Indian Journal 'OfTextile ResearchV'Ol.II, March 1986, Pp.I-6

/Polarizing Microscopic Study of the Origin and Growth ofE tinction Bands in

Native Cotton Fibres

A V MOHARIR

Nuclear Research Laboratory, Indian Agricultural Research Institute, New Delhi II 012, India

B C PANDA

Division 'OfAgricultural Physics, Indian Agricultural Research Institute, New Delhi 110012, India

and

V B GUPTADepartment 'OfTextile Technology, Indian Institute 'OfTechnology, New Delhi 110016, India

Received 7 October 1985; accepted 22 October 1985

Native cotton fibres in never-dried and dehydrated states show under crossed polarized light characteristic extinctionbands along their lengths. The cause 'Ofthe 'Originand growth 'Ofthese bands is still unresolved. This p per reviews critically thepublished literature in the light 'Ofthe present author's experimental observations 'Onthe 'Originand g owth 'Ofextinction bandsSome pl-ausible mechanisms for the 'Origin'Ofthese bands from 'Ourobservations with a polarizing icroscope 'Ondevelopingcotton fibres 'Ofknown ages are discussed in detail. The classical hypothesis 'Ofreversing spirals 'Ofcellulose fibrils in thesecondary layers is refuted to explain the 'Origin'Ofthe extinction bands. .

Keywords: Cellulose fibrils, Cotton fibre, Extinction bands

1 IntroductionCotton fibre has a fibrillar structure and the bulk of

its cellulose is laid in the secondary diurnal growthlayersl-9. These growth layers have been shown toarise as a result of diurnal temperature cycles duringthe growth period rather than due to changes inillumination cycles as shown earlier9-

2'O. Even cotton

fibres grown under constant illumination andtemperature, though without discernible growthlayers, show extinction bands and their cellulose alsohas been shown to be fibrillar+".

Cellulose fibrils in the individual secondary layersform a spiral about the fibre axis 1,2,4,22 - 26. Someworkers 2 7 - 34 believe that the orientation {)f thesefibrils in the .successive secondary diurnal growthlayers to be similar, whereas others believe that theindividual secondary growth layers can both be left-handed and right-handed and also could differ in spiralstructure, cellulose chain length and crystallitesizes 1,4,2'0.27-33,35-39. Some wor-kers 1,2,9,25,26,35,4'0 -44 have argued that in each layerthe sense of the helix reverses several times along thelength of the fibre: These so called reversal points areidentified as extinction bands in a polarizingmicroscope under crossed field. Several electronmicrographs of fragments of secondary layers of cottonfibres by negative staining, shadow casting techniquesand by surface replicas. have however not shown anyclear evidence of the existence of an abrupt change in

orientation in secondary la ers 13,35,45 - 52. Parallely-laid cellulose fibrils only re however seen. Someevidence of cross-orientatio s in successive secondarylayers deposited in definite w ve patterns in developingcotton fibres have been r ently shown by freeze-fracture replica technique i a transmission electronmicroscope+",

Extinction bands have een mapped by severalinvestigators 1,16,29,33,38,49,53 - 55, and their frequencyis reported to be influenced y genetic32,53,56-58 andenvironmentaI1l-13,5'O,53,58 59 factors and by irri-gation+". The number of e tinction bands has beenshown to be more in Gossy ium barbadense varietiesthan in the Gossypium hirsu um, Gossypium arboreum,and Gossypium herb ace m species of cottonfibres 22,28,55,6'0 - 62. Positio s of extinction bands areagain shown to be as weak points by someworkers 63 - 68, but as s rong points by someothers65,69 -72 . If the revers I points are weak points,their increased number in ossypium barbadensee+"?varieties cannot be easily re onciled with higher fibrestrengths of the fibres of this species55,61,7'O. Thenegative correlation betwee the number of extinctionbands and the strength of otton fibres observed bysome workers'P may be misleading because the

.strength data on these fibr varieties were taken bythem from a general bookie 63 of fibre characteristics,rather than from the actual easurements on the samefibres used for counting ext nction bands.

INDIAN 1. "FEXT. RES., VOL. II, MARCH 1986

Reversals of cellulose fbrils in individual secondarygrowth layers are believ d by some32,7i to coincideexactly or nearly so. If h wever true, the intensity ofblackening of extinction bands should progressivelyincrease with the depositi . n of each secondary diurnalgrowth layer. No such re ort exists, nor have our ownobservations reported b low shown any such effect.Superposition of revers Is in individual secondarygrowth layers therefore a pears to be only speculative.In a system like the c tton fibre, where internaldimensions are changin with each deposition ofdiurnal-secondary growt layer39,73, such a possibilityseems doubtful. Reversal in secondary growth layersare again different from the occasional reversals inexternal convolution twi ts on cotton fibres, and yetcoincidence in tensile ractures with reversals inmorphological studies with a scanning electronmicroscope 74-77 and a olarized light microscope=have been shown. Ho ever it has been clearlydemonstrated 50 that sue acoincidenc~ 'i's limited toonly the plane of obse vat ion and should not bemistaken as a morpho log calor structural coincidence.The evidence in resp ct of the hypothesis ofreversibility of cellulose fibrils in secondary growthlayers, therefore, contin es to be derived from thepolarizing microscope . although .riot withoutdoubtsii,7s.78 -80.

