imitated for a good many years,
The first man outside Nathorst's own department, to take up
Nathorst's methods enthusiastically was Hamshaw Thomas. With
Bancroít (1913) he studied Cycadophyte leaves and reduced their
grievous coníusion at a stroke by dividing them into two sharply
contrasted groups - the Benneltitales and the rest (<<Nilssoniales>})
and it is remarkable that their crítería though reexaminedhave
stood the test of time and infact have been sharpened, This was
the first successful use of cuticles in classification: Nathorst
himself had worked on the Cycadophyte leaves, but the divisions
he proposed were so numerous and vague that no one adopted
them,
The fírst to use cutic1es to furnish characters díagnostic of
species was Nathorst himself who demonstrated differences between
the species of Ntlssonia, and in other genera. Both Gothan and
Thomas made some effective use of cuticles in separating species,
but there have been astonishingly few descriptions of floras where
cuticles and spores have been systematícal1y descríbed and floras
are still descríbed where this side is neglected altogether. ln my
opinion the value of the cutícle for characterising a ley species is
about the sarne as the value oí all its macroscopic Ieatures. The
cuticle enormously enhances lhe interest of the specimen and
171
anyone who has seriously studied cutic1es will never again be
content with a specimen lacking one,
Today, then, the palaeobotanist can approach a flora of weU
preserved compressions confident that he can write an altogether
better account than could have been wrítten sorne time ago.. There
should not be a single indefinite and ílldefined species of Gymnosperm leaf. Then also, every little black seed and every smaU
male cone becomes the subject of numerous illuminating figures
instead of a meaningless black mark. Classification is considerably altered and we hope, improved.
I will illustrate the effects of the new methods on the c1assic
Jurassic flora, the flora of the Lower Oolites of Yorkshire. Professional and amateur col1ectors working hard last century col1ected
very many fine specimens and Seward wrote a catalogue on them
in 1900, just before the new methods began, This catalogue was
a sound piece of work and remarkably complete. It was difficult
for anyone to find anything worth a description in the c1assic
collectíng grounds;. in several visits made recently I found little
which would have been of value in 1900.
Now with the new methods all. that is changed and there is
not a single species, indeed hardly a specimen that does not repay
investigation, and collecting again has become worth while, A week
in the field gíves material for a year's laboratory work. ln fact
Seward had reached a blank wall, which seemed the end of progress: now that wal1 is down and we can go ahead freely. True
our progress is not unlimited but there is an immense amount of
ground to be covered before we reach the end of what the cutic1es
can teU us; more indeed than alI that was explored previously.
We are back at a pioneering stage.
There is an urgent need that knowledge of this flora should
be brought up to date. Specimens from alI over the world have
been identifíed wíth Yorkshire Types. Where the descriptions of
these íoreígn specimens are on old fashioned lines, like the 1900
catalogue, there is not much to be said, but where modem methods
are used the results are often deplorable to me as an English
Palaeobotaníst. Thus Oishi described a number of Gínkgoalean
leaves from Manchuria which resembled the published figures of
Yorkshire ones and 80 he ídentified them with Yorkshire species;
172
he also íígured the cuticles whieh were previously unknown. The
Yorkshire material has now been examined and proves specifically
distinct. ln this way out of date knowledge causes mistakes and
hampers progresso
The new methods have drawbacks. One is that they are
slow, It takes many times longer to describe a fossil if microscopie features are included. Another is that this and similar
technique makes fossil botany diffieult or even írightening to a
begínner and almost excludes the untrained amatenr. ln 1900
anyone who collected a Iossil plant could match it against the
figures in Seward's catalogue: the tecnhique was the common-sense
use of a pickaxe, chi sel or needle,
Today the same collector would be deíeated by the lack of
any comprehensive work; he cannot possibly know the scattered
Jurassie literature. Even if he deals with a flora where such an
up to date work exists he may find himself facing a classifieation
which uses microscopie detaíls, He must be reasonably skilled
with a mieroscope and he must then master the technique of preparing a cuticle and though this is the easiest of techníque it is
doubtless forbidding at first. This increasing difficulty which
fossil botany shares with so many sciences is a problem created
by progresso Increasing difficulty may be inevitable but I believe
its ill effects could be lessened if writers of monographs bore in
mind the needs of the untechnical.
Anyone who prepares cuticles at once begins to split establisbed species and sometimes established genera. The multiplication of species certainly, is troublesome but I cannot deplore it
because it is but the recognition of fact.
I will illustrate the effects of the study of cuticles, the
mistakes corrected and new problems created on one specíes,
Czekanowskia rigida Heer. The original specimens from the
uppermost Jurassic of Siberia were well figured but no microscopic
details were available, ln the course of time many similar specimens were found at all stages of the Jurassie and were identified
as C. rigida, Nathorst described the cuticle of a Swedish specimen
and everyone continued, quite reasonably to suppose that C. rigida
was a good species. A good many years later a Czekanowskta
resemblíng Heer's type was found with a cuticle distinct from
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Nathorst's specimen: the two had equal claíms to be regarded as
true C. rigida. Now Florin (1936) has described a third type of
cuticle, in a leaf he identiíies as C. rigida. Without a cuticle
there is now no reason why a specimen should be identified with
one rather than another of these species. The correct name where
no microscopic detail can be seen i~ «C. sp. fndet}} or perhaps
«cí, C. riglda» but certainly nothing more definite. However
nobody like to see such vague designations in his works and no
doubt specimens showing only features visible to the naked eye will
in future be named C. rigida as they were in the past. We shalL
thus have the name C. rigida used in two ways. Some will use
it in the less deíined sense due to Heer alone; others in the more
defined sense of Heer supplemented by Florin,
Some difficulty n:íáy always remain..M.aterial is not alL well
preserved, waiting to have its microscopic structure disclosed•
.M.uch of it has been oxidised or otherwise altered and no trace of
cuticle remains. Then too, ill preserved cutic1es are very often
met, such cuticles are prepared with great difficulty and are then
obscure. There is every intergradation between an excellently
preserved cuticle and a quite worthless one or none, I cannot
see how such untidy data can ever be completely sorted and stowed
into neat categories, but progress results mainly from the best
material and we need not worry unduly how to treat poor material.
