THE PRIMARY WALL OF GROWING CELLS 179 



has been found that, in thin transverse sections, the birefringence of the 

 walls amounts to about 0-001 g. Now, taking the percentage of cellulose 

 as 25 % (Table I) and ignoring small density corrections, we may say 

 that 



«/-0-25«y+0-75/?„ 



«/ =0-25/7^ +0-75«, 

 and 



«y'—«a' =0-25 («y— «J, 



where n^ and n^ are the major and minor refractive indices of the wall 

 in transverse section, n^ is the refractive index of the (isotropic) con- 

 taminant and A2y and «„ are the refractive indices of the cellulose 

 micelles as seen in transverse section of the wall. Taking the angle of 

 the chains to the transverse at the maximum value of 16° (and therefore 

 calculating the minimum possible birefringence) then «y— «„ should be 

 about 0-047 assuming the micelles to lie all parallel, and hence 



„;-«/ =0-012. 



Since the observed figure is so much lower than this then there must be 

 either high angular dispersion or a very low crystalline to non-crystalline 

 ratio. 



While we are considering the orientation in the cambium let us note 

 in passing that the primary wall is still present around the mature 

 tracheid, where with care it can be stripped off and examined. This has 

 been done (58) and it is found that, as judged both by the m.e.p. and 

 striation direction, the cellulose chains make on the average an angle 

 of about 10° to the transverse both in Pinus radiata and Pinus longifolia, 

 the only two cases in which a direct measurement has been made. Van 

 Iterson had, indeed, said somewhat eariier that the m.e.p. of such walls 

 were almost transverse in a variety of conifers. 



Returning to the X-ray diagrams, however, there are some other 

 features of considerable further interest. It will be noticed that the arcs 

 are much more diffuse radially than are those presented hitherto in 

 cellulosic specimens. Such broadening of arcs implies less perfect 

 crystallinity, but it is not easy at the moment to define the type of 

 imperfection involved. On the original Micellar Hypothesis, it is 

 possible to say that the micelles here are much smaller than in the 

 secondary walls, and even in the modified hypothesis the same inter- 

 pretation could be advanced. In these terms, it can be calculated that 

 the micelle width is of the order 20-30 A. and so Hes towards the lower 

 limit of the size of crystallite which can give crystalline X-ray diagrams. 