X-ray diffraction patt rns taken by Wakeham etal. 70of single fibres at the reversal position ha ve showna high degree of orie tat ion and crystallinity. Acomprehensive review of tructural reversals and theirrelation with mechan cal properties has beenpublished":

Several attempts have e~n made over the years toexplain the origin and gr wth of extinction bands, andyet the exact cause andinanner are not definitivelyknown. In the earlier 'p blications, extinctions havebeen attributed to the resence of pits in the cell-wallis. Kuhn, as quoted by Balls 2, ascribed them tothe mystic gyration of the protoplasm in the drying cell,and Balls? explained it a due to the movement of thegrowth centre within t e fibre, during secondarycellulose deposition. Iyen ar 78, observing moist fibresfrom ripe cotton bolls, C und the reversals to appearmidway between the adj cent bends of the fibre. Eachflattened portion develo s into a structural reversal;more often than not t e bend is also a reversal.However, none of these xplanations is convincing inthe light of experimental observations.

Whatever may be the eal cause of the origin of theextinction bands in cottor fibre, it is clear that these areformed during the secon ary thickening-period. It isalso now definite that. t e cellulose fibrils, find theirorientation in individual econdary growthlayers or inassociation with severa underlying layers' have a

2

bearing on the origin of extinction bands. How andunder what conditions the extinction bands shouldappear, need explanation.

In this study, we have made attempts to explain theorigin of the extinction bands in the light of some of ourobservations on developing cotton fibres of knownages from the dates of flowering, with the help of apolarizing microscope.

2 Experimental ProcedureA large number of flowers in a Gossypium hirsutum

variety 'Bikaneri Narma' were tagged for a week.Green cotton bolls of known ages were collected indouble-distill-ed water in a stoppered bottle. A fewdrops of chloroform were added to prevent bacterialgrowth. Fibres were carefully unlocked under water bycutting open the green bolls, and were examined in wetcondition on a Carl-Zeiss polarizing. microscope undercrossed polarizer and analyzer condition.

3 Results and DiscussionFibres from 6,7-day-old bolls showed complete

extinction from the tip to the base under allorientations in the crossed field, which is acharacteristic of an isotropic fibre. Photographingthese fibres was rather difficult for lack of sufficienttransmission of light through fibres. The first resultpresented in Fig. 1 is a photograph of fibres collectedfrom 14-day-old boll after flowering, as seen under acrossed-polarized light. The body of these fibres is darkand remains so even if the specimen stage is rotated,indicating that the fibres are essentially isotropic. Onlythe wall edges are seen illuminated and this isapparently an edge effect.

The brightness in the body of the fibres generallyincreases with age and the clear distinction between theedges is lost after about the 20th day from flowering asseen from Figs 2 and 3. The first dark extinction bandwas seen in a fibre from a 21-day-old boll. Fibres

Fig-.1-14 ~day-old fibres of Bikaneri Narma cotton varietyundercrossed-polarized field

MOHARIR et af.: ORIGIN AND GROWTH OF EXTINCTION BANDS IN NATIVE C TTON FIBRES

collected from bolls on later days showed theappearance of more bands and the interband spacingsbecame proportionately smaller for fibres from bollscollected still later. The appearance of new bandsbetween the existing ones would be expected to resultin this decrease of band spacing. Similar observationswere made by Betrabet et al. 30,54,58,60. The spacingswere generally not periodic, although in some fibresstatistically regular spacings were observed. Random-ness in extinction band spacings has also been shownby several workers1,9,33,54,79 and-also a fall in thenumber of extinction bands per milimetre in the secondpicking of fibres, which Iyengar." attributes to the risein the atmospheric temperature with the advance oftheseason. This decrease in the number of extinctionbands in fibres of second picking may be due to theshorter boll maturation period (secondary thickeningperiod) offered by the place of growth to the bolls fromlate flowers, owing to the advancing season. Raes etal. 66, however, observed a statistically significantperiodicity in extinction band distribution and theypresumed this to be inherent in the generation ofextinction bands. Figs 4 and 5.show some illustrations

Fig. 2-17 - day-old fibres of Bikaneri Narma cotton variety undercrossed-polarized field

