PLATE VIII 



Fig. 1. X-ray diagram of immature sisal 

 fibres (outer layer only of secondary wall). 

 Note that the general disposition of the arcs 

 resembles that in Plate VI, fig. 4, indicating 

 the presence of flat spirals. 



Fig. 2. X-ray diagram of mature sisal fibres 

 (outer and inner layers both present). The 

 diagram shows the presence of steep spirals 

 only — corresponding to the structure of the 

 inner layer — with no definite sign of the flatter 

 spirals which should still be present in the 

 outer layer. 



Fig. 3. Transverse section of bamboo fibres 



between crossed Nicols. Note the alternation 



of bright and dark lamellae. 



Fig. 4. Enlargement of an X-ray micro- 

 photograph of the (single) wall of a vessel 

 element of Queiciis (oak) (length of vessel 

 parallel to longer edge of page). The diagram 

 consists of two arcs corresponding to spacings 

 3-9 A., lying to the top and bottom of the 

 diagram. The cellulose chains in the wall 

 therefore lie almost transversely. 



[Facing p. 160 



