Coalification of Woody Tissue as Deduced From a Petrographic Study

45 Coalification of Woody Tissue as Deduced From a Petrographic Study of Brandon Lignite WILLIAM SPACKMAN

Downloaded by EAST CAROLINA UNIV on December 12, 2017 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch045

The Pennsylvania

State University, University Park, Pa.

ELSO S. BARGHOORN Harvard

University, Cambridge,

Mass.

The Lignogene suite of coal macerals is derived largely by vitrinization, fusinization, and micrinization of plant cell walls composed of cellulose or a cellulose-lignin complex. Early in coalification, unlignified cellulosic ray cell walls commonly transform with little evidence of developing a plastic phase.

However, the lignified fiber-tracheid and

fiber cell walls vitrinize to develop a semiplastic, sometimes almost fluid phase. The various fiber-tracheid and fiber wall layers respond differentially to vitrinization, and any single piece of wood appears to change heterogeneously. Sclerotic walls respond variously, yielding either high reflecting, dark-colord vitrinitic macerals, "normal" vitrinitic macerals, or low reflecting, cream-colored materials in lignitic coals. Source of at least three of the major maceral series was woody tissue of Brandon plants.

Coalification and carbonification are terms often used to describe collectively the processes involved in the formation and metamorphosis of coal substances. T o be more definitive, such words as vitrinization, fusinization, or micrinization have been used. It is clear, however, that these terms describe sets or sequences of normal chemical reactions, in some instances accompanied by physical changes. O u r ignorance of the details of coal substance formation and evolution has stimulated the use of these terms and the creation of others to indicate what we think we know concerning coalification. Vitrinization is 695 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


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thought to involve a sequence of reactions different from fusinization, a n d both of these sets or sequences are probably different from the reaction sequence involved i n forming macérais of the exinite series. O u r present knowledge permits only general statements concerning the chemical changes occurring during maceral evolution and, in our opinion, this significantly deters advancing knowledge i n many aspects of coal science. W h a t is the solution? First and foremost, the slow progress in this realm can be attributed to the coal petrographers. They undoubtedly have the initial role, for until the units comprising a heterogeneous mass are known, such units obviously cannot be sampled and studied. Fear of criticism has led the petrographer to adhere to simple concepts of coal seam constitution which regard a l l coals as consisting of less than a dozen common "macérais" and even fewer "lithotypes/' Such reputed macérais and lithotypes often have been supplied to the chemist for study and were accepted as well-defined samples of high purity. Normally, the sample purity presented no problem, but the analytical data have often varied widely from sample to sample and when one investigator's study was compared with another. This has led to confusion and even disbelief in the value and importance of pétrographie data. O f course, it is simply the result of including too many different entities in a single category. It is useful for the petrologist to point to possible differences between coal substances as the first step in advancing basic coal science. H i s role, in this connection, is that of creating hypotheses which ascribe existence to certain coal substances and certain maceral-mincral aggregates so that the chemist and physicist may validate, refute, or amend the hypotheses to increase our knowledge of coal composition. T h e penologist's job must be done first, simply because coal seams are rock masses, and the most expedient way to understand a rock mass thoroughly is to base one's study on that which can be perceived readily concerning entity distribution within the mass. Because we are concerned with altered plant materials, the petrologist, as well as the chemist and physicist, should be aware of the hypotheses generated by paleobotanists concerning coal substance derivation and evolution. As in the case of the petrologist, the paleobotanist's contribution may often be restricted to creating hypotheses that w i l l require validation, refution, or amending. This paper describes one type of contribution to coal petrology that can be made by the paleobotanist. Materials

Studied

The vitrinitic substances found in coal seams are commonly asserted to be derived from " w o o d " or " w o o d y " tissue. T h e statement is usually made without any intention of being specific about what is meant by wood a n d , in fact, several non-wood tissues are often included intentionally in this broad generalization. It is generally agreed that various different substances are contained in these woody source materials and that several different biochemical, chemical, a n d physical processes may influence their alteration. In spite of this, our coal research has led many individuals to think of the vitrinites as forming a single maceral series. T h e coalified woods in the Brandon lignite

Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


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provide us with an unusual opportunity to evaluate this concept a n d to examine some of the first products of the coalification of wood. The Brandon lignite is a small deposit of Tertiary coal occurring i n west central Vermont ( J , 2, 7 ) . It consists of a poorly stratified mass, composed of a mixture of coalified fragments of stems, roots, bark, seeds, fruits, flowers, a n d other plant parts in a matrix of organic and mineral debris. Prior reference has been made to the significance of this deposit with respect to coal petrology ( 2 ) , and we would like to amplify the subject at this time with particular reference to the discrete fragments of coalified woody material. F o r the most part, we shall be concerned with the fragments of secondary xylem that are common i n the deposits. These wood fragments are often identifiable as wood produced by plants belonging to the genera Cyrilla, Persea, and Gordonia. This being true, it is possible to associate paleobotanical and neobotanical facts, adding to one's knowledge of the original chemistry of the source materials. ( T h e botanical identities of the coalified woods were determined by extensive comparative study using the Harvard W o o d Collection. Identifications were confirmed by studying the other fossilized plant parts contained in the deposit.) Each wood is, of course, a complex tissue consisting of several cell types and several substances. T h e manner in which each has reacted to the coalifying conditions associated with the Brandon deposit may be valuable in understanding genetic interrelationships within the Lignogene maceral suite. Description

of

Observations

The lignitized materials w i l l be discussed by comparing coalified products observed i n each of the three wood types. As i n modern species (Figure 1), the secondary xylem found i n the species of Persea represented in the Brandon deposit is composed of four basic cell types: (1) a matrix of fiber-tracheids, (2) vessels that commonly occur singly or i n pairs, (3) ray parenchyma with "inflated" marginal ray cells, a n d (4) vascicentric xylem parenchyma. In the mature tree the first two cell types form the bulk of the wood. Both of these cell types are dead at maturity, hence devoid of protoplasm in the cell cavity. Their cellulosic cell walls are heavily impregnated with lignin, and the individual cell units are held together by an intercellular substance. T h e latter is initially composed of pectic compounds, and these are either heavily impregnated or replaced with lignin at maturity. Both primary and secondary walls are present, the latter being relatively thick. This cell wall material and the intercellular substances are about all these cell types offer to the coalifying processes. They form the bulk of the wood, and therefore these source materials are volumetrically important. The cell walls of the vessels in Persea wood have reacted to the coalifying processes i n a w a y that is different from the response of the fiber-tracheid walls, even though the two wall types may lie i n juxtaposition. Microscopic study of thin sections, cut on a microtome, show that the vessel walls are yellow-tan i n color, anisotropic, and morphologically intact though often deformed. In contrast, the fiber-tracheid walls have been converted to a yellowbrown to orange-brown, isotropic material that exhibits little of the original



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anatomical structure (compare Figure 1 w i t h Figures 2 a n d 3 ) . This is interpreted to mean that much of the original cellulosic framework remains i n the vessel element walls and that it has been destroyed i n the walls of the fibertracheids. T h e anisotropy exhibited i n Figure 4 also suggests this, a n d further confirmation has been obtained by extraction studies ( 2 ) . T h e vessel cavities may contain a dark, reddish-brown, isotropic material (Figure 2 ) , a yellowbrown, granular, isotropic material (Figure 3 ) , a red-brown, granular, isotropic material, or the cavity may be devoid of any solid substance. A common feature of this wood is the presence of tyloses i n the vessel. These are portions of adjacent parenchymatous cells that have grown into the vessel cavity through perforations i n the side walls. In the wood studied they have become "sclerotic." Secondary w a l l layers have been deposited by the tylose cell, a n d these have become lignified. These tylose walls have responded to coalification i n a distinctive manner, often yielding both a light yellow and a dark red product (see Figure 5 ) . T h e primary wall (and probably the outermost secondary w a l l layer) commonly gives rise to the light-colored product, a n d the inner layers of the secondary wall frequently are converted into the dark-colored substance. In many instances there remains only a dark red product with no evidence of the original laminae that composed the w a l l . However, i n a l l instances observed, the coalified secondary wall of the tyloses contrasted markedly with the light yellow or buff color of the vessel elements' secondary w a l l . This is shown i n Figure 6. T h e tylose wall yields another product that deserves description—a granular material that ranges from a red-brown color to an optically dense state (see Figures 7 a n d 8 ) .

