48 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



planes lying parallel to the molecular chains of cellulose are therefore 

 much broader than are those from planes perpendicular to this direction. 

 Explanations of this broadening are well recognized (Fig. 23); it implies 

 that in any crystalline region of the cellulose the number of planes lying 

 parallel to the chains is less than that of the planes lying perpendicular to 

 them so that, in the sense that the regions of strictly regular arrangement 

 of the chains are limited in extent, we may speak of the existence in 

 cellulose of "crystallites" or, to use the older term "micelles". It should, 

 however, carefully be noted that this evidence from the X-ray diagram 



Fig. 24. The micellar structure of cellulose originally propounded. (Reproduced from 

 Protoplasm, by W. Seifriz, McGraw-Hill, 1936, by courtesy of the author.) 



does not necessarily imply the existence of discrete micelles, in the sense 

 originally used by Nageli(17). All that it says is that there are periodic 

 interruptions in the lattice, and it is now generally agreed, on evidence 

 of many other kinds some of which will be discussed later, that the 

 original hypothesis of the existence of distinct and separate micelles as 

 first postulated by Nageli (Fig. 24) must be modified in such a way that 

 the distinction between "micelle" and "intermicellar space" is merely as 

 between ordered and less ordered cellulose chains (Fig. 25). This newer 

 hypothesis has the obvious advantage that one no longer needs hypothe- 

 tical long-range forces to hold the micelles together and it also yields a 

 better explanation of the tensile properties of fibres (see 3, 29). It has 

 been calculated (22) that the "micelles" are about 50 A. in diameter and 

 at least 600 A. long. It will be seen later that the molecular chains of 

 cellulose themselves are far longer than 600 A. 



Perhaps, before leaving the principles of the X-ray method, one 

 further point may be made, which will come under discussion again 

 later on. Adopting the dimensions for the unit cell proposed by Meyer 

 and Mark (or the one by Sponsler, for the result will clearly be the 

 same) the density of the cellulose within the crystaUine regions can be 

 determined. This is done by noting that each unit cell has two cellobiose 

 residues in it (the one at each edge being shared between four unit cells). 



