Investigation of the Effect of the Fibre Inter-Node Length on the Tensile Strength of Maize Stem Fibres at Different Growth Stages

This study describes an investigation into the variation of the tensile strength of maize stem fibres at different growth stages of the plant. In this context, the fibres were all extracted manually, and in some cases preceded by a water retting process for 10 days. The variation of the tensile strength and chemical functional groups among four common maize varieties were determined. The fibres were characterised by performing tensile test, density & linear density tests, Fourier Transform infra-red spectroscopy (FTIR), X-ray diffraction, Thermo-gravimetric analysis and surface morphology (SEM image analysis). The thermal analysis, FTIR and X-ray results showed that, in general, the fibres from the different maize varieties and from the different growth stages are semi-crystalline in nature. Furthermore, the SEM micrographs revealed the presence of equi-spaced fibre‑nodes along the fibre, that are believed to be due to the growth stresses induced in the plant stem. This fibre inter-node distance varied in relation to the growth stage of the plant, and yielded a good correlation (coefficient of 0.91) with the tensile strength of the fibres. Finally, a better fibre yield was obtained from the stem at the senescence stage of the maize plant.


Introduction
Maize is the second highly grown agricultural crop in the world, with about 50 maize varieties available on the world market [1]. On the African continent and other SIDS-African countries such as Mauritius, there are hybrid maize varieties which have good tolerance to maize diseases as well as good resistance to droughts and heat. Four maize varieties chosen for this study, notably, local (this variety has been locally planted for more than 30 years, but no code has ever been assigned to it), SC602, SC608 and SC719, were those available locally in Mauritius. SC602 and SC608 are both medium maturing maize strains producing yellow maize grains while SC719 is a late maturing maize strain which produces white maize grains [2]. Both SC608 and SC719 are among the leading maize varieties on the African continent.
Following maize cob harvest, the remaining maize stover has little secondary uses such as livestock feed and green manure or is simply considered as waste. However, this can be a good sustainable source of cellulosic fibres. Furthermore, the few experimental-based research studies [3][4][5][6][7][8][9] on maize fibres done till date have all shown that maize fibre has proved to be interesting by reason of its appreciable mechanical and chemical properties. From literature, it has been noted that extraction of maize fibres has been done by using either chemicals accompanied by boiling or milling equipment [3-5, 7, 9]. Thus, the economical aspect of such fibre extraction was not considered-which would seriously affect the scaling up of such a process. However, there are no explicit characterization of maize fibre in terms of its mechanical and chemical properties. Hence, studies on the maize fibre, itself, are relatively scanty.
There is significant studies in the literature about the effect of fibre defects and fibre deformation particular to wood pulp fibres on the mechanical strength. However, to the best of the knowledge of the present authors, there are no known study about the evolution of fibre deformation during the growth stages of the maize stem, and the impact on the tensile strength (TS) of the fibres. Moreover, the evolution of the mechanical properties of maize fibres during the growth of the maize plant has not been investigated either. This would be useful given that baby corn cultivation is very profitable by virtue of its high nutritional value, good taste, short harvest time and its ability to give good returns in short periods of time [10]. Furthermore, the difference in mechanical and physical properties of fibres extracted from different varieties of maize stems is not available in literature.
This study aims at presenting the findings for the room temperature maize stem fibre extraction process, and the variation of the fibre yield, tensile strength and chemical functional groups of maize fibres extracted from different maize plant species at specific stages of their development. Furthermore, the effect of fibre deformation in terms of nodes/crimps has been evaluated for the four stages of the stem growth and related to the fibre tensile strength.

Supply of Plant Material
The Food and Agricultural Research and Extension Institute (FAREI), Mauritius, supplied the necessary maize plant at the four different growth stages of the maize plant. The first stage, called the 'Young Stage', occurs at 1 month after sowing. Then, at 2 months, baby corns start to be produced and thus, this is named as 'Baby Corn Stage'. At 3 months, the baby corns grow into mature corns, denoted as 'Mature Stage'. Ultimately, at 4 months whereby all the corns have been harvested, the maize plant becomes dry: this stage is termed as 'Dry Stage'.
The maize plants were planted in a split plot design over an area of 474 m 2 . 612 maize plants for each of the four varieties were cultivated.

