Table Of ContentIpomoea purpurea pollen grain.
Fluorescence micrograph of immature grain treated with highly
dilute primuline solutions. The foot-layer and nexine-2 layers are
becoming differentiated.
SPOROPOLLENIN
Proceedings of a Symposium held at the
Geology Department, Imperial College,
London, 23-25 September, 1970
Edited by
J. BROOKS, P. R. GRANT,
MARJORIE MUIR,
P. VAN GLJZEL AND G. SHAW
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EDITORS1 PREFACE
The concept of a meeting to discuss Sporopollenin began
simply at an informal discussion held at the BP Research Centre
in March, 1970. The participants at that meeting decided that
it would be interesting to meet again in September, and to invite
a few more people to contribute. It was estimated that perhaps
about 20 people might attend. However, interest in the subject
proved to be more widespread than initially supposed, and within
a few weeks of its inception, the potential for a full inter
national Symposium to be held under the aegis of the International
Palynological Committee became apparent.
As we soon decided that the Symposium could provide a
valuable publication, it became evident that supporting funds
and accommodation would be necessary. It is with sincere thanks
that we here acknowledge the generosity of the British Petroleum
Company Limited, Shell Oil Company, and the Royal Society,
Professor J. Sutton of the Geology Department and the authorities
of Imperial College London for their kind hospitality and
co-operation.
The staff of Academic Press are to be thanked for their
aid and encouragement in producing the Proceedings Volume. We are
also grateful to all the individual secretaries who laboured to
produce the camera-ready copy; special thanks must go to Miss
Wendy Coleman for her forbearance in making amendments and
minor alterations to the scripts.
Any points in the discussion, adequately covered in papers,
have been omitted from the transcript of the discussion.
We do not feel that it is necessary for us to make a formal
introduction to the subject of Sporopollenin, as this is covered
comprehensively in individual papers. The brief foreword by
Professor Dr. R. Potonie and the summary by Professor Dr. P.P.
Jonker are as effective as anything we could achieve here.
October 1970
v
FOREWORD
A Symposium on Sporopollenin was held on September 23rd -
25th, 1970 in the Geology Department, Imperial College, London.
The meeting was excellently organised by Dr. M. MUTR and Mr. P.
GRAM1 of that department, in co-operation with Dr. J. BROOKS,
from the British Petroleum Company, Sunbury, Middlesex, and
Dr. P. van GIJZEL, Afd. Fluorescence Microscopy, Faculty of
Science of the Catholic University of Nijmegen, the Netherlands.
We must thank the organisers very much for having provided a
forum for discussion on the many different aspects of fossil
and Recent spore-coats. Many of the papers published in the
following pages are important to the further investigations
of sporopollenin in pure science and in industry. Botanists
long ago were aware that the substance of the coats of spores
and pollen grains was very resistant. They called it cutine -
a name which was also applied to the material which forms the
outermost layers of the epidermal cells of the leaves of most
land plants.
So that they could recognise this cutine, botanists
proposed a series of microchemical reactions. Only when
these reactions were positive was the substance called cutine,
and indeed, these positive reactions also occurred with the
spore coats of Carboniferous plants (R. POTONIE, 1915, 1920).
From this, we also believed that these exines were not much
altered compared with spores of Recent plants. This was the
vi
starting point for a more detailed chemical study of fossil
spore coats.
About the year 1920, Dr. F. ZETZSCHE came to the Geologische
Landesanstalt in Berlin and asked me to give him fossil material
such as I had examined in my work in 1915 and 1920. We gave
him then, amongst other things, spores taken from the Lower
Carboniferous Brown-coal of the Moscow Basin, and further,
the Australian Tasmanite, because at that time, we believed that
it contained spores, and not algae as we now know. The summary
formula given by ZETZSCHE and his collaborators then, were
very interesting, but they told us little more than did the
microchemical reactions. In every case, however, we could
see that the composition of the Carboniferous spore coat
substance had not changed fundamentally compared with Recent
spores.
It was only with the work of G. SHAW & A. YEADON (1964) that
the investigations took a new trend. These authors gave us
structural formulae of Recent Sporopollenin, and it became
possible to compare these formulae with the corresponding ones
of Carboniferous spore coats of the Lycopsida (POTONIE & REHNELT,
1969)> and we could see that coalification consisted of a very
slow aromatisation of the Sporopollenin.
However, this is only one of the aspects which are treated
with, and the reader must look to the following pages for many,
varied investigations into the subject.
Professor Dr. R. Potonie,
KREFELD, Germany,
October, 1970. vii
SPOROPOLLENIN IN THE BIOLOGICAL CONTEXT
J. Heslop-Harrison, Herefordshire.
Introduction
The unifying theme bringing together chemists, botanists,
geologists, pedologists and geochemists in this symposium is the
concept of sporopollenin. The word itself we owe to Zetzsche
(1932), whose work will be reviewed in some detail in later papers.