The use of cuticles in separating allied species has been
entírely happy. It is always helpful and so far as I know it has
never yet misled. Even closely allied species wich have been
described as having similar cuticles show perceptible differences
on dose examination. Palaeobotanists often disagree about determinations but only when very little is known about the specimens,
When plenty of facts are available as when both form and cuticle
are known they agree very well. ln view of the difficulties of
taxonomists of recent plants, it surprises me how well the species
concept fits fossil plants, Perhaps the very imperfection of the
fossil record makes our task easier.
tãenera. The value of the ley cuticle for the classification of
higher taxonomic groups is less clear and will remain so until the
position of these groups themselves is made clearer as a result of
general palaeobotanical progresso
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ln the Conifers, the genera have, as a rule, fairly definite
and recognisable types of cuticle, ln some of the Iarge genera,
the genus is varied, but the recognised subgenera are well characterised, so that a palaeobotanist studying them would no doubt
separate the subgenera: similarly certain closely allied genera
have similar cuticles and a palaeobotanist would unite them,
These discrepancies are not serious. The value of the cuticles in
the Conifers is real1y providential: microscopic characters is just
what is needed to help dassify the foliage of these very small
leaves plants.
Among the fossil Ginkgoales there is a fair range of cutícle
structure, though hardly as great as in the Conifers. Florin (1936)
has made several new genera of Gínkgoales based very largely on
the characters of their cutícles. As little is known about the
reproductive organs of fossil Ginkgoales it is useless to discuss
whether the Ginkgoalean leaf genera are natural but it does appear
to me that their definitions are precise and make c1assification easíer,
ln the Bennettitales, the Gymnosperm class set apart from
all others by its stomata, a high degree of uniformity prevaíls,
Thus the phylum cuticle type is welI defined, but no subordinate
groups are very marked until we reach the level of the species,
which again is definite. To use the cuticle to distinguish a Bennettilalean genus, as Thomas (1930) has done for Leptopterophyllum
is thus an undertaking of considerable difficu1ty. Perhaps thetrouble is that not only are all Bennettitalean cuticles rather similar,
but the genera recognised on leaf form, particularly Pterophyllum
are highly composite.
Aí a still higher level, the family, the cuticle seems less
helpíul, The only recent phylum provídíng sufficient families is
the Coniferae and here the larger families show many types of
cuticle, thus making it hard to define any family type, Here
general progress with the fossil conifers might help. It is already
clear that certain families with diverse cuticles, such as the Taxodiaceae are the scattered descendents of a once great and abundant
group. It may be that progress will establish that the Taxodiaceae
is a natural family and trace the origin of some of the divergent
cutícle types; but it may also be that ít will prove to be polyphyletic.
Still higher, at the level of the main Gymnosperm phyla-
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Conifers, Ginkgoales, Cycadales and others the eutic1e is again
sometimes of the very greéitest value, sometimes oí. very little,
The Bennettitales are marked off from all other Gymnosperms
(exeept Gnetales ) by their stomata. ln the Bennettitales the
subsidiary ee1ls show, by their position, that they,.. are elder sisters
of the guard cells, formed from the sarne mother eell in the young
epidermis. ln other Gymnosperms the guard ce1ls come from one
mother cell, the subsidiary cells from different ones. Thus the
cutic1e has the utmost value in distinguishing a Bennettitalean
leaf from all other Gymnosperms, íncluding Cycads and so far this
test has stood the most rigorous enquiry. The remaining Gymnosperm phyla, agreeing in this important eharacter, are harder to
distinguish and it must be admitted that though a typical Conifer
or Ginkgo or Cycad are'dístinguished by a fragment of cutic1e, it
would be easy to point to a less typical member which might cause
mistakes. The Pteridosperm cuticles are very like those of the
Cycads. The isoIated leaves of these groups are at present distinguished primarily on their general architecture and this basis seems
likeIy to remaín.
At a still higher leveI, the cutic1e may again be of service,
It is a fact that a cutic1e can be prepared with fair ease from the
mature leaf of every recent Gymnosperm, whevers a recent fern
leaí, Yields either no cutic1e or a mere wisp. 1 have never seen a
good íern cutic1e preparation prepared by HNOs KClOs followed
by aIkali as for the fossils. Close comparisons have been made in
the past between fossil Ieaf cutic1es (prepared by this maceration)
and the whole epidermis of a leaf sueh as a fern (sliced off by a
razor), but there comparisons have all been unfortunate. Some
ferns do possess a resistant layer in their outer epidermaI walls but
this layer (isolated for example by maceration in concentrated
H 2SO",) is not made of cutin. Perhaps some fern will prove to
possess a well developed true cutic1e, but 1 am sure sueh a fern
will be most exceptional.