Fibre Extraction and Measurement of Fibre Yield
The maize stems were cut at their stem-nodes. The chopped stems were immersed in fresh water, and the pH was monitored over several days. The objective of this preliminary investigation was to determine the duration of the water retting process which would allow the fibres to be easily separated from the remaining stem tissues, manually. The pH of the fresh water decreased from 7 to 4.6 after 24 h, and a further drop to 4.4 after an additional 24 h and remained constant thereon. This reduction in pH is, most probably, due to the release of organic acids produced throughout the decomposition of substances like sugars, hemicellulose, gums and pectin during the water retting process. After 10 days of water retting, it was observed that the fibres were very easily detached by a simple manual peeling action.

3
Thus a 10-day water retting process was adopted for the fibre extraction process.
Subsequently the chopped inter-node maize stems for the different maize varieties and for the young, baby corn and mature growth stages were weighed and then water-retted for 10 days. However, there was no need for water retting for the dry stage fibres since the fibres from the dry stage were easily peeled out of the stem. After 10 days of water retting, fibres were manually extracted. They were subsequently oven dried at 60 °C for 24 h. The dry mass of the fibres was then weighted, and the fibre yield was calculated.

Measurement of Density and Fibre Linear Density
The fibre density for each maize variety (at the dry stage) was measured by using a Micromeritics Accupyc II 1340 fully automated pycnometer. The instrument used has a 10 cm 3 sample cup with 3.5 cm 3 insert. Before the tests were conducted, volume calibration was done using the 3.5 cm 3 insert. After the calibration the sample was packed into the sample cup with the 3.5 cm 3 insert so that it was at least 2/3 full and the weight was recorded. The sample was put in the instrument sample chamber and the volume of the sample was measured. Helium was used as gas. The density reported for each sample is an average of five measurements. All the density tests were done at the Institute of Applied Materials, University of Pretoria, South Africa.
A Crimp Tester (Model: Eureka; Type: EY07) was used to measure the linear density of the fibres. The length of each fibre for a set of 20 fibres for each sample was first measured. The mass of the bunch of 20 fibres was then measured. The linear density was then computed as follows:

Tensile Testing
Tensile tests of the fibres were carried out using a universal machine Testometric M500-50AT equipped with a 10 kgf load cell and having a gauge length of 25.4 mm, as per the international standard ASTM C1557-03. A minimum of 50 individual successful tensile tests were performed for fibres from each stage of each maize species.
To determine the cross-sectional area of the tested fibres, 2-3 imprints at five different locations along each fibre were made on plasticine at room temperature. The images of the imprints were captured using a microscope (model: DigiMicro Profi at 5 Mega-Pixels) at a magnification of 300X. The areas of the imprints were computed using the ImageJ software. The mean area for each tested fibre was then computed based on the areas of the imprints obtained. It is to be noted that for the two species, most commonly grown locally, i.e., Local and SC608, maize fibres from the young stage were also tested in order to find any trend in the strength at the specific stages of growth of the plant.

