134 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



will occur at different angles of tilt, and the spiral angles can be 

 measured. This is the principle upon which the observations in conifer 

 tracheids were made. 



A series of sections were cut at various angles to the longitudinal 

 radial plane so that the walls examined were the tangential walls. In 

 each section a sufficient number of measurements of birefringence were 

 made to give a reasonably accurate average. This was done by measur- 



FiG. 50. For explanation, see text. 



ing the phase difference in a de Senarmont compensator as already 

 described (p. 69) using sodium light, and subsequently measuring the 

 thickness of the section (always of the order of 5 fx) by turning over a 

 few cells on their edges. The results for both outer and central layers 

 are given graphically in Fig. 48 for tracheids oi Picea sp. and in Fig. 49 

 for the fibres of Nothofagus cunninghami (48(a)). In each case a curve is 

 also included which gives at any angle of sectioning the birefringence 

 which could have been expected, calculated from the observed maximum 

 birefringence. This curve can be calculated in the following way. Let 

 ABCD, Fig. 50, be the trace on the wall surface of the index ellipsoid 