This fact is most valuable for separating Pteridosperm leaves
from these of ferns: if the fossil has a well developed cuticle it is
probably a Pteridosperm, if none it is either a fern or just ill
preserved. The associated Conifers and Ginkgos shouId throw
light on the preservation.
+
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jurassic floras and Stratigraphy.
Palaeobotanists have a
double task; first to recognisezonal floras and then to relate the
floral zones to the main time scale, ln West Europe, at any rate,
plant bearing Jurassic rocks are so isolated that there has never
been any temptation to construct an independent floral time scale,
Progress has been very unequal. At the beginning of the
Jurassic three floras have been investigated on modem lines and
these three floras (Sweden, S. W. Germany, E. Greenland) have
been fully correlated. Two plant zones were recognised and it
has been claimed that these zones are recognisable as far away
as in Japan.
ln the Lower Oolites of Yorkshire there is a considerable
thickness of plant bearing rocks which has been proved to corres- .
pond to a large number of faunal zones, but so far no real plant
zone has been distinguished. This is because many of the species
are ill known, because collectors concentrated on a very few localities, and because they stuck abominably vague locality labels on
their specimens. These causes have concealed any zones which
may occur there, but my own collecting though very incomplete
has convinced me that recognísable zones existo The Lycopod
megaspores are particularly helpíul.
We then pass ríght through the Upper Jurassic to the Lower
Cretaceous before we come to a flora described in detail. The
intermediate stages are by no means without floras in Europe, and
Asía. appears to be richer still, so that we may hope one day to
have every stage of the Jurassic covered by a large and fully described flora.
There is no problem here, but just a need for a great
deal of careful descriptive work along lines which are well understood,
Oeographicai distributlon. The view that in the Jurassic
there was an almost uniforme flora, world wide indistribution and
almost stable in time is the product of the Old Palaeobotany which
confined itself to the naked-eye form of plant impressions. The
use of microscopic features has modified it very much, at any
rate in the minds of those willing to examine their fossils. with a
mícroscope, It appears that many genera of Jurassic plants did
indeed last the wholeperiod and were of wide distribution (though
177
byno meansalways as wide as was once suggested ). The species
as precisely defined with the help of a cuticle had a restricted
range. It was the confusion of a dozen similar looking but specificalIy distinct leaves which gave the old, widely distributed species.
Yet it does remain true that the Jurassic flora is much more uniform
than that of the Permian for example.
A perplexing thing about the Jurassic period is that there is
remarkably little to suggest that the plants in one place enjoyed
a better climate than in another. The reader is probably welI
aquainted with the celebrated maps of the ancient world prepared
by Kõppen and Wegener: their map of the Permocarboniferous
(perhaps because based largely on polaeobotany) I find convincing ;
but their map of the Jurassíc seems to me to fit palaeobotanical
facts scarcely better thai' the present world map would do. This
lack of convincing fit is not the fault of Kõppen and Wegener, for
the continents were like putty in their hands, moulded to fit any
fact they had. The fault is that the Jurassic flora has provided
too few facts.
The Jurassic flora is too different from the flora of today for
us to get much from direct comparison with modem plants, There
was once, for example, a suggestion that the cIimate of Yorkshire
was sub-tropical because of the - presence of Cycads: the plants
referred to now prove to belong to an unrelated family and the
argument has lost alI cogency, Nor is the case much better for
comparison between say Matonidiam and Matonla. When the
Matoniaceae were a large family there might be members in many
climates, and in any case the plants have had time to change,
A little rather uncertain help might be given by comparison of
life forms, if we knew more about the stature of Jurassic plants,
Facts about leaves do not carry us far; the Conifer an Ginkgo
leaves are more xeromorphic in some floras than others; that is
about alI.
I believe the information we want is likely to come forward
of itself, once the world's Jurassic floras are described by the
methods -oí Nathorst, Thomas, Florin. My hope lies in the precisely defined leaf-species or seed-species (based on form supplemented by cuticIe structure). So far as we can now telI, such
species may have fairly extensive distribution, but by no means
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do they occur in alI localities of suitable age. I believe that the
ranges of a good number of such species would indicate c1imatic
zones. We would not at first know what climates the zones belonged to, but if we traced them widely enough we should either
see the position of the North and South poles. Perhaps on
the other hand evidence of continental dríft and distortion might
be obvious.
Several years ago I did in fact suggest that two c1imate zones
might be traced in Liasso-Rhaetic times, one from E. Greenland,
at Lat, 70° N traversing S. Sweden, Poland and across Asia to
Japan at Lat. 35° N and another about 1000 miles south of this,
including the floras of Persia and Tonkin. These belts have an
obliquity in relation to the earth's axis which is quite entrancing,
but I cannot take my own suggestion very serioúsly as there was
too much arbitrary acceptance of one specific determination, rejection of another, indeed I would have rejected the whole hypothesis
for a contradictory facts about the cutic1e of Asiatic plants. Such
facts alone we need and such .facts are now easy to get for those
who are willing,
Cutictes and morphology. Although it is near1y fifty years
since Nathorst made a beginning with the cuticle morphology of
reproductive organs: there is still much that we do not understand,
particular1y in seeds, No palaeobotanist has ever made an exhaustive survey of the cutic1es of seeds and pollen sacs and we have
gone almost blindly to work on the Iossils. We may not have
made any serious mistakes, but I am sure we have missed much
through lack of appreciation. It is probably too much to expect
that any one but a palaeobotanist will make a general study of seeds
from the point of view of the study of fossil compressions, Such
a study should take into account a good many angíosperms as well
as all living Gymnosperrn genera and might be even more massive
than Florin's study of Conifer cutic1es. Till this work appears all
one can do is to mace rate a few selected seeds, hoping to recognise
the sarne structure. as in a fossil, and hopíng too that this Iittle
knowledge is not going to mislead.