FTIR, TGA, SEM and XRD Testing
The chemical functional groups of the extracted fibres were analysed using a Bruker FTIR Spectrometer in the range of wavenumbers from 400 to 4000 cm −1 . The FTIR graphs obtained were normalized before performing any interpretation.
The SEM micrographs were acquired using the Ther-moFischer Apreo Volumescope FESEM at 2 kV accelerating voltage and 0.20 nA probe current using T1 and T2 trinity detectors with OptiPlan use-case. SEM micrographs were captured in TIF format at a resolution of 3072 × 2304 pixels. Image processing was carried out using the ImageJ software. Measurements were conducted after calibrating the software to each individual scale bar of the SEM image to ensure accuracy. All the tests were performed at the Central Analytical Facilities, University of Stellenbosch, South Africa.
The fibres were first ground to a powdery form before being tested for the percentage crystallinity. X-ray diffraction patterns were obtained using a Bruker D2 Phaser diffractometer fitted with a 1-dimensional LynxEye detector. A copper X-ray source (Kα = 1.54184 Å) was run at 30 kV and 10 mA, with K β radiation suppressed by means of a 0.5 mm thick nickel filter.
Patterns were recorded over a range of 5°-60° (2θ) with a step size of 0.02° and an equivalent step time of 57.6 s per step. Sample rotation was set at 15 rpm.
Bruker's proprietary Eva 5.2 software was used to evaluate the percentage of crystallinity defined as the percentage ratio of measured crystalline area to amorphous area under the curve, as follows: where X c = Percentage/Fraction of crystallinity, A c = Area of crystalline phase, A a = Area of amorphous phase.
The XRD tests were done at the Department of Materials, University of Loughborough, UK. (2)

Fibre Yield
Based on the results of Fig. 1, in general a higher mean percentage fibre yield was obtained at the dry stage, with a lower extent of variation. Thus, it is economically more viable for users to allow the stem to dry one month after the harvest of the maize cob before extracting the fibres. After the corns are harvested, farmers may leave the mature plants in the field, for a period of about one month, until they are dry and before the following planting cycle starts. Then the dry stems may be taken to the fibre extraction process. It is to be noted that the fibre yield for only the local variety at the dry stage is low because spongy materials were still present at this stage, thereby decreasing the fibre yield. It is also interesting to note that a relatively high percentage fibre yield was obtained for the baby corn stage, with only a difference of 4% when compared with the Dry stage for the SC608 variety and a difference of 14% for the local variety.

Fibre Linear Density and Density
In this study, the fibre length is, in fact, the length from one stem-node to the other. Thus, the longer the stem inter-node distance, the longer are the fibres. The typical average fibre length was observed to vary between 11 and 23 cm, depending on the maize variety and on the growth stage. It is observed that SC608 dry fibres are generally longer than fibres from other maize varieties ( Fig. 2) with a lower variation as compared to SC719, and the fibres tend to be finer ( Fig. 2) with a linear density of (12.4 ± 2.2) tex. These results are comparable to those obtained by Yilmaz [9], who treated corn husk fibres by alkali method and obtained a minimum linear density of 16.1 tex. In fact, fibre fineness is important as it helps to decide whether the fibre is fit for use in apparels. The average fibre fineness for the fibres in this study is 16.4 ± 4.3 tex. The fibre fineness for yarns used in the textile industry ranges from 1.6 dtex (for cotton) to 3.5 dtex (for wool) [11]. Thus, it can be inferred that maize fibres require treatment to improve the uniformity and fineness.
Based on the pycnometer method, the SC 608 maize variety exhibits the highest density ( Fig. 3) with the lowest coefficient of variation of 0.048%. Despite having the highest density among the four maize varieties, SC 608 fibres are more appealing from an industrial point of view since the fibres tend to be longer and finer.

Cross Sectional Area of fibres
The analysis of the cross sectional area of the fibres for the four maize varieties revealed that the local variety has the smallest average area of 0.029 mm 2 , followed by the SC 608 with 0.031 mm 2 , SC 602 with 0.034 mm 2 , and SC 719 having the largest area of 0.036 mm 2 . These observations support the results of the linear density where the local and SC 608 varieties have lower values as compared to those of SC 602 and SC 719. It is further observed that there is a small variation of 3-8% in the average cross sectional area for the local, SC 608 and SC 602 varieties across the different growth stages, except for SC 719, there is a larger variation of 16%. This shows that generally speaking there is no significant change in the fibre cross-sectional area during the growth period of the plant.

