MICROSCOPY OF THE COTTON CELLULOSE FIBER
There i s given in this paper a brief review 0f. the general grmth process by which the cottonseed hair i s formed. This i s followed by a discussion o f the composition of the primary cell wall and the microscopy of the growth of the secondary cell wall, with a n outline of the mechanism by which these processes may take place. There i s a short section devoted to the subject of the so-called "dead hairs" and a brief statement of the possible theoretical mechanism of their .formation. The subject of the desiccation of the cotton fiber is also discussed. A short section of this paper i s devoted to the cuticular layer. A section i s concerned with a general review of the present conception of the cause and formation of the spiral form of the desiccated fibers. The work done on the study of the porosity of the cottonfiber i s also presented. Some of the literature dealing with the correlation bemeen the areas of strain i n the livingfiber with the hvists of the desiccatedfiber i s reviewed. Finally, the theoretical considerations attempting to explain why a desiccated fiber cannot be restored to its original fomz are given.
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With the advent of cotton as a source of cellulose for the production of a considerable number of widely used technical products, the attention of many investigators has been directed toward the physico-chemical and colloidal aspects of its structure. As a background for these considerations, an understanding of the development and characteristics of the cotton fiber may be helpful, forming as il,does abasisfor the work in X-ray structure and micellar hypothesis. In the manufacture of a yam of fabric, the physical characteristics of the ultimate hair or fiber determine the character of the finished product. The factors of shrinkage, strength, elasticity, or ability to resume its shape after deformation, surface character, and water absorption are all very important to the textile manufacturer. In this connection $he British Cotton Spinners Association established a research group. To them, particularly W. L. Balls andH. J. Denham and their co-workers, belongs credit for much of the available detailed knowledge of the internal architecture of the cotton fiber. Growth of Cotton Fiber The cottonseed husk, which composes the tough, horny covering of the seed itself, when seen in cross-section has several layers of cells. The palisade cells which give the toughness to the seed coat consist in their upper part almost entirely of cellulose. It is the uppermost layer of epidermal cells, however, that figures prominently in the formation of the cotton fiber. This layer is made up of cells with thick, laminated walls containing a brown nuclear substance. I n the course of development the 114
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outer wall of a cell here and there begins to bulge. The swelling enlarges to about twice the diameter of the original cell and the nucleus passes out into it and keeps a short distance behind the tip of this growing hair or fiber. According to Balls (I), the fiber grows on an average of about twice its diameter every hour and attains its full length in about tluee weeks. At this period in its growth the cell of the fiber is very thin and to casual examination quite structureless. Only by careful staining can evidences of the cuticle be shown a t this period in its growth. Composition of Primary Cell Wall
It is generally conceded that the composition of this primary cell wall is chemically different from that of the rest of the fiber and t o i t the term "precellulose" has been assigned (2). Confirming this difference in composition, it has been found (3) that cotton boiled in alkali loses weight rapidly for the first half hour, then remains without change for a t least Z1/* hours and begins t o rise again after seven hours. This supposedly indicates two distinct reactions toward alkali by components of varying composition. Secondary Wall At the cessation of the growth in length of the fiber, the secondary thickening begins, the protoplasm in the cell laying down concentric layers of cellulose against this primary wall. Growth inward continues for another period of about three weeks, a t the end of which time the fiber has assumed the shape of a very thick-walled t8be with a comparatively small central lumen. Through a study (19) of the fibers from dated bolls it was anticipated that growth rings should be present to the number of about twenty-five, which would make each daily layer approximately OX034 mm. thick, which is below the resolving power of the microscope for ordinary light. These rings, however, were subsequently shown by swelling the fiber with other reagents (4) to about five times its original dimensions but arresting the action short of complete solution. These rings can be shown experimentally (5) by boiling a small quantity of cotton with 1 per cent sodium hydroxide, washing, then souring with acetic acid, washing again, and drying. A small amount of this dried cotton is then treated with a few cubic centimeters of 10 per cent sodium hydroxide and carbon bisulfide and allowed to stand for a few hours. Under this treatment the hairs swell and the growth rings may be observed under the microscope a t a suitable magnification, as with a 4-mm. objective. The possibility of a difference of composition within this secondary cellulosic wall was scouted by Denham (6) who felt that the periodic cessation of growth duringthe sunshine hours of each day was not sufficient cause for such an effect. It has recently been shown by Maskell and Mason (7)