Figure 1. Transverse section of secondary xylem of Persea borbonia —• showing characteristic form of vessels, fiber-tracheids, vascicentric xylem parenchyma, and ray parenchyma in uncoalified wood. 220X Figure 2. Transverse section of coalified Persea secondary xylem for comparison with Figure 1. At least three coaly products are shown: dark colored cell inclusions, vessel wall derivatives, and fiber-tracheid wail derivatives. 304X Figure 3. Transverse section of coalified Persea wood showing: yellow-tan vessel wall material, orange-brown fiber-tracheid wall material, pale tan granular material in vessel lumen (just above center of picture), and dark colored tylose walls within vessel cavities. 88X Figure 4. Same as Figure 3 but with light partially polarized. of vessel walls, suggesting retention of cellulosic framework.

Note 88X

anisotropy

Figure 5. Transverse section of coalified Persea wood showing a single vessel packed with tyloses. Note that the tylose walls are composed of a dark colored material that contrasts with the vessel wall residue. 284X Figure 6. Longitudinal section showing the yellow-tan anisotropic material that typically forms the remnant vessel walls in the coalified Persea wood. The contact between two vessel elements is shown with the dark red-brown tylose wall material in contact with the upper vessel element wall and extending across the perforation plate area into the lower element. 784X %



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In addition, the parenchymatous cells i n the xylem appear to yield at least two lignitic materials. O n e is a dark red-brown material that may be derived from inclusions originally present i n the cell. This material is common a n d may be observed i n Figures 2, 3, 5, 6, 7, a n d 8. T h e other coalified product associated with this cell type is yellowish i n color a n d derived from the wall of the cell (Figures 2 and 6 ) . Ray parenchyma and longitudinal xylem parenchyma typically form only a primary w a l l , a n d usually the wall is devoid of lignin. It is of interest that this wall layer is preserved and that it is frequently possible to observe the simple pits that are characteristic of this cell type (Figure 2 ) . Contrasting with the response of the Persea wood, is that of the wood of Cyrilfo. A s previously mentioned, the vessel walls of Persea often have remained "morphologically intact." T h e comparable walls of CyHlla exhibit responses ranging from i n situ minor alteration to complete destruction or transformation to either a granular or a textureless residue. A s Figure 9 shows, the vessel element walls frequently have become swollen as a result of coalification, a n d the innermost surface of the w a l l often exhibits a tuberculate or granular appearance. In some instances the swollen wall appears to have reduced the cell cavity to a fraction of its original size or to have eliminated the lumen completely. A l l transitions can be observed from an anatomically undistorted wall that has merely taken on some color as the result of coalification, through various degrees of swelling a n d granulation, to situations i n which the original cell outline is faithfully preserved, but no recognizable wall or lumen remains. In the latter case, the area formerly occupied b y the wall and lumen is now filled with either a vitreous, cream-colored material or a pale tan to brown granular substance (see Figure 9 ) . It is difficult to envision the manner i n w h i c h the large volume of material required to occupy the wall and lumen areas could be derived from the original wall substance. L o n g i tudinal sections show no evidence that material flowed from one level i n the vessel to a collecting site. It is possible that the vitreous a n d granular materials represent lignitized products of substances that occluded certain vessels in the living tree. Such vessel inclusions are not typical of Cyrilla w o o d , but they do occasionally occur. T h e frequency of this phenomenon i n the lignitic wood casts doubt on this explanation, but the perfection of the cell outline argues for an emplacement of the material inside a walled chamber. If, during coalification, the vessel wall were i n the process of swelling and approaching a plastic state and if the encasing matrix of fiber-tracheids were also i n a plastic or semiplastic condition, it is difficult to conceive of a synchronous filling of the lumen without distortion of the cell outline. One possibility is that the thin primary wall of the vessel resisted any appreciable degradation until after the secondary wall was altered a n d the lumen eliminated, either by being filled with the by-products of secondary wall coalification or by infiltration of a foreign substance, derived, perhaps, from fiber-tracheid degradation. Regardless of origin, four materials are found i n the lignitic wood i n the place of the vessels. These are: (1) buff colored, friable, isotropic material clearly derived from vessel walls, (2) yellow-brown, friable, isotropic material derived from vessel walls, (3) pale tan to brown, granular, isotropic secondary



