## 3 Results and Discussion
### Quality Marker Regions and Their Significance  
Comparing the stem spectra with those of a cellulose standard, we identified several spectral areas with high variability among samples. Particularly, we identified three regions present in all four species analyzed that were either weaker or missing in the cellulose standard. These were situated between 1245-1252 cm\({}^{-1}\), 1508-1509 cm\({}^{-1}\) and 1738-2740 cm\({}^{-1}\). Because these were very weak or absent in the cellulose standard sample,they can be attributed to other constituents. The comparison of FT-IR spectra from bamboo stems and isolated cellulose nano-fibers has also shown the presence of several peaks in stems that are not due to cellulose's presence but are attributed to other constituents. In the case of bamboo, these were situated at 1230 cm\({}^{-1}\), 1456 cm\({}^{-1}\), 1506 cm\({}^{-1}\) and 1735 cm\({}^{-1}\)[43]. The investigation into lignocellulosic of bamboo proved the existence of a strong correlation between their concentration and IR spectroscopic measurements [8], a fact that further confirms that the intensity of spectral absorbance can reflect the quantity of those components in the sample. The study conducted on bamboo defined characteristic FT-IR spectral regions for cellulose and hemicellulose between 1363 and 1893 cm\({}^{-1}\), and between 1315 and 1942 cm\({}^{-1}\) for lignin. Within this overlapping range, specific bands for each of the three constituents were selected and defined as characteristic fingerprints of lignocellulosic [8]. The proportion of cellulose in plant fibers plays the definitive role in fiber characteristics, and due to this the study of cellulose from fibers is important before being considered for use [39]. Defining criteria for cellulose characteristics are the distribution of functional groups within the repeating subunits and along the chains, the number of inter and intra hydrogen bounds at the molecular level, the length of the chains and their distribution and crystallinity [40]. Based on band assignment, for cellulose key marker ranges proposed for species analyzed in the current study correspond to a signal for the amorphous region from cellulose at 896-898 cm\({}^{-1}\), for the crystalline region at 1420-1428 cm\({}^{-1}\) and, respectively, inter and intra hydrogen bonds occurring at \(\approx\)3000 cm\({}^{-1}\). Hydrogen bonds are responsible for certain properties of lignocellulosic fibers, considering that closer packed cellulose chains due to stronger and more hydrogen links would translate into increased mechanical strength [40]. Because the quality criteria sought to meet for fibers are high crystallinity and high tensile strength, the specific bands from the last two spectral ranges mentioned should be associated with desired characteristics with increasing corresponding band intensity. During the processing of fibrous plant materials, various constituents such as lignin, hemicellulose, pectin and extractives such as waxes are removed because they are responsible for undesired qualitative characteristics. However, lignin is a constituent that increases in plants with the ageing process [39], and when present within plants it has been associated with pathogen resistance [60]. From this perspective, an increased absorption signal for lignin in stems of plants could indicate the older age of the plant or a higher resistance to pathogens. However, for fiber quality, lignin is not a desired constituent, and should not be present after cellulosic fiber extraction. Additionally, for extractive processes, lower levels and thus a weaker signal for this constituent are sought. Based on the four plant species analyzed, the signature lignin regions were assigned to spectral intervals 1245-1252 cm\({}^{-1}\) and 1508-1509 cm\({}^{-1}\). Other constituents that are undesired in fibers are hemicellulose, which displayed a characteristic signal in region 1738-1740 cm\({}^{-1}\), and extractives such as fatty acids, which were assigned to spectral region \(\approx\)2800-2900 cm\({}^{-1}\).