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that the nitrogen content of the leaf of the cotton plant increases by day and decreases by night and that in plants without bolls the nitrogen in the bark shows a similar fluctuation, while for plants with bolls the nitrogen content of the bark just below the foliage level shows no appreciable variation. From these data it is reasonable to postulate that there may be a difference in the nitrogen content of the cellulose laid down on this inner wall from hour to hour during the day sufficient to give i t slightly different optical characteristics when swollen. A more elaborate view of this structure is given by Liidtke (8) who conceives that plant fibers in general are divided into a series of concentric cylinders by skins of furfuroid substances and that these are further divided by membranes radiating from the axis. Photomicrographs showing this structure and the actual separation of the cell wall into its various cylindrical layers in the case of elm wood fibers have been presented to the Cellulose Division of the A. C. S. by G. J. Ritter (9). Dead Hairs When, through some functional disorder in the plant, the cellulose deposits on the inside of the primary cell wall cease prematurely, an immature fiber results, being characterized by a large central lumen, thin side walls composed mostly of "precellulose" and probahly containing tannins. These fibers which are usually, to some extent, present in all cotton have been inappropriately designated "dead hairs." As a matter of fact, all cotton fibers are dead after the boll breaks and these immature fibers might well be designated by some more"accurate name. Desiccation The fibers, up until the boll breaks, have maintained a cylindrical form with a well-defined central lumen. With the breaking of the boll, the fibers fluff out and, losing their water by desiccation, take on the form of a more or less flattened ribbon with a collapsed lumen and with characteristic convolutions. Under polarized light they assume a variegated appearance of light and extinguishing areas with finer detail of the surface. Treatment of the fiber with various reagents serves to bring out other structural features. Cuprammonium solution dissolves the cellulosic components with a great deal of swelling, producing the characteristic "sausage" effect owing to the insolubility of the cuticle. Treatment with a dilute alkali swells the fiber, showing spiral fibrils, and various stains and dyes expose other features of its internal structure. Cuticle In common with the epidermal cells of most plants the cotton fiber has an outer cuticular covering. This layer can be demonstrated by the use
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of Scharlach-R which has an affinity for fatty substances with which this layer seems to be impregnated. Analysis of this waxy material extracted from cotton fibers (10)shows them to be principally gossypyl and montanyl alcohols and some acid esters as carnaubic, palmitic, stearic, etc., together with other compounds. A study of the particular properties of the cuticle presents a good many difficulties. It is distinct from the primary wall but molded so closely to i t that some characteristics attributed to it are actually markings in the wall beneath such as the stomata described by deMosenthal (15). Balls ( 2 ) states that there is evidence of a spiral structure in the cuticle probably corresponding with the pit spirals of the primary wall. The r81e of the cuticle in mercerization of the fiber gives information regarding its characteristics (11). The fiber shrinks in length and also in its perimeter with the breaking of the boll. During mercerization there is a further decrease in length, together with an increase in the circumference, and during the drying after mercerization there is another decrease in circumference. Through these changes the cuticle acts as a semi-elasticretaining membrane conforming to the changing shape of the fiber. The studies of mercerization have further indicated that the inhibition power of the cuticle is less than that of the cellulose beneath and that there is a selective inhibition along its spiral markings (13).
