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 Herzog test uses a full wave compensator plate, which displays the retardation as a colour change as explained above. It is the manner in which the wavefront ellipse of the top wall of the fibre overlaps with that of the compensator, which determines whether additive or subtractive compensation occurs. In bast fibres, the microfibrils are oriented at a slight angle to the longitudinal axis of the fibre. Following our model this means that the wavefront ellipse of the front and back walls of the fibres will be slightly tilted with respect to the longitudinal axis, as illustrated in Figure 7. The expected colour change can be found by using the Michel–Levy birefringence chart. Figure 6 corresponds to a fibre with no twist. For Z- and S-twist fibres, the expected colours can be determined by shifting the graph in Figure 6 slightly to the right or left as illustrated in Figure 9. It can be seen that S-twist fibres are predicted to show a yellow hue for and blue for Z-twist fibres show the opposite behaviour. This is in exact agreement with the modified Herzog test.
Affiliation: Department of Physics and Technology, University of Bergen, Bergen, Norway.