WALL STRUCTURE IN THICK CELL WALLS 135 



corresponding to a maximum birefringence of w^— «„, and suppose the 

 section to be cut at any angle (f> to the major axis of the ellipse and 

 therefore to the preferred orientation of the cellulose chains. Then 

 when the wall is examined in the direction marked by the arrow, at 

 right angles to the surface of the section, the major refractive index is 

 «y'. From the equation of the ellipse, we can say: 



(«/)^cos^<^ (n,7sin^0 _ J 



it ^n ^ 

 or («/)2= 



•y "O 



Hence, since the value of the minor refractive index is invariate (the 

 wall being uniaxial), the birefringence at this angle of sectioning is 



«y«a 



«y -w„= /— ^ — , ."^ .. ■ „ ■ -n^ 



Va;„2^(«/— w„2) sin2 <f> 



The birefringence at any angle ^=90°— (0+ a) where a is the angle 

 between the plane of sectioning and the transverse plane, can therefore 

 be calculated. 



The first point to note is that the angles of maximum birefringence 

 are not the same in outer and central layers for either type of cell. In 

 Picea, the micelles in the outer layer are, on an average, oriented at an 

 angle of about 50° to the length of the cells while in the central layer the 

 angle is 18° (comparing favourably with the value of 20° calculated 

 from the X-ray diagram). In Nothofagus it is somewhat difficult to 

 determine the precise angle in the outer layer — it must lie between about 

 90° and 60° to the length of the fibre— but it is certainly much greater 

 than the 10° (the value also determined from the X-ray diagram) of the 

 central layer. It seems therefore to be established, by the only type of 

 observation which could firmly establish such a result, namely by 

 observation of the physical properties of undistorted walls, that the 

 micelles in the outer layers of tracheids and wood fibres while not trans- 

 verse, do at least form a spiral much slower than that in the central layer. 



On the other hand it is equally clear that such a change in the net 

 orientation from layer to layer is not the only factor involved in deter- 

 mining the optical properties of these walls. Thus, in the central layers, 

 the birefringence of the cellulose is of the order of 0-04 as compared 

 with the value 006 to 0-07 recorded for cellulose in ramie, cotton, etc. 

 This lower value can be largely attributed to the high lignin content of 

 the walls. Thus, chemical analysis of wood samples shows that the 

 cellulose content is about 60 % on a dry-weight basis. From this figure, 



