INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 67 



The significance of path difference. Birefringence 



Thus far, optical methods have been shown to yield information as 

 regards the orientation of refractive indices in a wall. It should be 

 noted that the m.e.p. gives the cellulose chain orientation only if the wall 

 is homogeneous in chain direction along the direction of propagation 

 of the light, so that interpretation of the m.e.p. has to be made with care. 

 Examples of the pitfalls awaiting the unwary will appear later in 

 abundance. Optical methods allow us to do far more than this, how- 

 ever. It has been seen that if the cellulose micelles are in fact all strictly 

 parallel to each other in a wall, then n^ is high and n^ is low, so that 

 «y— «„ is high. If the micelles are displaced from this parallel position, 

 or if foreign bodies are introduced (even air) which are less strongly 

 anisotropic or even isotropic, or if the amount of non-crystalline 

 cellulose is increased, then n^—n^ decreases. The value ofn^—n^ there- 

 fore gives us some information as to the precise condition of the 

 cellulose complex. Further, reference back to Fig. 27 will show that if 

 Wy and «„ for the cellulose are known, and n^' is measured for any 

 section, then the angle of tilt of the section to the cellulose chain 

 direction can be calculated or, conversely if n^' and d can both be 

 measured, then n^ can be calculated. Determination of n^', the major 

 refractive index in a section, is therefore often very useful and it should 

 be pointed out here that, since «„ is relatively constant the measurement 

 of the difference n^' — «„ is equally useful. This value, which is clearly 

 a measure of the degree of crystaUinity of a wall, is known as the 

 birefringence or the double refraction and can be determined rather 

 easily without determining the separate refractive indices individually. 

 This is often fortunate since, with cellulose, it may take several hours to 

 determine one set of refractive indices whereas the double refraction of 

 one wall can be determined in a few minutes. 



It will already be clear from Table II that the path difference can be 

 determined approximately by noting the colour of a wall when in the 

 45° position between crossed Nicols or, better by noting the addition 

 and subtraction colours when rotated over a Red I plate. Neither 

 method is capable of any great accuracy, even if only in view of the 

 difficulty of matching colours in a microscope against printed colours, 

 as presented in Plate III, Fig. 1, or against descriptions given in Table II. 

 Some much more exact method is needed. In principle the method used 

 is simple. The object to be investigated is placed in the 45° position to 

 the vibration directions of the crossed Nicols as described above. A 

 device called a compensator is then inserted in the microscope a variety 

 of which is available though space will not allow anything but a bare 