Facture Load
The load at fracture for the Young stage of the local and SC 608 varieties are higher than the other growth stages for each of these two respective maize varieties, and the fibres at the Baby Corn stage yielded a very low facture load. Thus the TS for the baby corn fibres for the local and SC 608 varieties is very low, 74% and 77% % lower than the TS at the Dry stage for the respective variety. On the other hand the Baby Corn fibres of the SC 602 and SC 719 varieties exhibited the highest fracture load as compared to the other growth stages. For example, the TS of baby corn is 60% higher than the TS at dry stage for the SC 602 variety.
Given that, it would be beneficial to extract the fibres after harvesting the maize cob, then from a practical perspective, the mature or the dry growth stages are the one of higher interest. The results showed that there is no significant different (3-15%) in the tensile strength between the mature and dry stage of local, SC 608 and SC 719 varieties, keeping in mind the standard deviation. For SC 602, there is 30% difference in the TS between the dry and mature stages. Given that it is much easier to extract fibres from the dry stage, without the need to have water retting, then it is better to allow the rooted stems to dry a few more weeks in the field before extracting the fibres.
In general there is no major difference in the tensile strength (TS) at the dry stage of the four maize varieties; with the mean fracture load varying between 2.47 N and 3.47 N, and the resulting TS varying between 91 and 111 MPa (Fig. 4).
The analysis and interpretation of the TS results through FTIR, XRD, TGA and SEM are discussed in the next sections.

FTIR Results for Fibres of the Four Varieties at the Dry Stage
The dry stage for the four varieties has been analysed in terms of the FTIR spectra in order to determine whether the presence or proportion of any specific chemical function group can explain the TS results. From Fig. 5, it is noted that peaks at 1730 cm −1 , which corresponds to Hemicellulose, are of the same low level for all the four maize varieties at the dry stage. Thus, this shows that the amount of hemicellulose at this stage, is almost negligible. Furthermore, at around 1510 cm −1 , the peaks corresponding to lignin for Dry SC719 and Dry SC602 are most prominent and they are almost of the same height (0.04).
As for wavenumber 1220 cm −1 , the peaks for all dry species almost overlap (absorbance height for SC608: 0.031; Local: 0.024; SC602: 0.033; SC719: 0.035). Thus, the extent of lignin is almost similar for the four varieties which, in turn, explains their relatively similar fibre yields (ranging from 4 to 7%).  The absorbance of the peak at 897 cm −1 is almost the same for the four varieties, ranging between 0.02108 and 0.02667. It would imply that there is no significant difference the relative percentage of crystalline cellulose I for the four varieties of maize fibres. However, the peaks at 1420 cm −1 corresponding to amorphous cellulose are most prominent for SC719 and SC602 varieties (same absorbance height of 0.060) and least prominent for SC608 (absorbance height of 0.036) and local (absorbance height of 0.043) varieties. This shows that the relative extent of amorphous cellulose is greatest for the two maize varieties (SC719 and SC602) and least for SC608 variety.
Given the narrow range of the TS for the four varieties of dry maize fibres, and the range of standard deviation, it is deemed that there is not a major difference in the TS of these varieties at the dry stage. However, it seems that the relatively lower % of amorphous cellulose (lower peak at 1420 cm −1 ) does lead to a slightly higher TS for SC 608 fibres.

