168 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



the cellulose matrix of dried fibres. Now since density here therefore 

 depends on the closeness of packing of the constituent cellulose complex, 

 it will be clear that a higher density, involving a closer packing, impUes 

 a higher proportion of crystalline material. There was sufficient 

 encouragement to send samples to Dr. P. H. Hermans of Utrecht, 

 Holland, who has perfected a method of determining the crystalline/ 

 non-crystalline cellulose ratio by X-ray examination. The impUcations 

 of the density determinations are fully justified, as Table XVI will amply 

 bear witness. Unfortunately a second sample gave somewhat discor- 

 dant resuhs, but there was here clear evidence that the fibres had not 

 been purified quite so thoroughly as had those of the first batch so that 

 perhaps less reliance is to be placed on these later results. Although, 

 therefore, it is not possible at the moment to be very certain about it, 

 there remains the strong possibility that some part of the variation of 

 refractive indices with length may be associated with a change in the 

 amount of crystalline material in the fibre. 



Relationships in other fibrous cells 



In both conifer tracheids and bamboo fibres, then, there is the 

 clearest possible evidence that the molecular architecture of the 

 secondary walls depends in no small measure on the dimensions of the 

 cells. Somewhat the same situation would appear to obtain also in the 

 other fibrous cells we had occasion to examine in the last chapter 

 although, unfortunately, these have not been worked in sufficient detail 

 to allow the facts to be ascertained with the precision thus far revealed. 

 These other cells can therefore be dealt with very briefly. 



In both sisal and cotton the evidence we have refers to the outer- 

 most layers only of the secondary walls, but comparison with the data 

 available in conifer tracheids makes it reasonable to assume that a 

 similar relation could be established for all secondary wall layers. We 

 have seen that, in sisal, fibres can readily be obtained with only the 

 outermost layer present, and that such cells yield an X-ray diagram 

 quite different from that of the more mature tissues which carry also 

 the inner wall layers, even though the two groups of cells are not 

 significantly different in length. This we have seen is due to a diff"erent 

 orientation in the outer and inner layers. If, now, immature cells of 

 different length are compared, then it becomes quite clear that the spiral 

 in shorter cells is flatter than it is in longer ceUs. The observations are 

 too few to attempt any quantitative relationships, but it is satisfactory to 

 note that the figures would be in general harmony with the quantitative 

 results presented above for tracheids and bamboo fibres. 



