Fossil Spores From New Zealand Coals
Terminology. Spores are highly specialised, reproductive cells of plants. The term spore, used in a broad sense, includes spores (sensu strictu), microspores, pollen grains, and megaspores (or macrospores).
Spores (sensu strictu), are the asexual reproductive bodies of the lower groups of plants such as fungi, mosses, and many ferns.
Microspores are developed in the microsporangia of heterosporous plants such as small club mosses (Selaginellaceae), gymnosperms and angiosperms. They are the smaller spores of heterosporous plants and they usually give rise to male gametophytes. Although the terms microspore and pollen grain are used in the broad sense as synonyms, there is a distinction in their strict botanical application—the microspore is the immediate product of division of the microspore mother cell whereas the pollen grain contains within its walls the micro-gametophyte developed from the microspore.
Megaspores are the larger spores of heterosporous plants, and they usually give rise to female gametophytes.
Without knowledge of the type of plant to which a spore belongs it is very difficult to determine whether a spore is a spore (sensu strictu), a microspore. a pollen grain, or a megaspore. In the following account the term spore is used in the broad sense unless otherwise stated.
Production and Distribution of Spores. Plants produce prodigious numbers of spores. An idea of the immensity of their output is given by the following figures (Erdtman, 1943, p. 176):—
|Pollen output of ten-year-old branch systems:|
|Pinus silvestris (a pine)||346,000,000|
|Quercus sessiliflora (an oak)||111,000,000|
|Fagus silvatica (a beech)||28,000,000|
Some spores, especially those carried by the wind, may be transported for considerable distances (up to 1,000 km. or more [Ibid., p. 179]), but these fartravelled spores are of minor importance when compared with the vast numbers which settle at no great distance from their points of origin.
Spores as Fossils. It is well known that peats, coals and other sedimentary rocks (from the Silurian to the present day) contain fossil spore remains.
The protoplasmic contents of a spore are surrounded by a membrane, the intine, which in turn is enclosed by a stout outer coat or membrane, the exine. The exine is composed of a number of resistant substances such as sporonine. cutin, esters of higher fatty acids, oils and resins. All of these are extremely resistant to chemical reagents and to chemical and biochemical decomposition caused by bacteria, fungi, and other lowly organisms. Because of their resistant nature, although the inner less resistant parts are decomposed, spore exines remain unattacked in relatively large numbers (Thiessen, 1935). The exines of many spores are finely sculptured and ornamented and the preservation of these intricate markings in material millions of years old is indeed remarkable.
After their aerial journey spores may settle in peat bogs or lakes and bays where sediments are being deposited, and become entombed and preserved in the accumulating peat or sediment. “In this way the peat bogs and sediment banks become archives of vegetational history imprisoning pollen grains, season after season, millennium after millennium.” (Erdtman, 1943.)
The Importance of Fossil Spores in Geology. The importance of fossil spores in geology is twofold—first, they can be used for the correlation of certain sedimentary rocks, and secondly, palaeobotanical knowledge is greatly increased by fossil-spore studies.
In many cases certain morphological characters (size and shape of the spore, sculpture and thickness of the exine, number or absence of germinal furrows and pores, and the nature, markings and extent of any appendages) make possible the identification of the family, genus or even species to which a fossil spore belongs. A valuable key to the constitution of floras of the past is thus provided. Geological information regarding palaeogeography and palaeoclimatology is obtained from knowledge of the nature and oecology of plants in these fossil floras.
Spores from the Ohio Coalfield.
The presence of spores, sometimes in considerable abundance, in the attires of Ohio coals was noted during the examination of thin sections. Except in a very few cases of extremely large spores, spore detail could not be determined successfully in thin sections of the coal owing to the nature of the matrix.
Microscopic study of the morphological detail of the spores required the separation of the spores from their attrital matrix and special preparation of the material thus obtained.
Preparation of the Spores for Mioroscopic Examination. A modification of the method outlined by Raistrick and Marshall (1939, p. 130), gave good results. The separation of the spores from the coal was accomplished by oxidation of the bulk of the coal constituents with Schulz solution, followed by alkali treatment to dissolve the oxidised material, leaving a residue consisting mainly of spore exines, with small amounts of other resistant plant debris. This residue was stained and mounted as required.
The coal sample to be examined was first crushed to pass an 8-mesh sieve*. Two grams of this crushed coal were then mixed with 2 gm. of finely crystallised potassium chlorate. This mixture was added, little by little, to 30 cc. of concentrated nitric acid in a small beaker. (It is necessary to mix the coal and chlorate very gently, and to add the mixture to the acid slowly, to prevent explosive reaction. Coal samples fresh from the mine usually contain a little moisture which slightly retards explosive reaction, but even their use demands considerable care.) After gentle stirring the beaker was covered with a watch glass and placed in a fume cupboard.
