128 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



to these by Bailey and Berkeley (47(^)) who regarded them as evidence 

 for the presence in wood of transverse cellulose chains which would, of 

 course, produce arcs in exactly this position. We can see now, however, 

 that it is possible that these arcs represent nothing more than the fused 

 ends of the widely spread equatorial arcs. This has been shown, in 

 fact, to be the case (47(e)). We can verify that such arcs are spurious here 

 in a most convincing way by comparing Plate VI, Fig. 3 with Plate VII 

 Fig. 1 . In the former the lateral arcs are widely spread and the meri- 

 dional arcs are very evident. In the latter the lateral arcs are much more 

 sharp and now there is no trace of any meridional arc. The layering of 

 the cells as seen under a polarizing microscope in transverse section, 

 which Bailey associates with the presence of transverse cellulose chains 

 (p. 114) and therefore with the meridional arcs, is equally evident in 

 both samples of wood. It is therefore certain that these arcs have no 

 connection with the structural features underlying the optical hetero- 

 geneity, and re-examination of the three diagrams obtained from 

 different regions of one piece of wood (Plate VII, Fig. 1) makes it quite 

 clear that the meridional arcs are indeed spurious. For, although the 

 make-up of the cells in the three samples of wood must be very much 

 the same, a meridional arc is noticeable in the diagram of wood from 

 the third and fourth annual ring but not from the tenth. Here again it 

 is clear that the presence of a meridional arc is associated with a spread- 

 ing of the lateral arcs. In all these cases the apparent intensity of the 

 spurious 3-9 A. is intensified by the fusion of the 021 arcs along the 

 meridian and the meridional arc itself is in reality rather weak. 



There is thus no satisfactory evidence for the presence of fiat spirals 

 in conifer tracheids, and this lends support to an earlier statement (47(c)) 

 that in X-ray diagrams of single tracheids there is no sign of transverse 

 orientation. There can therefore be no doubt but that the X-ray 

 diagram corresponds to only one spiral set of cellulose chains, and we 

 shall see in a moment that this is a most puzzling feature of structure 

 here. For consideration of the detailed optical properties of the 

 tracheids wall has now made it quite clear that, although chains oriented 

 in the transverse, or even approaching the transverse, plane as postulated 

 first by Bailey are absent, the outer and inner layer in the wall do diff'er 

 markedly in chain direction from that in the central layer. 



Crossed fibrillar structure in tracheids and fibres 



Although the X-ray evidence and to a large extent also the optical 

 evidence thus seemed fairly clear, there were still some unsatisfactory 

 features of the optical properties of tracheids which had been noted 