The Iossil microspores of the Jurassic períod, as those of
other periods, provide their problems. While Wodehouse's comprehensive theories about the comparative morphology of Gymnos-
179
. perm pol1en stood almost alone most people no doubt accepted his
condusions without further worry. This is no longer possible as
Florin has put forward a very different interpretation of the winged
pol1en of the Pinus type. The situation invites everyone to form
his own judjement if he wants, but more pressingly to look at
ear1y microspores. The evidence for this particular problem must
be largely pre-J urassic, but no doubt some microspore development
took place in the Jurassic aiso.
Fossil seeds provide abundant scope for the preparation and
study of cuticles : Nathorst again was the first to begin this work
with his investigation of the seeds oí: Nilsson ia. He attempted to
compare the cuticles in the fossil seeds with structures of recent
Gymnosperms but wasnot- entirely happy in his results. It would
have been easier for him if he had macerated recent seeds in the
sarne. way as the fossils, he would have obtained preciselysimilar
membranes, and the interpretation of the fossils would have been
easier, íuller and more accurate.
Again Hamshaw Thomas was the first to take up Nathorst's
methods and develope them íurther. He macerated fruits and
seeds of the Caytoniales by ordinary methods as well as in other
ways and prepared very good cu lides of interesting and elaborate
structure.
Among fossil seeds there seem to be two main types of
cuticle structure. I do not know how far these two types may
intergrade, and for all I know there may be other main types,
These two types are contrasted below under the names Cycad and .
Caytonia types.
Integument : outer cutide.
Integument: inner cutide.
Nucellus: cuticle of free parto
Nucellus: chalazal openníng,
Megaspore: cuticle,
Cycad type.
Cayionia type.
very thick.
rather thin,
rather thin.
large,
very thick.
rather thin.
almost absent.
exceedíngly thick,
very smalL
thin or absent.
I
The two types agree in possessing a stone layer in the
integument and in having a rather thin cutinised lining to the
micropylar canaL
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The Cycad type inc1ude some living cycads, Nilssonia seeds
(= Beania}; Oinkgo and the fossil Oinkgo seeds so far investigated; several presumed Pteridosperms and several fossil conifers.
The Caytonia type includes some at least of the Bennettitales and
probably als o some recent Angiosperms.
The Cycad type seems understandable; the occurrence of
cuticle apartfrom the thick megaspore membrane is as would be
expected. Frankly, I íind the Caytonia type difficult to explain :
. what does the very thick nucellus cuticle mean? - with its small
chalaza it makes an almost complete coat. Maybe the thick megaspore membrane of the Cycad and the thick nucellus cuticle of Caytonia agree in perfoming the sarne function in seed development
orgermination. I do not know what this function is; that is a
problem for plant physiologists but I refuse to believe that the
thick nucellus (rnegasporangium ) cuticle is just an anachromism,
a relic of a freely exposed sporangium or that the megaspore membrane is a relic of the protective coat of a freely exposed megaspore,
Those thíngs are too remete.
Much of this essay has consisted in praise of the leaf cuticle
but no one must suppose that the preparation of cuticles is a
technique perfect in its way, like the transíer method, It is not;
it is fundamentally bad. ln preparing a cuticle we take a leaf
which once developed elaborate structure and then was partly spoilt
by the ill treatment of a hundred million years. We pick oíí
fragments and put thern in a destructive solution, removing them
at a time when the cuticle is still largely intact, but the interior
has gone. We have continued the work of destruction; a little
longer and the acid would dissolve everything.
The cuticle is valuable because it reflects an important tissue,
the epidermis, but it is not as good as the whole epidermis.
It shows nothíng at all of inner tissues and we would particularly
like information about the vascular tissues; the xylem has characters
not less valuable than the epidermís, some indeed (though working
in complete ignorance of the epidermis) have rated it infinitely
higher,
ln an ordinary fossil compression the interior of a leaf is
reduced to a coaly material, or at best a slightly translucent dark
brown substance of most umpromising aspect. Veins can be seen
181
only as surface ridges or as dark strands by transmitted light,
The thicker substance of stems and petioles is an opaque, seemíngly
structureless coaI.
The old fashioned attitude was just to call this «coaI»regarded as the element «C» associated vaguely- with impurities.
There have however been several who revolted agaínst this defeated
attitude and have gone to work with the faith that all the structure
. is preserved and available and ready to be displayed by methods
within the grasp of human ingenuity. Indeed I heard Thomas say
something like that in his lectures when I was a student.
Success in this direction has been rather small, and though
not entirely discouraging, hardly commensurate with the efforts
made. How to achieve.... this success is the great problem for the
future.
Nature sometimes gives us what we ask in a way that seems
derisory. Leaves are sometimes found which have rotted a good
deal before preservation and almost the whole interior has disappeared, perhaps through aerobic bacterial action. The cuticle
remains and the cells of the wood remain but these have become
macerated and have separated more or less. Such a leaf is preserved as a very translucent yellow film consisting of the cutide
and these xylem cells alone, and the thickenning of the xylem is
perfectly dear ; I have se~n scalariform tracheids in the smallest
veins of both Sagenopteris and Ctenis leaves. Unfortunately such
tracheids are of little morphological use, they represent the xylem
in its feeblest form. It is the well developed wood of the stern or
root that is most valuable. However since such as ill-preserved
leaf shows lhe structure we need, I cannot doubt that better preserved material still hasit.