After eighteen hours the contents of the beaker were poured into a porcelain evaporating dish and the liquid portion containing fine suspended matter was poured off. The remaining lumps, now quite soft, were crushed with a pestle to expose the coal material more thoroughly to the oxidising solution. The solution which had been poured off was then poured back into the evaporating dish containing the crushed coal and the mixture was returned to the beaker, which was again placed in the fume cupboard.
After eighteen hours as much liquid as could be removed conveniently without disturbing the sediment at the bottom of the beaker was drawn off by means of a 10 c.c. pipette fitted with a rubber suction bulb. A little distilled water was added to the sediment, and after thorough stirring the mixture was poured into “heavy duty” glass centrifuge tubes. Water was added as required to balance the tubes. The material was centrifuged until the supernatant liquid was quite clear. Washing of the residue with water, followed by centrifuging, was repeated three times. Water was then added to the residues in all tubes and their contents were poured into a beaker. More water was added to make the volume up to about 50 cc. The mixture in the beaker was stirred vigorously and then strained through a 36-mesh sieve to remove the larger debris. The material which had passed through the sieve was left in water until the finest sediment had settled to the bottom of the beaker. This settling process usually took about twelve hours.
The water was then drawn off with the pipette and 20 cc. of 10% potassium hydrate were added to the sediment. After thorough stirring the mixture was left to react and settle for twenty-four hours. The liquid, almost black in colour, was again drawn off from the sediment with the pipette and the sediment was washed and centrifuged several times until the supernatant liquid was no longer tinted brown.
After centrifuging, 25% alcohol was added to the residue in the tube, and this was left for twelve hours. Treatment with 50% alcohol for six hours, followed by 75% alcohol for three hours, was then carried out as a preliminary to staining. Staining was effected by adding a small quantity of a saturated solution of basic fuchsin in water to the residue in the tube. The intensity of staining could be varied very simply by the length of time the stain solution was left on the residue. Staining for fifteen seconds was found to produce a useful intensity. After staining the material was washed with water and centrifuged several times until the pink colour was no longer visible in the supernatant liquid.
A little of this residue was mounted in glycerine jelly and the stained spores were ready for microscopic examination. The remaining residue was stored for future use in a solution of 10% glycerine, to which a small crystal of phenol had been added.
Discussion of the Method. As Schulz solution is a particularly vigorous oxidising agent, the exines of some of the less resistant spores may be attacked with obliteration of morphological detail, while others may be completely de-
[Footnote] * The sieve sizes refer to the number of meshes to the inch.
stroyed. However, using Schulz solution, the writer has separated thousands of spores whose major morphological characters are plainly visible and in many cases the most intricate detail of exine markings can be recognised. It is suggested here that the temperatures and pressures to which the spores have been subjected for millions of years have induced chemical changes which have increased the resistance of the original spore exine materials. The insolubility and chemical composition of fossil resins, compared with present-day resins, may be cited as support for the above suggestion. The resistance to chemical reagents of spores in Tertiary and older coals is thus considered to be far greater than that of spores from Pleistocene and Recent peats and other deposits of much younger age.
Treatment with Schulz solution followed by potassium hydrate has the great advantage that the bulk of the attritus in which, the spores are embedded can be removed in solution, allowing preparation of spore concentrates with very little foreign material. The chlorination acetolysis method recommended by Erdtman (1943, p. 34) for peats does not remove sufficient debris from coal to make bulk treatment profitable. Other methods which the writer attempted had the same disadvantage. It is possible, however, that spores separated by methods using less reactive solutions may be better suited for observations with magnifications of about 1,000 diameters.
Gentian violet and methyl green used as staining solutions proved quite effective. Staining by using methyl green and fuchsin jellies as mounting media did not give staining of sufficient intensity to facilitate counting although the light staining produced by this method showed sculpture and markings to advantage. The use of basic fuchsin solution provided a simple method of effective staining. Staining for about fifteen seconds was found to be sufficiently light to show major morphological characters and sufficiently intense to permit easy counting. Lighter staining for the examination of intricate morphological detail could be effected conveniently by dilution of the fuchsin solution.
By the preparation method outlined above mounts were made from the spore concentrates with about two hundred spores to the square centimetre. With crushing in two stages, the numbers of broken and torn spores appeared to be less than the number of damaged spores found when coal which, had been ground to pass a 36-mesh sieve was used at the beginning of the preparation.
Spore-sampling of banded coal in lump form is considerably aided by mechanical separation of the spore-bearing attritus from the anthraxylon and fusain, which do not contain spores, before chemical treatment. This will ensure maximum concentration of spores with minimum occurrence of other plant debris. Mechanically separated spore-bearing attritus, obtained by using a small hammer and chisel, a scalpel, forceps, and dissecting needles gave excellent results for the preliminary examination of the Ohai coals.