The term, a compound of "sporonin" and "pollenin", also used by
Zetzsche, was adopted as a collective appellation for the resistant
wall materials found in the spores of pteridophytes and the pollen
grains of gymnosperms and angiosperms. "Sporopollenin" therefore
conveyed from the beginning a generalisation, namely that all these
materials belong to the same family of compounds, chemically speak
ing. The early evidence for this conclusion came largely from
similarities in the empirical formulae of spore and pollen-grain
wall substances from different sources. Recently evidence of a
more sophisticated kind, also to be reviewed later, has appeared
to support this proposition. Dr Shaw and his colleagues have
indeed extended the chemical generalisation to include the walls
of the propagules of thallophyte groups, and this has been coupled
with the suggestion that the "sporopollenins comprise a distinct
and unique group of substances": unique presumably in the sense
that they are chemically distinct from all other major classes
of plant cell wall polymers.
I
2 J. HESLOP-HARRISON
In this introductory contribution I propose mainly to offer
some thoughts on the functional roles of spore and pollen grain
walls, and to say something about how plants deal with the business
of handling sporopollenin as a wall material. However, because of
the extraordinary interest of recent chemical findings, I begin
with a few prefatory comments on this topic.
The Sporopollenin Concept
The conception of sporopollenin as a class of substances is
a chemical one, and as such it must obviously be immune from res
triction or re-definition on the basis of biological criteria.
Yet without querying the validity of the chemical generalisations,
it is legitimate enough to look critically at the biological impli
cations .
I suspect botanists, while acknowledging a common function
for the walls of spores, have been as much impressed by their
diversity as by their community of character. I do not refer
here wholly to structural diversity, which surely needs no emphasis,
but to chemical variation as revealed by cytochemistry, including
the cytochemistry of staining procedures, empirical and imprecise
as this may have been. As Professor Faegri's findings showed
(Faegri, 1935) the exine of the mature pollen grain has strata
with markedly different staining properties, and developmental
investigations have shown that these differ somewhat in the way
that they are formed. Furthermore, the exine during its develop
ment shows progressive changes in its stainability and reaction to
chemical fixation agents such as osmium tetroxide (Heslop-Harrison,
SPOROPOLLENIN IN THE BIOLOGICAL CONTEXT 3
1968a). When major taxa are compared, differences in the stain-
ability of spore walls are often found, and even within a group
such as the Angiospermae exines show some variation. In a survey
of the responses of the pollens of several species of flowering
plants from a range of families to some half-dozen common histo-
logical stains, we found that almost every species could be
characterised by the stainability of the exines of the mature,
fresh freeze-sectioned pollen grains.
Some of the differences in stainability of fresh exines
certainly result from variation in the amount and character of en
crusting and impregnating materials, notably those added by the
activity of the tapetum at the conclusion of wall development.
This factor aside, however, it remains true that cleaned exine
material from different sources very often does show cytochemical
diversity. No doubt much of this is attributable to differences
in degree of polymerisation and saturation, in amount of cross-
linking, or in the proportion and order of monomers, factors
insignificant enough so far as the broad chemical generalisation
of the nature of exine materials is concerned. But the point is
that there still remains a comparative chemistry of exines to be
looked into, for such factors may not be without phylogenetic or
adaptive significance.
I am aware from the abstract of Dr Chaloner's paper that he
will be commenting on some of the phylogenetic issues, but I feel
impelled to make some comment on the suggestion that sporopollenins
comprise a unique group of compounds, peculiarly associated with
4 J. HESLOP-HARRISON
the protection of propagules in all of the major plant groups.
As many evolutionary morphologists of the first half of this
century abundantly demonstrated, the development of the protected
spore was a key event in the spread of life over the land surface
of the earth. Bulky plants are by their very nature non-motile;
and the abandonment of water as a medium for existence by organisms
which could achieve photosynthesis on a massive scale on dry land
only became possible with the development of the spore as an agent
of dispersal. But the early spore-producers remained tied to water
as a medium for fertilisation, and liberation from this limitation
followed only with the telescoping of the haploid generation which
turned the microspore into the pollen grain, the agent of gametic
transfer. Protected unicellular or paucicellular propagules were
not, of course, the exclusive property of vascular plants. The
meiospores of bryophytes are invested with thick exines, and among
thallophytes zygotes commonly bear heavy walls, constituting then
zygospores or oospores. These latter structures represent quite a
different phase of the life cycle, but their role is comparable with
that of the archegoniate spore - namely, to attain dispersal and
to maintain the species through adverse conditions, primarily of
desiccation. Again we see that these are indispensable functions,
even for species whose vegetative life is pursued in water or satu
rated atmospheres, when suitable habitats are scattered or subject
to periodic drying, the conditions of life'for many algae and
fungi of the land surface.
Clearly, then, there is no particular difficulty in accepting