Spiral Structure The spiral structure that may be seen in a cot$on fiber has been studied by many investigators. Denham (13) enumerates four distinct classes of spirals. This structure does not proceed always in the same direction but suffers more or less regular reversals, although in the majority of cotton hairs it starts a t the epidermal layer in a sinister spiral. Statistical studies have not shown definitely the factors which determine them. Intimately connected with this structure are many of the other properties of the cotton fiber such as its convolutions on drying, the reaction to polarized light, the fibrillarmicellar arrangements and its X-ray structure. A careful study (12) has shown that the pattern and the reversals of this spiral probably were determined during the growth in the length of the fiber and that as each successive layer of cellulose was deposited on the outer wall it conformed to the pattern that was already extant, first on the primary wall and subsequently on each secondary layer so that the structure of the inner wall, in addition to being laid down in concentric cylinders, is subdivided axially into fibrils. In the growing hair the fibril boundaries were located by rows of pits. This picture of the fibrils in each successive growth ring lying parallel to each other is questioned by Denham (13) as always being the case. He conceives that while they may grow parallel a t certain places, in others
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the striations in the different growth rings may be at angles to each other and suggests that this arrangement may be the reason why a t some points in the hair the growth rings may be shown more readily than at others. In any case the question, while an argumentative one based on microscopical evidence, can probably be clarified by X-ray studies. Porosity of Fiber
It would seem from these investigations that there might be pits or crevices between the fibrils of a fiber extending from the outer wall to the lumen. Such have never been demonstrated but there is conclusive evidence of the porosity of the cell wall of the fiber. Clegg and Harland (14) in a unique investigation arrived a t a fairly accurate approximation octhis "pore space" in different cottons. The density of cotton found by weighing dry and weighing in water is close t o 1.5, that is, one cubic centimeter of cotton weighs 1.5 g. or one gram of cotton has a specific volume of approximately 0.66 cc. By determining the volume of a number of cotton hairs having a known weight a direct measure of the density was computed directly from the formula d = m/(n
+ a)
in which d is the density, m is the weight of n number of hairs 1 cm. long, and a is the average cross-sectional area of the hairs. I n the determination of these values the sample was divided. From one portion a sufficient number of hair%,wasselected a t random to give a representative average and, from their central portions, sections 1 cm. long were cut and weighed on a micro-balance From the second portion of the sample cross-sections were made a t the center of the hairs with a microtome. The image of these sections was projected a t a known magnification and drawings made on a sheet of paper, each drawing cut out, the same number weighed as from the first portion of the sample, and their weight converted into cross-sectional area by means of a standard paper of known weight. Making due allowance for errors introduced in cutting the sections, they found that in a typical instance the computed volume was 0.88 cc. per gram while the specific volume found by the immersion method was 0.63 cc. per gram. Thus 0.25 cc. or 30 per cent of the volume was "pore space." Subsequent measurements on different types of cottons showed that this porosity varied from 30 per cent to 40 per cent of the volume of the hair, being greatest in the coarsest cottons. I n connection with porosity of the wall Denham (13) further notes the existence of abnormally permeable areas in a more or less regular pattern. This is seen with a hair which has been soaked in an aqueous dye solution and then subjected to a slight pressure under a cover glass, whereupon small bubbles spaced a t regular intervals appear upon the cuticle, very
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suggestive of the position of the spiral lines of weakness. He also noticed a similar position of gas bubbles when the fiber was treated with critical strength sulfuric acid. Cotton Fibers in Polarized Light Cotton, together with most vegetable fibers, shows strong double refraction in polarized light. As far back as 1891 Chardonnet and Liebschutz described the change in polarization colors of nitrated cotton with the change in percentaxe of nitrogen.' Later, however, deMosenthal (15) found that cottons of the same nitrogen content, but preparedunderdifferent conditions, had different colors and concluded that the appearance of the fibers in polarized light was a function of the method and not the degree of nitration, being a change in the anisotropic condition of the fiber brought about by physical influences as well as chemical behavior. Phillips (16) bas since shown that the colors are dependent upon the degree of division