Examination of Spores from the Ohai Goal. Striking morphological differences were revealed by microscopic examination of the spores from the Ohai coal Differences were noted in the size and shape, in the thickness of the exine, in the presence or absence of exine sculpture (in the nature and extent of the sculpture when present), in the presence or absence of germinal furrows, pores, and tetrad-scars (in the number, arrangement, and nature of these when present), and in the presence or absence of appendages (in the number, size, shape and ornamentation of these when present). Several compound types and a few peculiar spore aggregations were also noticed.
Attempts to illustrate these morphological differences of the spores are given in Plates 14, 15, and 16. The illustrations were prepared using a magnification of about 450 diameters. The figures in Plate 14 are camera lucida drawings, but the remainder were drawn to scale using measurements made with an eye-piece micrometer. While every care was taken to represent the appearance of the spores accurately the sketches are only very rough approximations, for the detail of the finer morphological characters can only be observed by microscopic inspection of the actual material.
All the spores have been considerably flattened by compression which the coal seams have undergone. The flattened nature of the spores was observed by mounting a little of the spore concentrate in water. The spores were pulled across the field of view by touching one edge with a small piece of blotting paper. During the resultant movement of the water towards the blotting paper many of the spores turned over several times and their flattened nature was observed. Attempts to expand the spores by soaking in glycerine were unsuccessful.
The apparent shape and size of the spores is thus a distortion of the original form. However, if the spores have been compressed vertically without horizontal distension (Walton, J., 1940, p. 8), the distortion seen in proximal, distal, or side views may not be very great.
Stratigraphical Correlation by Means of Spores.
Spore studies have been used successfully for stratigraphical correlation in many countries (Thiessen and Staud, 1923; Thiessen and Wilson, 1924; Raistrick and Marshall, 1939, p. 127; Erdtman, 1943, p. 171). Peats, coals, and other sediments containing spores have been correlated by this means.
In the correlation of coal seams “two assumptions are fundamental to the work, and results in recent years have justified the use of these assumptions as working hypotheses. They are that:
The spores, and especially the microspores, were distributed from the parent trees by wind, and, like the pollen of modern trees and plants, were capable of being carried for great distances. The result was that at any one particular time in the coal swamp the myriads of microspores produced would be mixed and wafted about in the wind so that a fairly uniform scatter was obtained, and the mixing would be such that at most localities a statistical average of all the types of spores being produced in the coal forest at that time would be found.
It is also assumed—and the assumption is again supported by observation—that there would be sufficient difference in the make-up of the flora of successive coal-seam swamps to give the spore average a recognisably different proportional make-up. On a larger scale, this assumption is proved in the succession of floral zones now being used by Dix and others for the correlation of the Coal Measures.” (Raistrick and Marshall, 1939, p. 129.)
A roof-to-floor sample of the seam to be examined is collected and after crushing and prolonged mixing a small amount of this is treated chemically to separate the spores. A spore diagram for the seam is made by counting some hundreds of spores in a preparation and then expressing the number of each different type as a percentage of the total of all types. The percentages are graphed as successive vertical blocks “giving a pattern which has been found in the case of many seams to be constant over the whole area of a coal-field. It is also found that the patterns of adjacent widespread seams are generally different, and that in a vertical succession of several seams, taken in one part of a coal-field, the same differences from seam to seam are found as at another part of the same field. In this way the seam correlations can be spread over a whole field, and an isolated coal can be fitted into the diagrams in the correct place…
“In addition to the general types just mentioned, each coal contains a varying proportion of microspores present only in very small quantities, which may be called ‘accessory’ spores. These accessories may be just as important as the more abundant general spores for correlation, and may contain among them one or more spore types of very restricted range. … As in all types of correlation, identification of the coal will be based not only on the rare types of spore present, but also on the varying proportions of the commoner types—i.e., the general spore diagram—supplemented by the accessory spores.” (Raistrick and Marshall, 1939, p. 130)*
Possible Correlation of New Zealand Coals by Means of Spores. The abundance of spores in coals from Ohai has been mentioned previously. Separation of spores from Waikato, Greymouth, Charleston, Mataura, and Kaitangata coals, using the method described earlier, has been carried out by the writer, and shows that abundant spore material (the basis for correlation), is present. Future work will very probably show that, nearly all, if not all, New Zealand coals contain sufficient spore material to make correlation investigations possible.
Similarity in the spore content of seams from the same field (e.g., Ohai), and great difference in the spore content of coals from widely separated districts (e.g., Charleston and Ohai), have been noticed. Further investigation in New Zealand may show that spore correlation of seams in the same field can be made. Spore studies may also show similarities and differences in age of coals whose chronology is at present imperfectly understood.
