WALL STRUCTURE IN THICK CELL WALLS 



127 



first place, there is good qualitative agreement between the diagrams 

 obtained and the predictions from theory, in that the lateral arcs do 

 split up into 4 spots symmetrically disposed about the centre of the 

 diagram and that in the flatter spirals these fuse into two meridional 

 arcs. Table VII, giving the values of the angle ip as actually observed on 



TABLE VII 



The angular distance, %p, between the arcs on the X-ray diagrams 



of model spirals 



the diagrams and as calculated from equation (1), p. 123, makes it further 

 clear that the agreement is good also in a quantitative sense. There can 

 therefore be no doubt as to the general validity of the considerations put 

 forward above. The fusions of arcs referred to do not make themselves 

 evident in these model spirals because the sample of artificial silk chosen 

 to give the best check against the theory had naturally as little angular 

 dispersion as possible; but parallel diagrams of an inferior material did 

 in fact give such fusions in which, in particular, the four spots became 

 fused into two equatorial arcs although these cannot be recorded here 

 for limitations of space. 



Such fusions are, however, very evident in the less perfectly crystalUne 

 material found in wood. Here, the lateral 3-9, 5-4 and 6-1 arcs are 

 sometimes separated into four spots (Plate VI, Fig. 2), although often 

 these are caused to fuse, as a result of high angular dispersion, into two 

 equatorial arcs. One feature of these diagrams is always, however, 

 most striking. Whenever these lateral arcs are wide, i.e. when the spiral 

 is relatively flat, then two meridional arcs also appear at the same 

 distance from the centre of the diagram. These are very evident in the 

 diagram presented here in Plate VI, Fig. 3. Attention was first called 



