178 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



parallel walls. The very obviously greater intensity of the arcs visible 

 along the meridian therefore, indicates immediately that the molecular 

 chains of cellulose are oriented more or less transversely in the cells. 

 The continuation of these arcs around a complete circle shows further 

 that this orientation is far from perfect: that while the general trend is 

 for the chains to he transversely, there is a good deal of spread about 

 this transverse position. Figure 60 will give some idea of the type of 



structure which may be deduced 

 from such observations. It will 

 further be noticed that if the chains 

 had been truly transverse then the 

 X-ray diagram resulting when the 

 beam is directed in turn along B 

 and C (Fig. 59) should have 

 differed; along C a ring diagram 

 should have been obtained since 

 the beam would then point along 

 the chains, whereas position B 

 should have yielded a good fibre 

 diagram. Since, in fact, these two 

 diagrams are identical (Plate IX, 

 Fig. 3) then this implies that there 

 must be some considerable dis- 

 persion about a nearly transverse 

 general direction. From these two 

 diagrams it has been calculated 

 that the average angle between the 

 cellulose chain direction and the 

 transverse plane in the ceUs cannot 

 be greater than 16° (57). We may note in passing that, even if the cells 

 had not completely flattened on drying, comparison of Plate IX, Fig. 

 2, with the diagrams of the model spirals (Plate VI, Fig. 7) show quite 

 unequivocally that the cellulose chains must still be directed in a very 

 slow spiral. 



Observations under the polarizing microscope lead to precisely the 

 same conclusion. In face view, the m.e.p. of single walls lies almost 

 transversely (the exact direction being difficult to ascertain in view of the 

 low birefringence) and the wall is nevertheless birefringent even when 

 viewed in optical longitudinal section, i.e. when the side walls of a whole 

 cell are examined. This must mean that the cellulose chains lie almost 

 transversely with probably considerable angular dispersion. Again, it 



Fig. 60. Diagrammatic representation 

 of the run of fibrils (full line) in a 

 primary wall. The length of the cell is 

 supposed to lie parallel to the edge of 

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