[Footnote] * Description and illustrations of the method outlined above are given by Raistrick and Marshall (1939, pp. 127–136).
Notes on the Vegetation which Formed the Ohai Coal.
Following Lillie (1945), an autochthonous origin of the Ohai coal is assumed. As the Ohai coal-field is approximately thirteen square miles in area the further assumption is made that the majority of spores in the coal are those from plants which grew in the original coal swamps, minor spore contributions being made from vegetation adjacent to the swamps, and still smaller contributions from plants farther afield.
Spores of pteridophytes, gymnosperms, and angiosperms were found in the Ohai coal.
A coniferous gymnosperm element was obviously dominant in the Ohai coalswamp flora for spore counts of roof-to-floor samples from the Birchwood, Linton, and Wairaki Mines showed that approximately 55% of the total number of spores were winged conifer pollen grains. (Wing-like bladders, sometimes called air-sacs or wings, are only found in pollen grains belonging to some genera of the tribes Abietineae and Podocarpineae in the Coniferales [Wodehouse, 1935, p. 243]).
One particular type of pollen (Plate 16, Figs. 5, 6, 7, and 8), made up about 85% of the total winged-conifer pollen. This pollen is more similar to the pollen of Phyllocladus than to any other living N.Z. conifer. The Ohai pollen has, however, peculiar knob-like protuberances which are not known in Phyllocladus*. Further, the pollen differs slightly from descriptions and figures (Cranwell, 1940) of Phyllocladus pollen in size, in. the width of the furrow, and in some wing features. The “knobs” were first thought to have resulted from collapse of the pollen grains (Plate 16, Fig. 7), but grains such as shown in Plate 16, Fig. 5, prove that this is not the case. Apparently knob-like protuberances, one associated with the attachment of each wing to the grain, are a morphological feature of this particular pollen from Ohai. Wodehouse (1935) and Erdtman (1943) do not mention “knobs” as morphological features of the pollen of any living (or extinct) conifer. It is therefore assumed that this conifer pollen with knob-like protuberances has been derived from an extinct genus.
The majority of the other 15% of conifer pollen is very similar to that of the genus Podocarpus, and a small amount resembles Dacrydium pollen. The three winged pollen shown in Plate 16, Figs. 1 and 2, is very similar to the pollen of Podocarpus dacrydioides, but the spacing of the bladders is a point of difference from Podocarpus dacrydioides pollen.*
The coniferous element described above is considered as the source of the abundant resin associated with the Ohai coals. The search for Agathis and Libocedrus pollen has as yet proved abortive, but as the pollen of these two resin-producing conifers is not easily preserved (and not easy to identify), a conclusion regarding the presence or absence of these two genera would not be justified.
Pollen closely resembling that of Nothofagus is present in very small amounts.*
Lycopodium spores are also present in very small amounts.*
The diagram (Text Fig. 1), from which a very general idea of the early Tertiary Ohai vegetation can be obtained, is based on spore counts of roof-to-floor samples from the Birchwood mine. This diagram is constructed on principles given by Erdtman (1943, p. 156).
Many of the complex problems associated with the study of floras of considerable geological antiquity are better left to the skilled botanist, but the above intrusion into the purely botanical field clearly demostrates the wealth of “raw material” (in the form of spores) which awaits the botanist and palaeobotanist in New Zealand coal.
The preceding account is a portion of a Jacob Joseph Scholarship Thesis prepared at Victoria University College.
The author is indebted to Mr. W. G. Hughson, Director of the New Zealand Coal Survey, for providing numerous coal samples.
The generous assistance of Mr. W. F. Harris of the Botany Division of the Department of Scientific and Industrial Research, whose experience and knowledge of the spores of the present New Zealand flora and New Zealand peats were of great help, is gratefully acknowledged.
[Footnote] * Mr. W. F. Harris, an authority on spores of the present New Zealand flora, has confirmed these statements.
The author is also indebted to Dr. I. V. Newman and Mr. J. H. Warcup, of the Victoria College Botany Department, without whose assistance much of the microscope work could not have been carried out.
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Lillie, A. R., 1945. Geological Report on the Ohai-Nightcaps Coalfield (MS).
Raistrick, A., and Marsiiall, C. E., 1939. The Nature and Origin of Coal and Coal Seams. The English Universities Press, Ltd., London.
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Thiessen, R., and Sprunk, G. C., 1935. Microscopic and Petrographic Studies of Certain American Coals. U.S. Dept, of the Iuterior, Technical Paper 504.
Walton, J., 1940. An Introduction to the Study of Fossil Plants. Adam and Charles Black, London.
Wodehouse, R. P., 1935. Pollen Grains. 1st Edit. McGraw-Hill Book Co., Inc., New York.