140 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



that we may take it as rather a general rule that the outer layer of 

 elongated fibrous cells is wound with a molecular spiral of angle about 

 40° or so. Further, and giving some considerable confidence in this 

 calculated value, we see that the value of the refractive index n^ is 

 exactly what has actually been observed in other cells. Finally, it may 

 be noted here that unhke sisal and tracheids, bamboo fibres possess a 

 whole series of lamellae which are bright, not just an outer and an 

 inner one. This is illustrated in Plate VIII, Fig. 3. It is naturally 

 impossible to be precise regarding the organization of these intermediate 

 bright layers but, by measuring the birefringence in transverse section 

 and assuming the values of «y and «„ are the same as those calculated 

 for the outer bright layer, then the spiral angle can be calculated for 

 each layer from equation (2), p. 138. The corresponding values are 

 included in Table IX. The spiral in the bright layers therefore apparently 

 becomes steeper as the lumen is approached. 



By analogy with the fibres of wood and of sisal, we may conclude 

 that the extensive dark layers observable in the transverse sections of 

 bamboo are composed of cellulose chains lying in a much steeper 

 spiral, and for exactly the same reasons. This is, in fact, shown by the 

 X-ray diagram of fibres taken with the beam normal to the length of the 

 fibres which is not included here since it resembles very closely that of 

 hemp fibres (Plate II, Fig. 3) except that it is not so perfect. The m.e.p. 

 of the whole wall is also in harmony with such a view (Table VI). Again 

 there is the pecuUarity that the bright layers are not apparently re- 

 corded in the X-ray diagram, and this is now so general a phenomenon 

 that it merits some little attention. 



We saw in conifer tracheids and wood fibres that the cellulose micelles 

 in the outer layer had very considerable angular dispersion and, 

 possibly, low cellulose content. If we can take the case of sisal fibres 

 as analogous to that of the wood cells, then a tentative explanation of 

 the phenomenon might be attempted. In sisal, the cellulose content of 

 the outer layer in immature tissues is not markedly lower than in mature 

 ones, but it is to be remembered that from the present point of view we 

 are interested in the outer layer in mature fibres for which we have no 

 data. It is clearly possible that intensive lignification, involving as it 

 almost certainly does a "swelling" of the cellulose matrix by deposition 

 of the lignin in "intermicellar" spaces, would induce a higher angular 

 dispersion. 



There is in the literature a good deal of evidence that lignification 

 induces swelling in cell walls and we may perhaps note one particular 

 case closely analogous to the present one. In jute and hemp fibres it 



