On Some Methods in Palaeobotany

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  • On Some Methods in PalaeobotanyAuthor(s): H. Hamshaw ThomasSource: New Phytologist, Vol. 11, No. 4 (Apr., 1912), pp. 109-114Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2427204 .Accessed: 12/06/2014 22:28

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  • T H E

    tlEW PHYTOLLOGIST. VOL. XI, NO. 4. APRIL, 1912.

    [PUBLISHED APRIL 29TH, 1912].

    ON SOME METHODS IN PALAiOBOTANY.

    By H. HAMSHAW THOMAS.

    THE remains of the plants of past ages hav-e generally been pre- served in two ways. The first is by petrifaction, when the

    original cell outlines are preserved by the infiltration and solidifi- cation of lime, silica or some similar stubstance. The second method is by the formation of an impression or cast in a sandy or muddy matrix, the original plant-tissue being reduced to a coaly layer. But we have also in some cases a rather different form of preservation from the last mentioned; here the plant has been embedded in muid, little decay has taken place, and the remains which are now found are best described as " mummifled plants." In the case of leaves, the cuticles of both surfaces often remain, sometimes with portions of the tougher cells, the cellulose has become more or less converted into carbon, and we have here a stage in the production of the ordinary type of impression in which all the plant-tissue is reduced to a thin coaly layer.

    From the point of view of the botanist, of course, the petrifled plants are by far the most important. It has, however, rarely happened that the conditions were suitable for the petrifaction of plants, and eveni when we have petrifled remains they are often very fragmentary. Impressions are far commoner, and though we do not gain so much useful knowledge from them, they often exhibit the external features of plants with great clearness; they aid us in piecing together scattered fragments of the various organs of a plant, and sometimes furnish useful information as to the form of the reproductive organs.

    The mistakes of some of the earlier-and ever of the later-palaeo- botanists, whose observations were confined to plant impressions,

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  • 10 H. Hamshaw Thomas.

    have inspired in the minds of many, a profound distrust for much of the work based on the study of impressions. But where the latter can be correlated with petrifactions or some knowledge gained of their structure, a comprehensive study of their external form may be of the greatest importance. In the case of mummifled plants or of impressions in which the plant substance is not entirely destroyed, much information can usually be gained as to the form of the stomata, epidermal cells, spores, seeds, etc., by the use of suitable methods. This is especially the case with those Mesozoic plants which are preserved in fine shales or mudstones; the tissue has not yet become completely carbonised, and the remains of the plant can usually be easily detached from the matrix. These plants are especially worth study because we know of so few localities in which petrifactions of this age are found, and hence we have comparatively little knowledge of the affinities of many Jurassic plants, at the same time they are the key to many of the most discussed phylogenetic problems. In these circumstances a close study of the remains which have hitherto been regarded as structureless may lead to interesting results.

    The object of these notes is to give a brief account of some of the methods which have proved useful in studying Jurassic plants, chiefly of the mummifled or impression type.

    PREPARATIONS OF LEAF CUTICLES.

    These can be readily made from specimens of plants which originally possessed a fairly thick cuticle and which are preserved in a flne grained matrix fiom which they can be easily detached. It was shown by Borneman in 1856 that the carbonaceous material could be dissolved out by using Schultz's solution, and the method has since been used with considerable success by Schenk, Zeiller, Nathorst and others.

    The following method may be employed. A portion of the leaf is chosen in which the tissue shows a tendency to peel off, and some of it is removed with a needle. If a drop of wvater is previously placed on the spot, it often renders the removal of the tissue easier. The detached fragment is then placed in a shallow china pan of the type used by artists for mixing water-colours (these pans are circular and are made in sets, one fitting on top of another and acting as a cover), a little powdered Potassium Chlorate is sprinkled on it and a few drops of moderately strong Nitric Acid added. The pan is covered up, labelled, and left to stand for some time, Watch.

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  • On some Methods in Paleobotany. I I T

    glasses may be used instead of the china pans, but are not so stable and it is by no means pleasant to spill the strong acid solution. After a time, varying with different specimens from three or four hours to as many days, and depending on the strength of the acid and on the material of the leaf, the specimens become clear brown in colour. The acid is then decanted off, the specimen is washed with water and freed from any surface particles by the use of a fine brush. A few drops of ammonia solution are then added, which cause the cuticle to become black at first, but which later brings about the clearing of the specimen and the solution of the dark substances. If the process has been complete a more or less clear membrane is left, but care must be taken to separate the cuticles of the upper and lower surfaces at this stage, otherwise a clear view of neither will be obtained. This separation is usually a matter of some difficulty, but may be assisted with needles and by using the sharpened point of a match. If the specimeii does not become clear it may be washed and r eplaced in fresh acid and the process repeated. Some leaves may require treating for a considerable time before becoming transparent. The resulting cuticles are washed with water and may be stained or mounted directly. I often employ as a mountant pure Glycerine in which Zinc Sulpho- Carbolate has been dissolved (the latter substance improves the optical properties of the glycerine). The slides may be conveniently sealed with wax,' but if they are to be kept for a long time a coating of gold size should be added to prevent the wax from cracking.

    Preparations made in this way often yield a surprising amount of information about the form of the epidermal cells, the shape and distribution of the stomata. If hairs or papilke were present, they are clearly seen, while portions of the more resistent sub-epidermal tissue, fibres, etc., may occasionally be found. It seems probable that the study of the stomata may be useful not only in classifying the fronds from which they are obtained, but also in investigating their relationships and phylogeny.