With the less severely altered plants of the Tertiary and
Cretaceous (lignites) fossils have been successfully swollen in
alkali and then sectioned. The results have sometimes been very
good; the sections showed most of what could be expected in
recent material.
The great interest of Halle's sections of male fructifications
of palaeozoic Pteridosperms must not blind us to the fact that the
material was very intractable. .It was possible to see the pollen
in the pollen sacs and it was possible to reconstruct the form of
182
the whole fossil but no trace remained of hístology, We can be
sure that this was for no want of skilI.
ln the Jurassic, almost the only important effort has been
Thomas's wiht Caytonia. Thomas (1925) swelled Caytonia fruits
in alkali, microtomed them and then «etched» the sections by gentle
maceration, a mitigated version of ordinary maceration. What he
saw was not only alI the cutícles demonstrable by ordinary methods
but also certain thick walled ce1ls, chiefly the stone ce1ls of the
integument, Again he could get no light on the histology of vascular tissue. The methods was troublesome and the proportion of
success rather small.
It seems to me that the methods of the coal-petrologists may
hold hope. These methods do not rely on swelling the material
but instead use polished surfaces of virtually untreated coaI. The
polished surface is then etched to a minute depth and so developes
a slíght unevenness which shows structure to vertical illumination,
Details oI tracheids have been repeatedly seen in carboniferous
coals.
Such methods have not been seriously used for Botanical
ends, at any rate in the J urassic. I once spent a short time treating Caytonia fruits in this way with fair results: I saw the details
of the seeds very much as in Thomas' figures, and without spending
much trouble I saw a piece ofaxylem cell (it had scalariíorm
bars) - but I saw no more. My apparatus and methods could
have been ímproved. The reason why I have not taken up this
study seriously is this: there is still so much to do in applying
Nathorst's methods that I am disinclined to look for advance in
new directions whilefully occupied withestablished ones. However
the day will come when this advance is made and an ordinary
account of well preserved compressions will include figures
displaying the details of xylem and perhaps of less robust
tissues too. I hope to be spared long enough to read an essa
y by someone railing at the old fashioned method of the old
generation of fossils botanists who actually destroy all the interior
tissues of a fossil for the sake of a merecuticle. Possibly I may
be honoured by having my name included among such out-of-date
fossil botanists.
18~
Systematic review.
A review of the Jurassic fossils which provide problems by
being incompletely known would simply be the sarne as a list of
the whole Jurassic flora, for there is not a single Jurassic plant
which we know in the manner in which we have a right to expect
to know a fossil, Knowledge of Lepidodendron or Calamites may
not be complete, but no Jurassic genus yet rivaIs them.
I will therefore confine myself to brief notes on plants of
speciaI interest - ones which have provided a good many facts and
then withold the rest in a particular1y exasperating way.
Lycopodiales.
Heterosporous Lycopods were very well represented in the
Jurassic: at Ieast their megaspores are abundant and rich in species but Ieafy shoots are very rare. For instance in the Liasso-Rhaetic of Greenland there are .fiíteen kinds of megaspores, but
only one shoot, a Lepidodendron-like plant. ln the Yorkshire Lower
OoIite flora incomplete search has given about twenty megaspores
but only one Selaginella-úse shoot. It may be imagined that Lycopod megaspores are among the remains most adapted to being
washed out of inland forests and carried by water for very long
distances without injury; so perhaps these megaspores represent
the species common in the inland flora. But are we to take the
ratio of 15 or 20 to 1 as representing lhe ratio of the unknown and
almost unknowable inland flora to the known deltaic flora?
The Fillcales.
Ferns are so abundant that it is easy to imagine thern the
dominant or indeed almost the only herbaceous plants in the Jurassic
land; though in swamps the Equisetales were certainly dominant.
It is rather surprising that the Jurassic ferns are ali c1assifiable, naturally as it seems, in existing families. It surprises me
too that the transfer method of Walton which is so perfectly adapted
to expose the sporangia of a fóssil fern should have provided facts
which have merely supplemented and confirmed the conclusions of
184
earlier workers using less powerful technique : or at most have
modiíied their conclusions in detail. Thus the continued study of
Jurassic feros has had a most unusual result, possibly unique in
palaeobotany: it has solved problemsand created none.
However a praticaI problem remains and is troublesome:
how to improve the difinition of a fossil fero species. Fossil fero
species were once just as good or as bad as gymnosperm leaf speeles. Cutieles have improved gymnosperm species out of all recognition but have scarcely helped with the feros. The difficulties
are inherent in the feros. Their Ieaves are often very large and
thus preserved as relatively smalL Iragments, and the details of
one part of a fero leaE change great1y, though gradualIy as the eye
passes from one part to another. The result is great scope and
need for individual judgement and there is no check from the cutiele because most fero leaves have no cutic1e. OccasionalIy the
leaf is preserved a matrix fine enough to show the epidermaI cell
walls, but such preservation is too rare for the epidermis to be
much use in comparisons. More heIp is given by transferes:
sometimes the other side oE the leaf shows characteristic scales
but it is commonly smooth and featureless.
