120 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



m.e.p. is tilted in this way, must form a spiral round the cell. The spiral 

 is not, however, uniform from cell to cell, and within any small sample 

 of wood or any other fibrous tissue there occurs a wide range of spiral 

 angles. With this information regarding the structure of these cells we 

 can now proceed to examine the X-ray diagram; we shall return later 

 on to the optical properties when we wish to examine the wall in further 

 detail. 



The X-ray diagram of conifer wood. The spiral diagram 



When a chip of wood some 1 mm. thick is arranged in an X-ray 

 spectrometer so that the X-ray beam traverses the specimen in the 

 radial direction, then a diagram is obtained like those illustrated in 

 Plate VII, Fig. 1 . These are clearly very similar to those of ramie and 

 of hemp (Plate II, Fig. 3) where we know that the cellulose chains run 

 longitudinally. It is therefore immediately clear that here too the chains 

 often run almost longitudinally and that the closer resemblance of later 

 annual rings (Plate VII, Fig. 1(c)) to ramie and hemp means that in 

 these later rings the chains are much more nearly parallel to cell length. 

 This is found invariably to be true (see p. 159). The arcs are, however, 

 commonly much more diffuse than in ramie and in particular they have, 

 with the exception of the outermost wood of older stems, a much greater 

 lateral spread; in the innermost annual rings, in fact, they are spread 

 almost into a complete circle (Plate VII, Fig. 1(a)). 



At first sight one might conclude that the cellulose chains in wood, 

 while tending to lie longitudinally as in ramie and hemp fibres, have a 

 much greater angular dispersion about this direction, and there is often 

 nothing in the diagram to indicate that this interpretation is in error. 

 Now, however, that we have the fact recorded above that the m.e.p. 

 of single walls is usually tilted through a very considerable angle to cell 

 length, we can see that this interpretation cannot, in general, be correct. 

 In fact, as we shall see, the diagram of a spirally wound cell resembles 

 rather closely that of cells whose chains have a wide dispersion about 

 the longitudinal direction. In interpreting the diagrams both angular 

 dispersion and spiral organization have to be taken into account and 

 we can perhaps best appreciate the factors involved in the following 

 way. The argument here becomes rather mathematical. It is supported, 

 however, by X-ray diagrams of model spirals (Plate VI) which can be 

 consulted in lieu of the mathematics. 



The evolution of the fibre diagram can be followed with the aid of 

 spherical projection and the pole figure (Fig. 41). A crystal is imagined 

 as lying at the centre of a sphere, and the point at which the normal 