    THE STUDY OF SPORANGIA AND SPORES.

    Careful search in flne grained fossiliferous shales often results in the discovery of fertile specimens of ferns, conifers, etc. The sporangia of ferns are often quite clearly preserved though con- verted into carbonaceous material, and the annulus if present, can be clearly seen. The prismatic binocular microscopes made by

    I See NEW PHYT., VOl. X, P. 105, 1911.

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  • 112 H. Hamnshaw Thomas.

    Zeiss are of the greatest service in the examination of these and similar opaque objects. Even if the sporangia cannot be clearly made out, preparations of the spores can be readily obtained. The cutinised exospore wall seems to be almost indestructible and if embedded in a suitable matrix remains unaltered for an indefinite period and can be obtained by dissolving away the surrounding material. Thus spores can be obtained from Carboniferous andi Jurassic plants by treating them with Nitric Acid and Potassium Chlorate in a similar way to that used for the leaf-cuticles. If the sporangia are small it is best to remove them from the rock with a needle and place the fragments on a glass slide possessing a small concavity. A drop of Nitric Acid and a few tiny crystals of KCIO3 are placed on the fragments and the slide is put under a small bell-jar with a dish of water, or between two watch glasses the lower of which contains some water. In this way it may be kept for a day or two without the acid drying up. When the carbonaceous fragments have tur ned brown they are carefully washed with water and a drop of dilute ammonia is added. This at once dissolves away the opaque material and the spores are liberated. The spore-walls show the markings they originally possessed, and the tetrahedral or lateral scar indicating their mode of origin is usually seen. The preparation and permanent mounting of spores in this way requires very cautious manipulation, but by using a glass slide on which the whole process is carried out, the progress of the reaction can be watched under the microscope. The fact that microspores are preserved in this way is most useful, especially when we are dealing with structures which appear to be like sporangia, for we can easily test whether they are really of this nature, and whether they are micro- or macrosporangia. It appears as though the megaspore membrane were cutinised in the same way, and if we apply the same method of treatment to seeds it is remarkable how many tissues become visible and seem to have escaped destruction. In some such seeds the outside of the integuments, the megaspore membrane and remains of fibrous cells been visible; traces of thickened hypodermal cells and portions of tracheids may also be seen occasionally.

    CEDAR-WOOD OIL.

    The examination of indistinct or badly preserved opaque objects under the binocular microscope is often facilitated by the use of cedar-wood oil and strong illumination. A drop or two of oil placed

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  • On some Methods in Paleobotany. I 13

    upon the object has the effect of differentiating its parts more clearly, of bringing out any slight differences of colour and of making minute veins or markinigs more easily visible. As is well known, water has a somewhat simiiilar effect, but it quickly soaks in and often destroys the specimen, on the other hand the oil dries up slowly after use without damaging the specimen at all. This method was devised by Dr. T. G. Halle of Stockholm.

    The use of oil in the same way has proved very helpful in examining the polished faces of blocks of petrified material especially where the cell walls had lost their colour and were almost invisible without such treatment.

    THE COLLODION METHOD.

    In cases where the cast of the epidermal cells of a leaf have been retained in fine grained material such as tufa, the microscopical features of the original cells can often be made out by taking a collodion cast of the fossil. Accounts of this method have been published by Professor Nathorst' and Dr. Bather2. The process is very simple, a drop of collodion is placed oni the surface of the fossil and allowed to dry; as it dries, it peels off the rock, and if sub- sequenitly mounted and examined under the microscope it may show clearly the outlines of the epidermal cells. I found great difficulty at first in employing this method successfully. The collodion dried slowly to a whitish semi-opaque film which often adhered firmly to the specimen and could only be removed with difficulty. This seems to be due to the composition of the collodion, and the ordinary British Pharmacopoeia solution is quite unsuitable. It apparently contains too much alcohol to allow the rapid drying of the film, but by allowing a quantity of this original solution to evaporate to a jelly and redissolving it in ether, a solutioni was obtained which dried rapidly, peeled off the rock readily and gave a transparent film.

    I have not found this method very useful when working with Palkeozoic and Mesozoic impressions because it is seldom that the matrix was suitable for retaining a cast of the epidermal cells. It is, however, of considerable value in dealing with petrifled wood, etc. Films taken from freshly fractured surfaces, often show as much or more than a thin section cut in the same direction through the specimen. It is therefore of great use in a preliminary study of material and for comparison with sections. In order to get perfectly

    I Ark. for Botanik VII, p. 1, 1908. 2 Geol. Magazine, Decade V, Vol. V, p. 454, 1908.

    I

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  • 114 C. H. Ostenfeld.

    clean films from a surface, several preliminary casts should be taken and discarded, all the loose material will be picked off witlh the earlier films, and the surface left quite clean.

    It seems probable that a renewed examinationi of fossil plants by some of the methods just described will r esult in gr eat additiolns to our knowledge of their structure and affinities. The elaboration of these and similar methods is giving a new impetus to the study of what has been hitherto regarded as very unpromising material from a botanical standpoint. Most of the methods described in these notes were devised or improved by Professor Nathorst of Stockholm, who has done so much for the study of Jurassic plants and to whom the thanks of the author is...