Fertile fero leaves make just as good species as Gymnosperm
leaves because ou maceration they yield spores of characteristic
size, shape and sculpture. The spores have however not yet been
seriously used to separate species but their preparation has been
undertaken to help in morphological problems. Such fertile Ieaves
are unfortunately less common than sterile ones, and it is often
hard to-relate sterile leaf to the same species as a fertile one,
When we can examine sterile fero leaves with as :high
magnificatíons as we can the cuticles of Oinkgo leaves we may
clear up the muddle that is the fern-leaí-genus Cladophlebis, but
until then the muddIe is sure to grow. I can see no prospect of
this advance,
CaytoniaIes.
As far as we can tell, this order or rather this isolated genus
lasted all the time from the Upper Trias to the Wealden without
any important change, The Caytonia plant is full of problems of
185
gaps.in knowledge. We know the leaves and smallest twigs, but
nothing of its main stern or general build, We know the sporophylls but nothing of the inflorescence. We know nothing of the
vascular structure of the sporophylIs, nor of course of the stern
xylem. What we do not know but ought to really constitutes a
very respectable body of knowledge, and add to that all the details
we cannot hope for of gametophyte and embryology. Such detaíls
form about half the description of living Gymnosperms in present
day text books, and though I regard the proportion as over high, I
admit its importance. It is a tribute to the intrinsic interest of
Caytonia and to the way it was investigated that even with these
great gaps it remains.one of the most Iascinating of Mesozoic plants.
Bennettitales.
The Bennettitales are favoured by a series of petrefactions,
unique for the Mesozoic; but they have suffered gravely from a
forced comparison with the Cycadaceze. Little has been done with
the petrefactions for many years and I think the time is ripe for a
return to them. It would for example be well to know the phyllotactic relations of the ovules and interseminal scales, and also
their vascular attachments, Serial sections, prepared by the etching
and celloidin pull method should show thesefeatures clearly,
Then too the microsporangia are worthy of fresh study; I suspect
that they are folded leaf segmenta rather than simply Marattia-like
synangia The seed integument should be reexamined for it is
impossible to relate the structures seen in the macerated seeds
with those described in the petrefactions with any coníidence. There is no doubt room for greatadvance in the discovery
of new types of flower: several isolated microsporophylIs are known
which are worth generic rank and the same is true for certain
fragmenta of gyncecia. These are problems caused by incomplete
knowledge in which the missíng part is nearly the whole,
Cycadales.
The position of the Mesozoic Cycads is not as it appeared
a few years ago. ln ear1y days, the Mesozoíc and particularly the
186
Jurassíc was cal1ed the Age of Cycads. Then gradual1y one after
another of these Cycads was sh:own to be a member of the Bennettitales, a group of utterly different reproductive organs and the
status of the uninvestigated remainder seemed very insecure,
Work on cuticles however has gone far to rehabilitate the Cycads,
The Cycad-like leaves of the Jurassíc are the true Cycads, including
Bjuvia simplex with a Cycas-like megasporophyll and Nilssonia
with Zamia-like male and female cones. Thus limited the fossil
Cycadales remain a considerable and important Jurassíc family,
though less numerous than the Bennettitales, Conifers and Ginkgos,
It must however be borne in mind that the reference of such genera
as Ctenis and Pseudoctenis to the Cycads rests on a chain of
reasoning, a chain with many links, some perhaps doubtful1y
reliable, So; while accepting the classification as the most reasonable that can be put forward, I would not be very surprised if
Ctenis for example, proved to have very different reproductive
organs from Cycas. The only thing whích would overwhelm me
would be evidence that it belongs to the Bennettitales!
The classification of Jurassic plants is tidier than the facts
really warrant. I remember someone writing that it was satisfactory that the ranks of the incertce sedis were now very sniall.
They ha ve al ways been small: the pioneer fossil botanists classified nearly every specimen and so have writers at every stage in
history; we know that the pioneers made very many mistakes and
the later authors have already been detected in not a few.
Pteridosperms.
It has taken a long time for the idea that the Pteridosperms
were a major group in the Mesozoic to be accepted in spite of a
good deal of cumulative evidence. They are still a most problematic group.
I would regard as Mesozoíc Pteridosperms the leaves with
the fol1owing pair of characters : (1) The leaf is pinnately branched, tending at least to
become bi or tripinnate.
(2) There is a wel1 developed cuticle, with stomata of the
common Gymnosperm type.
187
The first character separates them from the Fossil Cycadales
(in the strict sense) the second from the feros and from the
Bennettitales. I cannot pretend that the separation from the Cycadales is fully convíncíng and such a leaf as Piilozamites might go
in either group were its microsporophyll (Hydtopterangium} less
different from that of the Cycads, Ou the other hand such diverse
types of reproductive organ as the sporophylls of Ptilozamiies,
Lepldopterls, Dicroidium seem to cause less trouble in the Pteridosperms because that group already is very diverse having
sporophylls with but one thing in common: they are all much
branched.
Thomas's work (1933) on the megasporophylls of the
S. African Pteridosperm family Corystospermacere seriously challenges even this. The sporophylls are remarkably small but a
good deal branched. He suggests that the ideas of morphological
homology should be extended generously enough to make it possible
to regard them as strictly homologous with both the richly branched
megasporophyll of a Paleeozoic Pteridosperm and with the cymose
bracteate inflorescences of various Angiosperms. The reader must
tum to the original paper for an adequate statement.