Fig. 3-21 - day-old fibres of Bikaneri Narma cotton variety undercrossed-polarized field

of the extinction band spac ngs. It is therefore clearfrom the above observations that the formation of theextinction bands takes place uring the secondary wallthickening period. A definite correspondence betweenthe number of days of second ry thickening period andthe number of growth layers as been seen5,9,2Q,53 .It isalso indicated that the perio of secondary thickeningcontinues for well over 30 ays depending upon theenvironment. A prolonge range of secondarythickening periods (boll aturation periods) forGossypium barbadense cotto from 51 to 74 days hasbeen observed for locations n Egypt and Sudan, andthis' has been ascribed t the differences in airtemperatures compared t the other economiczones81,82. Schubert et al. 3 and ltoh84, however,indicate an overlapping f secondary thickeningperiod to some extent over t e elongation phase, fromtheir studies with an e ectron microscope ondeveloping cotton fibres. Sin e in general the length ofthe fibres is determined before the secondarythickening begins, an in teres ing close correspondencebetween the mean fibre len ths per unit temperaturecycles for the secondary t ickening period and theinter-reversal distance for the fibre of Gossypium.hirsutum species reported in the published litera-.

Figs 4 & 5-Some examples of t e distribution of the extinctionbands in co on fibres

3

INDIAN J. TEXT. RES., VOL. II, MARCH 1986

ture60,85, has been seen y Moharir et al. 50,59. Theseobservations implied t at the diurnal secondarythickening periods ma be responsible for theformation of extinction ands.

Whereas the existenc of cross-oriented cellulosefibrils in the primary w lls of cotton fibre has beengenerally accepted and s en", the evidence for abruptchange in-the direction 0 the fibrils laid in the walls ofsecondary layers is not y t established. On this subjectthere are various school of thought. Some workersbelieve that the fibrils a laid parallel to each otherwithout any change in di ection. Others consider thatthe fibrils are deposited i Sand Z spirals in successivesecondary layers. Yet oth rs believe that the spirals aredeposited as the S spiral 0 ly. The spiral angle in all thesuccessive secondary gro th layers, irrespective of thespecies of cotton, is bel eved by some26,27,31 to beconstant at 22°. Others elieve that the spiral angleprogressively decreases towards the core of thefibre21,32,37,85.86.87. Inc ease in refractive index forn II with respect to n1.in a fibre refractometer has beenattributed to decreasing piral angle and to variationsin spiral arrangement among different fibres orsegments of the same fi reo Duckett and Ramey?",however, believe that the e is a rapid decrease in spiralangle in the outer di rnal layers, after which aconstancy in spiral angle is approached.

In the context of the discussion above, the mostimportant question that eeds to be answered is : in theabsence of abrupt spiral r versals in direction, how canthe extinction bands appe r in a crossed polarised light?To answer this question the following mechanismsare considered and discu sed.

3.1.1 The spirally deposited cellulose fibrils in eachindividual secondary layer may be oriented parallel tothe fibre axis at extinction points.

In considering this possibility, we are required torevert to the classical hypothesis of the cellulose fibrilsabruptly changing their sense of orientation from leftto right and while doing so, they align themselves in analmost parallel position to the fibre axis and producean extinction band. It is however very difficult toimagine a whole cylindrical di~rnal layer changingabruptly in orientation during its deposition. Also, it isdifficult to conceive of such patterns in cellulosedeposition being copied in toto in the same location insuccessive secondary layers deposited, particularly in adynamic system such as cotton whose internaldimensions are changing with each secondary layerdeposited. Here therefore two conditions arise. If thereversals in successive secondary layers deposited wereto coincide with the reversals of the first ever secondarylayer, then the number of the extinction bands seenalong the entire length of the fibre must be determinedin full number in the very first secondary layerdeposited, and the successive layers merely copy themby exact superposition. Experimental evidence30.54.58and our own observations reported in the early part ofthis paper however have shown that the number ofextinction bands increases after the 21st day offlowering. Secondly, if we assume that the reversals inevery individual secondary layer deposited do notcoincide with each other but are established separately,or marginally ahead of the ones in previous layers, thewhole fibre should appear dark under crossedpolarized light or at least the extinction bands shoulddiffuse into very wide zones. None of these twopossibilities has been experimentally observed andtherefore this postulate in explaining the origin of theextinction bands also seems implausible.

3.1 Mechanism of Formation f Extinction Bands

The polarizing microsc pic studies have shown thatthe transverse extinction ands statistically increase innumber with each diurn I layer deposited during thethickening of the cell wal , and that the conditions fororigin of extinction band are actually formed duringthis period of a develo ing cotton fibre. Althoughmany workers believed" that there are differences inthe cross-sectional dimen ions of cellulose microfibrilsfrom primary and second ry walls of cotton, Willisonand Brown+' found no di ferences in the dimensions offibrils from the freeze-fra tured replicas of these wallsin developing cotton f bres using a transmissionelectron microscope. hey however found theindividual microfibrils a d bundles of microfibrils insecondary layers deposit d in definite wave patternswith definite amplitude, avelength and relative phasedifferences.

