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136 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



using methods already described (p. 56) it can be shown that, if the 

 birefringence of the cellulose micelles is actually 0-06 then the maximum 

 birefringence of the lignified wall should be rather more than 0-04. 

 From such figures alone it seems possible that the low birefringence 

 here could be accounted for completely in this way. Other considera- 

 tions, however, show that other factors must also be involved. For 

 reference to Fig. 50 will make it clear that in transverse section the 

 major extinction position of the walls should always lie parallel to 



the wall surface, and this is a 

 point which has already been 

 discussed (p. 65). In tracheid 

 walls, however, it was observed 

 by the writer many years ago, 

 and confirmed during this par- 

 ticular investigation, that the 

 m.e.p. often lies well away from 

 this parallel position and, in 

 fact, is sometimes normal to the 

 wall surface. This could only 

 arise through a very consider- 

 able angular dispersion of the 

 micelles, and this must add to 

 the depression of the value of 

 the maximum birefringence. 



In the other layer the same 

 condition appears in a much 

 more exaggerated form, since the 

 birefringence is reduced to 0-02. At the moment, and until we know a 

 good deal more about the chemical composition of this outer layer, it is 

 not certain how far the observed difference in birefringence (004 as 

 against 0-02) is to be accounted for by a difference in cellulose content. 

 The low value in the outer layer, however, coupled with the observed 

 discrepancies between the run of the observed and calculated curves 

 in Figs. 48 and 49, makes it fairly certain that the cellulose here has 

 an unusually high angular dispersion. 



In these cells, therefore, the inner layer is dark not only because the 

 micellar spiral is a steep one but also on account of the angular dis- 

 persion which reduces the birefringence to a level much lower than it 

 would otherwise have been. Similarly the outer layer is much less 

 bright in transverse section than one would expect, again on account 

 of high angular dispersion and high lignin content. Our present 



Fig. 51. Diagrammatic representation of 

 the structure of the secondary walls of 

 conifer tracheids. The broken lines repre- 

 sent the run of the cellulose chains. 