I find these plants most perplexing. My own view, which
I give for what it is worth, is that it is not yet necessary to alter
the ideas of seed plant morphology 80 drastically. I think it
possible to regard these sporophylls (as Thomas does) as reduced
from Iarger and more elaborate ancestral forms, but I suggest that
in this reduction they have acquired a cyme-like appearance which
is purely secondary. Very few pinnules are developed and these
are small, some of them at the bases of ramifications have the
position of braets but have no more significance as bractsthan
the basal pinnules of fero leaves. I suggest thatthe Angiosperm
inflorescence evolved in another family and on different lines.·
Whatever else Thomas's work has done it emphasises the
need for more work on the Mesozoic Pteridosperms. The few
types 9f sporophyll we know so far have 50 little in common that
there is nothing to indicate that the family is natural, nor can we
easi1y relate them to Palseozoic types, There are many genera
and the commonest in the N. Hemisphere,' Thinnfeldia, Cycadopteris, Stenopterls still lack reproductí ve organs,
188
Conifers.
The Conifers ought to be much the most satisfactory of the
Jurassic phyla; their well-cutinised leaves make the best deíined
species of all; their male and female cones are common and very
often attached to leafy shoots and amenable to investigation by
various methods, new and old. Without the cuticles however they
make hopelessly aggregate species and almost meaningless genera
and this is the situation today apart from a few well described
but isolated species.
As far as I can judge from the scattered facts known to me,
the main families of the Coniíeree were all evolved by the end of
the Jurassic, and there were also other Conifer families which
have no lívíng descendants. The history of the family since the
Jurassic would then be mainly the history of mígration and slow
extinction. Two familíes of major importance in the N. Hemisphere were the Taxodiacese and the Araucariacere; both are now
reduced, the one family chiefly as relics in E. Asia; the other in
the S. Hemisphere,
Several very interesting papers by Florin on fossil Conifers
have recently appeared. ln one (1940) all the fossil Conifers of
the S. Hemisphere are reviewed and he reaches the conclusion
that they are mostly Podocarpus or Araucarians, but not members
of the N. Hemisphere families. Unfortunately íacts are very few
and he makes bold conjectures about the affinities of the numerous
Conifers described from the Jurassíc, His work is, rightly considered, a challenge.
. Other papers deal with the pre-Jurassic history of the Coniferre: Florin (1939) has produced the strongest evidence that the
<ovulíferous scale » is a reduced relic, for in the earliest Conifers
. it was a little shoot with many spirally placed leaves, and there is
already a series of forms connectíng these seed bearing shoots
with types of conescale known today. To me, at least, it seems
that the view that there has been progressive reduction in the
Conifer «ovuliferous scale » appears now inescapable as well
as entirely acceptable; so if anyone still holds a view in opposition he will have to go to work quickly to reorientate these
new íacts.
189
Glnkgoales.
Knowledge of Ginkgoalean leaves is as satisfactory as for other
phyla but knowledge of their reproductive organs is in its infancy.
They have been neglected and for this bad reasoru nobody has ever
supposed they are the ancestors of the Angiosperms.
Ginkgoalean leaves often occur in immense numbers, and it
is quite common to find reproductive organs with them, These
organs have well developed cuticles (often much better developed
than in the Conifer cones) and give evidence of structural agreement
with the associated leaf. Such few of the organs so far have been
studied suggest that the Jurassic Ginkgoales are very varied, and
it would not be at all. surprísing if they showed just as great
variety in reproductive organs as do the recent Conifers. Here,
then, is a field obviously ripe for study,
Anglosperms.
The Angiosperms have been very cruel to fossil botanists.
Once they appeared, there were far too many, and before that there
were none, They have been sought with a single-minded purpose
which has achieved success in other fields, but the only success
here has been in bringing to líght thíngs which are definitely not
ancestral angíosperms.
There are only two leaves in the Jurassic which in a later
formation would be regarded as Dicotyledons. One described by
Seward years ago from the Englísh Nliddle Jurassic, is isolated
and ill preserved and looks rather like an apple leaf; the other
I described from the basal Jurassic of Greenland.. It is better
preserved and looks rather like a willow leaf, but although it was
reasonably common there was no reproductive organ attributed to
ít, The present vaiue of such fossils as evidence of the Jurassic
history of the Angiosperms is to write plainly, nothíng: or at best
they serve as goats.
Two Nlesozoic phyla have been specially considered as
Angiosperm ancestors, namely the Caytoniales and the Bennettitales, and although no one holds that the fossils we know are
direct1y ancestral, a good many consider them descended from
190
some fair1y near common ancestor. Both have been elaborately
compared by others and will not be discussed here. They show
points of real agreement and real disagreement ; it seems to me
near relationshíp is possible with either (though scarcely with
both) but there is no convincing case. 1 suggest that relationship
between two plants should only be accepted when it is possible to
compare all the organs of the one with those of the other and to
recognise homology in detail. On this test all the genera of the
Cycadales would undoubtedly be regarded as related and the phylum
homogeneous. 1 think this would be true of the whole Angiosperm
group; or of the whole Coniferse with the doubtful exception of
Taxas. It would be completely untrue to say that it is possible
to homologise the gynrecium of Cycadeoidea and Magnolia in
detail, or their microsporophylIs either for that matter. It is conceivable that the two are related and that a series of intermediates
will be found which will make this homology clear and unequivocal, but till that happens it is best to regard them as unrelated.