With this knowledge, here are several possibilitiesin which the mechanis of the origin of extinctionbands may be worked ou . These are discussed below.

4

3.1.2 Formation of isotropic zones in the secondarylayers along the length of the fibre.

Assuming that the spiralling cellulose fibrils inincreasing orientation from the outermost to theinnermost sedondary layers create totally isotropicconditions at periodic intervals along the fibrethrough small cross-sectional matrix of the fibre, onecan see extinction bands at such isotropic locationsunder crossed polarized light. Such bands arising outof the isotropic disposition of cellulose fibrils betweensuperposing secondary layers must necessarily stay intheir positions without change in intensity under allorientations of fibre in crossed polarized field. This ishowever not experimentally true since the extinctionbands alternate from dark to bright on rotating thestage of the microscope. The conditions of isotropy

MOHARIR et al.: ORIGIN AND GROWTH OF EXTINCTION BANDS IN NATIVE TTON FIBRES

between layers does not therefore seem to be the causefor the origin of the extinction bands in cotton fibres.

3.1.3 Formation of conditions for complete extinctionmay be a localized phenomenon between twosuccessive or intermediate secondary layers in theupper half or the lower half of the cylindrical fibres,when observed vertically in a two-dimensional plane ina crossed polarized field. If this be true, and in view ofthe fact that the secondary layers are hot uniform inthickness along the cross-section of the fibres 5,6.48,87,

it implies that cotton fibres at extinction points in oneplane of observation should produce no extinctionwhen observed orthogonally. Our observations ofextinction bands in several cotton fibres in suchmutually orthogonal views, on a specially designedaxially rotating goniometer, show that the extinctionbands stay in their positions along the entire cross-section ofthe fibre on rotation about the fibre axis. Theformation of the extinction bands is therefore not alocalised phenomenon between a few intermediatesuperposing secondary layers but is disposed throughthe entire circular cross-section of thel fibres.

The above three arguments leave only onepossibility on the origin of the extinction bands asdiscussed below.

3.1.4 Freeze-fractured replica of secondary layersof developing cotton fibres'" have shown that theindividual cellulose microfibrils and bundles ofmicrofibrils are deposited in definite wave patternswith a definite amplitude, wavelength, and phasedifferences between them. These wave patterns madeby the bundles of microfibrils contribute to the variableorientation of the anisotropy revealed by polarizedlight studies. These bundles of wavy fibrils within anindividual secondary layer or in co-operation with anumber of underlying or overlaid layers form a narrowregion of crystallized matrix in which the orientingbundles of cellulose fibrils are held parallel or nearlyparallel to the fibre axis, thereby producing extinctionbands. If however these bundles had been laid exactlyparallel, the extinction bands would have beenexpected to change from dark to bright at each 90°rotation in a crossed polarized field. Close observationof extinction bands shows that they alternate fromdark to bright at each 20-27° rotation of the stage of themicroscope. This means that there is a cross-orientedcrystallized matrix of cellulose fibrils at the extinctionbands. Such orientations may therefore be possiblebetween the successive secondary layers.

The little periodicity observed in the distribution ofextinction bands must therefore be associated with theperiodic wave patterns in which the cellulose fibrils aredeposited within the secondary layers and their

periodic in-phase superpos tion. Assuming that thewave patterns of the deposit d cellulose fibrils in everysecondary layer remains the arne, the wave patterns inthe freshly laid layers woul be laid slightly ahead ofthe waves in the preceding I yers owing to the reduceddiameter of the fibre. As a c nsequence, the number ofextinction bands would be xpected to be more in theupper half of the developi g cotton fibre along itslength than in the 10 er half. Experimentalobservations of increased frequency of extinctionbands in the upper halves of otton fibres 5 support ourhypothesis on the origin of xtinction bands in nativecotton fibres.

4 ConclusionWe refute the classical h pothesis that the reversing

cellulose spirals in seconda y layers of cotton are thecause of the origin of extin tion bands. The origin ofthese bands is therefore a phenomenon betweensecondary layers deposite in a developing cottonfibre. Since the duration of the deposition of thesecondary layers depe ds greatly on severalenvironmental and growt conditions, besides thegenetic factors, varietal c aracterization of cottonfibres based on the freque cy of extinction bands isalmost impossible. Prolo ed secondary depositionperiods for the varieties of the long-staple Gossypiumbarbadense species82,83 als explain why the fibres ofthis species have an incre sed number of extinctionbands seen on them. Also, since the extinction bandsarise as a result of the orde ed regions in the matrix ofthe cotton fibres, their ncreased number shouldcorrespond to higher stren ths of the fibres as has beenobserved by some worker 4,22,28,55,60.

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