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: which causes the fibre to turn either slightly blue or yellow. The colour change is said to depend on whether the fibre is S- or Z-twist. A Z-twist fibre is said to turn yellow when parallel to the polarizer and blue when parallel to the analyser, while for a S-twist fibre the situation is exactly opposite. The modified Herzog test can also be used to distinguish between bast fibres and other plant fibres. For example, in cotton, a seed fibre, the microfibrils change their twist directions at short intervals (Peterlin & Ingram, 1970; Goodway, 1987) so that it will normally not be possible to observe any extinction and when using the compensator plate a rapid colour change along the the fibre will be observed (see Fig. 3). Of the authors listed above only Valaskovic (Valaskovic, 1991) provides any formal treatment of the Herzog test. Similar to us, he suggests that it can be modelled using the Jones Matrix formalism with each cell wall being treated as a linear retarder. He provides computer simulations for the light intensity in various configurations to illustrate this, but he does not derive an analytical expression for the light intensity distribution as we do. Further practical experience shows that the Herzog test does not always work and none of the authors listed above provide any explanation as to when the method works and when not.
Affiliation: Department of Physics and Technology, University of Bergen, Bergen, Norway.