114 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



the other two layers, so that the difference in wall thickness is to a 

 large extent governed by the thickness of the central layer. • This was 

 first pointed out by I. W. Bailey. Layering of an exactly similar type 

 is found in fibres, both in the xylem and the phloem, and in some 

 vessels in Angiosperms. 



This layering is brought out in a much more striking way when 

 observed between crossed Nicols under a polarizing microscope (Plate 

 V, Fig. 3). The outer and inner layers are invariably bright (the inner 

 layer being often less bright than the outer), and the central layer is 

 dark, or at least much less bright. The outer and inner layers, therefore, 

 as seen in transverse section, are highly birefringent and the central 

 layer is almost isotropic. This calls to mind very strongly the condition 

 in the alga Cladophora, where a similar phenomenon was found to arise 

 from layers alternately cellulose-rich and cellulose-poor. Further 

 examination shows, however, that the two cases do not run parallel. 

 When conifer wood is examined in thin longitudinal section the bulk 

 at least of the wall is highly birefringent; it is difficult to be sure about 

 the outer and inner layers but at any rate the central layer, which was 

 isotropic in transverse section, is now birefringent. All three layers 

 therefore contain abundant cellulose and explanation of the optical 

 heterogeneity must be sought in some differences in the cellulose 

 matrices themselves. 



Looking back on the structural features of cellulose (Chapter V) and 

 the relation of structure to birefringence (Chapter IV) it will be obvious 

 that such a difference in birefringence in two neighbouring layers which 

 must have very similar chemical make-up could arise from any one or 

 more of the following: 



{a) Since the "micelles" are optically equivalent to positive uniaxial 

 crystals, then a variation in birefringence could arise from a variation in 

 the orientation of the micelles. Here there are two possibilities. 



(i) The change might be due to a difference in the net orientation 

 of the micelles, and this would mean that the micelles in the outer and 

 inner layers must lie more or less transversely in the cell forming, 

 therefore, either transverse circles round the cell (if the micelles are 

 quite transverse) or flat spirals.* The micelles of the central layer, 

 on the other hand, must lie more or less longitudinally. This was the 

 suggestion first made by Bailey and Kerr (47(a)), who called 

 attention to the whole phenomenon. They further succeeded in pro- 

 ducing strong corroborative evidence for this view. Thus, if isolated 



* More correctly a helix. The term spiral is, however, so widely in use in the 

 botanical literature that the term is retained here to avoid confusion. 