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cell wall degradation material a n d lumen fillings, and (4) cream colored, vitreous, vessel lumen fillings. The fiber-tracheids of Cyrilla yielded large quantities of an orange-brown, isotropic substance (Figures 9 a n d 10) with small quantities of a pale yellow anisotropic material that probably represents a remnant of the primary walls. " W o u n d tissue" is formed by Cyrilla under certain conditions. This consists of tangential layers of fibers that possess exceptionally thick walls and a very small lumina (Figure 1 0 ) . T h e massive, multilaminate, secondary w a l l is composed of a pale yellow, anisotropic material, except that the outermost layer often is altered to an orange-brown substance that is indistinguishable from the material so commonly derived from fiber-tracheid walls (see left half of Figure 10). In Figure 10, a red-brown to optically dense material can be seen occupying the lumen of one of the centrally located fiber-tracheids. I n the living tree the fiber-tracheids and parenchyma strands i n the zone of " w o u n d tissue" often contain dark colored inclusions. This granular material appears to have been derived from such a source. T h e ring of deep red-brown material i n the cell adjacent to that containing the granular product probably is a cell wall derivative. Although associated with Persea and Cyrilla wood i n the same lignitic matrix, the wood of Gordonia was altered in a w a y quite different from that characteristic of the other woods. In almost a l l of the specimens examined, the vessel walls and cavities have been destroyed completely. This process of vessel wall destruction is so effective that previous investigators (4, 5) mistakenly identified the wood as a conifer, calling it a Pittjoxylon. Hence, i n the case of the coalified Gordonia wood, virtually nothing remains that is identifiable as the derivative of vessel substances whereas i n the case of both Persea and Cyrilla four or more coal substances were readily associated with vessel source materials. Another interesting feature of the response of Gordonia wood to coalification is the development of a homogeneous appearing, amberbrown substance from the walls, or at least in the position of the walls, of ray parenchyma cells ( Figure 1 1 ) . This was unexpected because of the presumed nonlignified nature of these walls. Although the amber-brown substance was usually formed, occasionally a cell i n the ray yielded an almost colorless mass of material with a poorly defined wall remnant that also is almost colorless (see left half, Figure 11). These may represent cells that never reached maturity and whose walls were never impregnated with the source material, whatever it may be, of the amber-brown substance. Figures 11 and 12 show that the coalified product derived from the fibertracheid walls, although lighter i n color, is similar to that derived from such walls i n the case of the other woods. W o r t h y of note, however, is the colorless material shown i n both small a n d large strands i n Figure 12. These appear to be remnants of portions of fiber-tracheid walls (the small strands) a n d ray parenchyma cell walls (the larger strand in the right half of the photograph). Material of this type studied thus far has proved to be isotropic. Table I summarizes these observations by listing the coalified products derived from each cell type in the xylem. O n l y the more common materials are listed.



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Possible Relationships of Materials with Coal Macérais of Higher Rank Although it is premature to assign the materials that have been described and illustrated to particular maceral classes, it may be useful to suggest possible affinities. A t least four types of source materials are involved: vessel walls, fiber walls, parenchyma walls, a n d cell inclusions. This morphological classification of source materials is not a fully satisfactory basis for discussing coal substance development. More rational interpretations of maceral evolution could be made if the source materials could be differentiated o n the basis of their chemical compositions. L a c k i n g such knowledge, the four-fold differentiation of source materials suggested above does distinguish between the initial plant substances involved a n d , although the distinctions made are primarily botanical, there is a general implication concerning the chemical nature of the substances. F o r example, the vessel and fiber cell walls w i l l typically contain large amounts of cellulose and lignin with pectins between the juxtaposed p r i mary walls where the parenchyma walls are primarily cellulosic but are also surrounded with pectins, and the cell inclusions may be varied i n composition but devoid of cellulose and lignin.

Coalification of Woody Tissue as Deduced From a Petrographic Study

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