Determining the fibrillar orientation of bast fibres with polarized light microscopy: the modified Herzog test (red plate test) explained.
Bottom Line: The test has the reputation for never producing false results, but also for occasionally not working.However, so far, no proper justification has been provided in the literature that the 'no false results' assumption is really correct and it has also not been clear up till now, why the method sometimes does not work.We also provide an explanation for why the Herzog test sometimes does not work: According to our model, the Herzog test will not work if none of the three distinct layers in the secondary cell wall is significantly thicker than the others.
Affiliation: Department of Physics and Technology, University of Bergen, Bergen, Norway.Show MeSH
Mentions: The identification of fibres, in particular, textile fibres, is important in several scientific fields (Goodway, 1987; Petraco & Kubik, 2004; Brettell et al., 2011). Although it is relatively simple to separate between plant and animal fibres (animal fibres have scales), it is much more difficult to identify different types of plant fibres correctly (Catling & Grayson, 2007; Bergfjord et al., 2010). Most plant fibres used for textile production (apart from cotton) are bast fibres. The term bast is commonly used to describe bundles of tightly joint fibre cells found in the stem of plants like hemp, flax, jute, ramie and nettle or in the inner bark of wood. Each bast fibre cell consists of a cell wall, which surrounds an empty space (lumen). The cell wall is separated into two parts: the primary (outermost) and the secondary wall. The cell wall contains so-called macrofibrils, which in turn are made up of microfibrils. The microfibrils consist of chains of cellulose that are birefringent. In the primary wall, the microfibrils are arranged randomly, but with a generally longitudinal orientation in the outer part. In the secondary wall, the microfibrils are arranged in a corkscrew (helical) fashion, winding around the longitudinal axis of the fibre (Beck, 2005). In many plants the secondary wall consists of three distinct layers, commonly known as S1, S2 and S3, as shown in Figure 1. The microfibrils in these three sublayers can twist in different directions. It is the helical orientation of the microfibrils found in the thickest region of the secondary wall, which is used to designate the overall fibrillar orientation of a fibre as either Z or S-twist (right- or left-handed fibre). The spiral angle of the dominating layer is known as the fibrillar angle (ϕ) or twist angle of the fibre. Fibrillar orientation is a characteristic feature for a species and serves as an aid for identification (Herzog, 1955). For example, knowledge about the fibrillar orientation of a fibre and the presence of calcium oxalate crystals in the associated tissue makes it possible to conclusively distinguish nettle/ramie fibres from hemp, flax and jute (Bergfjord & Holst, 2010; Bergfjord et al., 2012). The composition of fibre cells in most common bast textile plants are in fact very similar. For example, in flax, hemp and ramie S1 is Z-twist and S2 is S-twist. S3 is Z-twist in flax, while in ramie and hemp the microfibrils in S3 are almost parallel to the fibre axis (Harris, 1954; Meredith, 1956; Lewin, 2006). However, the relative thickness of S1, S2 and S3 is different, making hemp overall Z-twist and flax and ramie S-twist. In flax, ramie and hemp, the magnitudes of the fibrillar angles are 6.5°, 7.0°and 7.5°, respectively. Jute is Z-twist (Chakravarty & Hearle, 1967; Lewin, 2006) and nettle is S-twist (Bergfjord & Holst, 2010).
Affiliation: Department of Physics and Technology, University of Bergen, Bergen, Norway.