WALL STRUCTURE IN THICK CELL WALLS 



121 



to any set of molecular planes intersects the spherical surface is called 

 the pole, P, of these planes. The conditions of reflexion of X-rays from 

 the set of planes are defined by the reflexion circle, PQRS, the locus of 

 the pole when the planes are inclined at the glancing angle d to the 

 X-ray beam. In Fig. 41 the pole P is drawn in a position for reflexion. 

 The reflecting positions are thus given by the points at which the 

 reflexion circle intersects the locus of the pole. If the crystal is rotated 

 about Mm the locus of the pole is two smaU circles LJ^ and LJ^, which 

 therefore intersect the reflexion circles at four points, P, Q, R and S 



Fig. 41. For explanation, see text. 



corresponding to four symmetrically disposed spots P', Q\ R' and S' 

 on the photographic plate. It will simplify matters considerably if only 

 those planes of most importance are here considered, i.e. planes 

 parallel to the direction of rotation. There are then only two positions 

 for reflexion, lying along the equator of the plate and equidistant from 

 the centre (Fig. 42(a)). 



If now the crystal is replaced by a bundle of parallel fibres, in which 

 the ceflulose chains run longitudinally, arranged to lie parallel to the 

 rotation axis Mm, then Fig. 42(a) gives also the derivation of diffraction 

 spots from planes lying parallel to the cellulose chains. The major 

 difference between this ideal geometrical pattern and that of a real 

 natural fibre is merely that no part of the fibre represents a real perfect 

 crystal. Its cellulose component consists of innumerable minute regions 

 — the micelles — in which the chains are strictly parallel and arranged 

 in a regular manner, separated by "intermicellar" spaces in which the 

 chains run from micelle to micelle in a more random fashion. These 