It is possible that some of the many isolated leaf types at
present included in the Pteridosperms or Cycadales may prove to
have reproductive organs closely resembling those of an Angiosperm,
but if so it will be an Angiosperm with very unusual Ieaves. It is
also possible that a perfectly normal Angiosperm will be found in
the Jurassic somewhere with organs such that its c1assification is
beyond doubt. We already know the Jurassic floras well enough
however to be able to say that such plants are but unimportant or
rare constituents of the deltaic floras. If my guess from Lycopod
megaspores is correct that there are about twenty times as many
common species inland as in the deltaic floras then there is roorn
for such Angiosperms to be widespread and common but still
unknown,
Some have tried to build such an ancestor from pieces
supplied by known plants living and fossil. Cycadeoidea had such
a queer flower that no one would have dared imagine ít, but since
it was evolved in the Bennettítales, a similar flower could be
evolved in other families and some parts of it can be borrowed.
The sarne is true of the Caytonialean fruit; such fossils though
not regarded as ancestors can still provide building blocks, Such
a hypothetical reconstruction is at best a bloodless thing.
191
However the quest remains worth while. An ear1y and primitive Angiosperm would be the most ínteresting of plants for its
own sake. It would also have tremendous effects on systematics.
The alternative classifications of the Angiosperms exist because
there is no accepted guíding principle, Such .-a principIe could
come for a fossil. Pteridophyte morphology was in no less coníusion till Rhynia made people think alike.
1 will end with my views on the problem of the object of
fossil botany. This is important because 1 think this problem lies
near the root cause of the decline which palaeobotany has suffered,
along with morphology generally, during the last thirty years, 1 am
sure this decline is real; there were more paIaeobotanists in Britain in 1910 than today. and there will be fewer soon as there has
been too little recruitment. 1 think there has been the same decline
over West Europe generalIy, but perhaps not in America. ln Russia,
India, China, Japan there has been growth.
.
1 think people undertake the labours of research on a fossil
plant for two objects which 1 dístinguish although almost every
research embodies both. I will call these pure and applied fossil
botany, By pure fossil botany, 1 mean a research which has the
simple object of bringing a plant to light. By applied fossil botany,
1 mean a research which tries to make the fossil plantinto a
stepping stone leading to the interrelations of living plants, or on
the Geologícal side to ancient land mas ses and ancient climates.
The balance of the pure and applied interests varies greatly; both
are perfect1y legitimate but the pure is safer.
ln pure palaeobotany, all the worker asks isto be able to
bring a plant to light and the pleasure of uncovering it is Nature's
reward of his labour, It does not matter if the plant proves quite
different from what he first supposed that reward at least is secure
and by his publication he can help others in pure and applied
palaeobotany past their difíiculties.
ln applied palaeobotany the ínvestigatíon may not be a pleasure and liable to be tiresome and is in danger of beeing treated as
tiresonie. More often than not when completed it gives no clear
indication about the matter which was the real object. The result
has been that some, realising their mistake turned to other sides
of Botany or Geology; but others were less critícal and the
192
reputation of palaeobotany still suffers from their published speculation.
The decline in descriptive morphology of living plants must
have been partly due to the material being worked out and this
may have turned young workers away from morphology at an
early stage and so they never reached palaeobotany. With wise
direction it should have had the opposite effect. I am convinced
that to some minds form is fascinating in what ever organism it
occurs and these people should become morphologísts and be
directed into those branches of morpholohy where progress continues. Palaeobotany offers the best prospect of all, for unworked
material is abundant and thanks to new technique a year's work
now gives more than at any time in history.
WORKS QUOTED
FLORlN, R.
1931-
Untersuchungen zur stammesgechichte der Coniferales und Cordaitales,
K. Sv. vet-akad HandJ. 3. 10.
FLORlN, R.
1936.
--
Die Iossilen Ginkgophyten vou Franz-Joseph-Land, Palaeontographíca
81 B, 82 B.
FLORlN, R.
1939.
The morphology of the female fructifications iu Cordaites and Conifers
of Palaeozoic Age. Bot, Not. Lund, 1939.
FLORlN, R.
1940.
The Tertiary Fossil Conifers of South Chile and their phytogeographical
signífícance. K. Sv, vet. akad, Handl. 3. 19. 2.
1931-1937.
HARRlS, T. M.
The fossil flora of Scoresby Sound E. Greenland.
85 and 112.
KÕPPEN, W.
and
WEGENER, K.
Die Klimate der geologischen Vorzeit.
OISHl,
Medd. om Gronland.
Berlin.
s. 1933.
A study on the cuticles of some Mesozoic Gymnospermous plants from
China and Manchuria. Sei. Rep. Tohoku Imp, Univ. Geology XII 2 B.
1943.
Ntpariostrobus, a new genus of Dacrydíum-Iike seed-bearíng cones,
RAO, A. R.
and other silicified plants from the Rahmahal series.
India.
Nat, Acad. Sei.
13.
SEWARD, A. C.
1900, 1904.
Catalogue of the -Mesozoic Plants the Jurassic Flora. I and II.
194
1925.
The Caytoniales. Phil. Trans. Roy. Soe. London B, 213.
THOMAS, H. H.
THOMAS, H. H. 1930.
Further observations on the cutic!es of Mesozoic Cycadean íronds.
J. Linn. Soe. Botany, 48.
1933.
On some Pteridospermous plants from the Mesozoic rocks of South
Africa. Phil. Trans. Roy. Soe. London B, 222.
THOMAS,
and
1913.
On the cutic!es of some recent and fóssil Cycadean fronds,
Linu, Soe. London 2, VIII.
THOMAS, H. H.
BANCROFT, N.
Trans.