FTIR Results for Maize Fibres from the Four Growth Stages of SC608
Given that SC 608 is the most commonly planted maize variety in Mauritius, the FTIR spectra for its four growth stages have been analysed in an attempt to understand the trend in the tensile strength. Figure 6 represents the FTIR spectra for fibres from SC608 variety from 800 to 1800 cm −1 .The peak at 1730 cm −1 , which corresponds to the C=O acetyl group in hemicellulose is hardly noticeable at the young stage (absorbance height: 0.00937). The peak at this wavenumber increases in prominence for the two later stages, Baby corn (absorbance height: 0.0176) and Mature (absorbance height: 0.0269) stages. Then it decreases for the dry stage (absorbance height: 0.0142), The same trend is also observed for lignin which is characterised at peaks of 1510 cm −1 and at 1220 cm −1 [15,16]. The lignin content is lowest at the young stage (absorbance height at 1220 cm −1 : 0.0111) as compared to the other stages (absorbance height at 1220 cm −1 for baby corn stage: 0.0240; mature stage: 0.0352; dry stage: 0.0346). A relatively comparable trend in the lignin content was observed by Longaresi et al. [17] for fibres from different growth stages of cornstalk. Lignin has a low degradation rate, and this can explain that proportionately it is higher at the Dry stage as compared to Young and Baby Corn stages. Furthermore, the peak at 1420 cm −1 , which corresponds to amorphous cellulose structure, is least prominent at the young stage (absorbance height of 0.0264) and proportionately more prominent at the other three stages (absorbance heights: baby corn = 0.032; mature = 0.035; dry = 0.038). This higher percentage of amorphous cellulose can be explained by the lower pectin and hemicellulose content particularly at the Dry stage.
The peak at 897 cm −1 is normally assigned to C-O, C-C and C-H stretching present in cellulose I structure [18]. From Fig. 6, the peak for the young stage (absorbance height: 0.023) is close to that of the baby corn stage (absorbance height: 0.022) showing that there is almost the same level of cellulose I at these two stages of growth. Moreover, the extent of cellulose I increases slightly for the two next stages, mature and dry stages, due to their peaks being slightly higher (absorbance height for mature stage: 0.028; absorbance height for dry stage: 0.026). This slight relative increase, particularly for the Dry stage could be explained by the lower content of hemicellulose. Thus proportionately, the cellulose I content is thus slightly higher.
It is thus observed that for the four growth stages of the maize SC 608 fibres, there is relatively a higher proportion presence of amorphous cellulose as compared to crystalline cellulose. Secondly there are no significant difference in the percentages of crystalline cellulose which could explain the major variation in the tensile strength across the four growth stages of SC 608 maize.

XRD Results of the SC 608 Fibres at the Four Growth Stages
XRD analysis revealed that there is no large variation in the percentage crystallinity of the SC 608 maize fibres among the four growth stages (Fig. 7). These values show that the fibres for all the four growth stages exhibit a semi-crystalline structure.
According to Mazian et al. [19] well defined peaks at around 14° and around 16° can clearly be observed for plant fibres with a relatively high crystalline cellulose content. On the other hand, when a natural fibre has a rather high proportion of amorphous content, these two peaks tend to merge and would appear as one broad peak.
Based on the current investigation, a well-defined peak has been obtained at around 2θ = 22° corresponding to the diffraction plane (002) of cellulose I type while a broader peak has been obtained at around 2θ = 16° corresponding to the crystallographic plane (101) of cellulose. No peak could be observed at 14°. This tends to confirm a rather high amorphous content.
Mazian et al. [19] reported that the percentage crystallinity (by XRD analysis) of field retted hemp fibres was higher than unretted fibre irrespective of the growth stage. Thus this can explain the lowest percentage crystallinity of the Dry stage fibres (Fig. 7), which were not retted (as was the case for Young, Baby Corn and Mature fibres). According to Mazian et al. [19] during the retting process the amorphous components of the fibres which exist between the cellulose microfibrils are better degraded leading to a higher % crystallinity.
The relatively low percentage crystallinity of the maize fibres for the four stages can generally explain the rather low values of tensile strength as compared to other natural fibres such as flax (71-81%), hemp (68%), and jute (61-71%) [20]. However, there is no direct correlation between the tensile strength obtained for the four stages of SC 608 (Fig. 4) and the percentage crystallinity of Fig. 7.