of the nitrocellulose which is a function of a variety of factors such as the percentage of water in the nitrating bath, the ratio of nitric to sulfuric acid, the time and temperatureof nitration, and others. Harrison (17), in his investigations on the r81e of double refraction in fibers, concluded that it was entirely due to internal stresses within the fiber similar to the double refraction in glass, gelatin, rubber, and other colloidal substances when deformed under load. By using a glass slide and grooved celluloid film he applied local pressure to the fiber while under observation and observed the interference figures in these areas. In support of this hypothesis he cites the fact that when treated with ammoniacal copper adfate only those portions which have not swelled, as in the constrictions, show double refraction and that a film prepared from a cuprammonium solution shows double refraction only when subjected to stress. In connection with his investigations concerning the spiral structure imd convolutions of the fiber, Balls (12) put the entire matter of the relation of polarized light to the internal structure of the fiber in a new light. He showed first that m-existing in the primary wall of young hairs, shown by treatment with hot alkali, staining with congo red, and then examining by polarized light was a series of fine spirals in both directions. These spirals, he postulated, are the determining factors of the spiral formation of the cellulose in the secondary wall, the map or mold to which it conforms as it is deposited by the protoplasm. The reason why the spiral pattern, first in one direction and then in the other, should be dominant isstill open to conjecture. By the use of elliptically polarized light he showed that with a fully developed fiber taken from the boll, before the loss of water had formed i t into a convoluted ribbon, the points of reversal of the spiral could be
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plotted for the length of the fiber from the areas of extinction in the polarized light and that, furthermore, if the fiber was now allowed to dry without tension the convolutions of the desiccated ribbon would match in detail the reversals of the fibril structure. He further showed that anisotropic structure of this cylindrical hair was weak in a direction a t right angles to the fibrils and hence gave an area of extinction when the fibrils were momentarily with the axis of the fiber, which was either parallel or a t right angles to the plane of polarization. These observations of Balls give to the arrangement of the molecular components of the fiber some of the orderliness of the arrangement of the molecules in a crystal and agree with the more recent X-ray conceptions. Desiccation of the Cellulose This concept of the internal structure of the fiber, made up of fibrils in long twists very much as the individual strands in a piece of string, save that occasionally the direction of the twist is reversed, agrees rather well with other characteristics of the fiber. The desiccation and accompanying twisting into convolutions of the fiber on the breaking of the boll is irreversible. By no method yet known can the fiber be brought back to its original state after having lost its constructional water. It is suggested by Urquhart (18) that since the cellulose is deposited in the cotton fiber in the presence of water, the hydroxyl groups will have water molecules attached to them and that on the loss oft* water there will be a tendency for the cellulose particles to rearrange themselves so that the residual valences of the hydroxyl groups will be mutually satisfied. He considers it probable that, in addition to the rearrangement of the groups in the micelles, there may be actual deformation of the micelles themselves in order to bring the free hydroxyl groups nearer to each other, which may explain the convolutions of the dry hairs. Literature Cited
(10) (11) (12) (13)
BALLS,"The Development and Properties of Raw Cotton," London, 1915, p. 74. BALLS, W. L., AND HANCOCK, H. A,. PTOC.Roy. SOC.,93B, 426 (1922). KACZKOWSKI, w., FEFERMAN. I., AND Z A B I C K I . S . , P ~ S ~11,206-8 ~ ~ S ~ C(192i). ~~~., CROSSAND BEVAN, British Patent 8342/18. HALL.The Dyer and Calico Printer, p. 66, February, 1926, et seq. DENHAM, J. H., 1. Textile Inst., 13, 105 (1922). MASKELL, E. J., AND T. G. MASON, Ann. Botany, 43,205 (1929). LunntE, M., Papier-Fabr., Vol. 28, 1930; Ver. Zellrt. Ing., 129-33. R~TTER, G.S.,Ind. Enz. - Chem., 20. 941 (1928). . . F A R G ~R.R G., , AND PROBERT, M. E., J. Textile Inst., 15,337T (1924) CALVERT, MARY A,, AND SUMMERS, F., ibid.. 16. 265T (1925). BALLS,W. L.. AND HANCOCK, H. A., Plot. Roy. SOC., 99B, 13047 (192.5-1926). DENHAM, J. H.. J. Tedile Inst., 14, 86T (1923).
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CLEGG, G. G . .AND HARLAND, S. C., I . Textilelnst., 14,489T (1923).
H., J. SOC.Chcm. Ind., 26, 444 (1907). (15) DEMOSENTHAL, (16) PHILLIPS, A. J., J. Phys. Chcm., 33, 118 (1929).
W., PYOC. Roy. SOC.,94A, 460 (1918). (17) HARRISON, A. R., I . Tactile I&., 20, 125T (1929). (18) URQUHART, (lg) BALLS,W. L., Proc. Roy. Soc., 90B,542-54 (1917-1919).
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