WALL STRUCTURE IN THICK CELL WALLS 115 



cells or longitudinal sections are swollen, then striations can often be 

 observed which run more or less transversely in the outer layer and 

 more or less longitudinally in the central layer; iodine crystals grown 

 in the wall take up two orientations, the needle-shaped crystals lying 

 transversely in outer layers and more nearly longitudinally in central 

 layers; and finally fungal attack develops cavities in the cellulose in 

 these two directions (47(ft)). Such evidence alone, however, is hardly 

 convincing since in all these observations the wall has been strongly 

 swollen, or subjected to other equally undesirable processes. 



(ii) The change might be due, not to a net change in orientation of 

 the micelles, but to a change in the angular dispersion while retaining 

 the net preferred orientation. This was the suggestion made by the 

 writer following largely the results of the X-ray analysis of tracheids 

 and other cells showing similar optical heterogeneity. This type of 

 dispersion change is one which undoubtedly does occur in these cells 

 and is certainly necessary to explain some of the physical properties 

 of the cells. 



As will become clear later on, it is certain that both of these factors 

 are operative. There are nevertheless other aspects of cellulose 

 structure which could also be involved, about which nothing can at 

 the moment be said with any certainty. These will be referred to 

 again later; at the moment we may perhaps notice them in passing. 



(b) The volume percentage of cellulose present might be suflBciently 

 different to play a part in the optical heterogeneity. Such a connection 

 is, however, difficult to establish until methods are devised for the 

 isolation of the separate layers in quantities sufficient for at least 

 microchemical analysis. The evidence which can at present be adduced 

 is conflicting. On the one hand, macrochemical analysis of wood has 

 shown that the percentage of cellulose is the same in both early and late 

 wood and, since the central layer is well developed only in the latter, 

 then it could be deduced that the composition of this layer cannot be 

 far different from that in the narrower layers on each side of it. On the 

 other hand Lange, using the method of infra-red microscopic analysis 

 developed by Steenberg, has produced results which suggest that the 

 outer layer at least has considerably less cellulose than the central layer. 

 There remains, therefore, the distinct possibility that the percentage 

 composition of the layers may be of importance. 



(c) Finally, and even if the percentage composition turns out eventu- 

 ally to be not sufficiently different, it still may be that the percentage of 

 crystaUine cellulose is different. This is a possibihty we can hardly 

 begin to assess at this time, but there is even now some small evidence. 