TGA Results of the SC 608 Fibres at the Four Growth Stages
From the DTG curve of Fig. 8, the first peak occurring below 100 °C represents the removal of moisture from the fibres, which is higher at the Young stage as compared to the other three stages for the SC 608 variety. It is observed that the second peak occurs at 282.1 °C, 290.9 °C, 329.9 °C and 301 °C for the Young, Baby Corn, Mature and Dry stages respectively. George et al. [21] mentioned that hemicellulose degradation occurred in the temperature range of 200-265 °C whereas Diez et al. [22] reported that hemicellulose degradation occurred in the range 200-300 °C (as a deformation of the cellulose degradation peak). George et al. [21] reported that cellulose degradation took place over the range of 265-400 °C, and Diez et al. [22] also reported a rather similar range of 250-380 °C.
Based on the above literature, and the DTG curves shown in Fig. 8a-d, it is observed that the second peak occurred at a relatively high rate varying between 7.6 and 9.9%/min for fibres of the four growth stages. This tends to show that there was a rapid degradation of the hemicellulose and cellulose in those fibres. Furthermore, the second peak for the mature stage fibres occurred at a higher temperature as compared to the young stage fibres, which could have indicated a higher crystallinity of the cellulose in the fibres at the mature stage but this is not supported by the XRD results of Fig. 7. The second peak for the fibres at the young stage occurred at the lowest temperature of 282.1 °C as compared to the fibres of the other three growth stages. Yet the fibres at the young stage exhibited the highest tensile strength and a higher % crystallinity as compared to fibres of the mature and dry stages. Moreover, a small peak at 372.2 °C is observed only for fibres from the Baby Corn stage. According to Fiore et al. [16], degradation of α-cellulose occurred at 370 °C, and this could explain the third peak in Fig. 8(b). Given that there are no peak at around 370 °C for the fibres from the young, mature and dry stages, this can thus also explain and substantiate the highest percentage crystallinity of the fibres of the baby corn stage from the XRD results of Fig. 7.
In most cases, a last peak is observed at around 430-455 °C which correspond to the degradation of lignin, and this degradation occurred at a much lower rate as compared to hemicellulose and cellulose.
According to Sacui et al. [23] molecular packing of cellulose extracted from wood samples are rather similar to the form I-α, but with more disordered and less uniform packing environments. The XRD results of Fig. 6 and the relatively lower degradation temperature of cellulose as exhibited in Fig. 8a-d tend to show that the SC608 maize fibres of the four growth stages have a lower proportion of highly crystalline α-cellulose, and a rather more disordered and less structurally packed cellulose molecules. However, it would seem that there is a variation in the percentage of the different native crystalline cellulose of the four growth stages of the maize fibres as well as a difference in the amount of amorphous cellulose (as shown by the FTIR results).
In summary, the FTIR, XRD and TGA analysis of maize fibres do not adequately explain the change in the tensile strength (TS) of these fibres over the growth stages.

SEM Image Analysis of fibres of the four growth stages of the SC 608 Maize Variety
The SEM images of SC608 fibres (Fig. 9) revealed a highly prominent rib-like structure in all the four growth stages of the maize fibres. These rib marks tend to be equidistantly spaced, and more frequent, that is, with the smallest mean inter-rib spacing for fibres from the Young stage of growth (71.58 ± 18.08 µm) as compared to the largest inter-rib distance for the Baby corn stage (123.89 ± 27.34 µm). Figure 10 shows the results of the average fibre-inter-node distance for the four growth stages.
Page et al. [24] reported different types of fibre deformation and defects from wood pulp fibres such as dislocations, and nodes. Dislocations or slip planes are regions where the alignment of micro-fibrils have been modified, whereas nodes (or crimps/kinks) are defects of a larger scale and are regions where failure in the fibre has occurred due to bending and compression. In the latter case, there is delamination of the fibe wall due to very high compressive strain.
The arrows on the SEM image of Fig. 11 clearly show that the small swelling or protrusion of the fibre wall are like kinks on the wall surface. These deformations of the fibre wall are on the micron level (Fig. 9), and could be categorised as node/crimps rather than dislocations. The crimped zone is rather similar to the stage IV, a triangular swelling (Fig. 12), as reported by Wathén [25], who cited Page and Seth (1980).
The absence of any cracks at these node/crimps locations would suggest that the localised stress level has not exceeded the ultimate tensile strength of the fibre but sufficient to cause a plastic deformation leading to delamination and a swelling at these node locations.
According to Thumm and Dickson [26], the occurrence of such types of fibre deformations is principally due to the method(s) of extracting and processing the fibres, and are more present in mechanically processed fibres as compared to chemically processed ones. These authors further suggested that fibre deformation has an effect on the fibre strength. Joutsimo et al. [27] reported that the fibre deformation could also occur during the plant development due to growth stresses, and that there is a change in the fibre wall as well as breaking of the hydrogen bonds at these localised defect zones.
Bourmaud et al. [28] have reported that there is a decrease in the tensile strength of fibres, which contain defects such as kink-band, knees, and dislocations. These authors mentioned that these defects occurred during the processing stages of fibres through the development of bending and compression stresses. Bourmaud et al. [28] also reported that defects observed on the surface of non-scutched flax fibres have probably developed during the growth of the plant.
In the present study, no mechanical method was used for the fibre extraction; only water retting and manual separation of the fibres, except for the dry stage fibres where no retting was required (since they were easily detached from the epidermal layer). Thus, it would seem that the presence of the fibre nodes/crimps could be principally due to the growth stresses generated along the maize stem. Therefore, it is important to differentiate fibre defects from fibre deformation, and in the present study, these nodes/crimps are defined as fibre deformation rather than fibre damage.
Based on the experimental data obtained in this present study and shown in Fig. 10, there seems to be a relationship between the fibre-inter-nodes distance/length and the resulting tensile strength of the single fibre for the four growth stages. From Fig. 13, it can be observed that at a smaller inter-node distance the fibre tensile strength is a maximum (young stage fibres). The correlation coefficient of the exponential equation (y = 139.54 e −0.003x ) is 0.91. This tends to show that the presence of the nodes/crimps developed during the growth stages of the maize stem improves the tensile strength. Bourmaud et al. [28] have mentioned that plants do response to external constraints such as wind or rain during their growth stage as well as due to the varieties in the gene pool. This response is known as thigmomorphogenesis and does have an effect on the mechanical properties of the fibres as well as affecting the quantity and stiffness of strengthening tissues.
Thus, it is believed that the nodes/crimps observed on all the fibres are due to the plant's response to environmental stimuli and/or due to the stresses induced due to specific developmental stages of the plant. One example of such a response is at the young stage where the plant tissues are more fragile and susceptible to stresses [30], and thus must respond strongly to survive difficult environmental stimulations such as windy conditions in the fields, leading to a strengthening of the tissues of the fibre wall, leading to the visible swelling at relatively shorter interval. This can explain the smaller fibre-inter-node distance on the fibres from the young stage (Fig. 9). On the other hand older tissues have a weaker thigmomorphogenetic response than young ones [30]. Hence, the fibre-inter-node distance could be longer in length.
According to Kouko et al. [29] the presence of microcompressions and dislocations along a fibre can cause a higher elongation before a break of the individual fibers. The authors have cited Hornatowska [31], who suggested that areas with disorders of the fiber structure such as dislocations or microcompressions behaved more elastically.
In the present study, the results of the individual fibre elongation showed that there was no significant difference in the percentage elongation of fibres from the four growth stages, with the mean values lying within 2.8-3.3%.

Conclusion
In this study, it has been observed that the maize SC 608 stem fibres exhibit a semi-crystalline structure throughout its growth stages until the senescence/dry stage. The XRD, FTIR and TGA analysis tend to confirm this finding, and this is reflected in a relatively low TS as compared to other common fibres such as jute and flax. The study of the fibre inter-node length has revealed a strong correlation with the TS, and the presence of the nodes can be attributed to the growth stresses induced by conditions such as wind regime, water uptake, type of soil, etc. The study also reveals no major difference in the TS of stem fibres from four different maize varieties, most of which are commonly used on the African continent. Finally based on the findings, it is recommended to extract fibres from the dry/senescence stage of the maize stem since generally a better fibre yield as well as a better tensile strength were obtained. Fibre inter-node length (µm) Tensile Strength (MPa) Fig. 13 Correlation between the fibre inter-node length from SEM images and Tensile Strength of fibres from different growth stages of SC608 variety