INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 45 



simpler crystals this can often be achieved by calculation from the 

 intensities of the various diifraction spots. Unfortunately cellulose is 

 far too complicated a substance for the usual crystallographic tech- 

 niques to be used here; the number of atoms in the unit cell is too large 

 and the number of diffraction arcs is too small for these calculations, 

 apart from the fact stressed earlier that the substance can be investigated 

 only in the form provided by nature. Hence recourse must be had to 



.8-35 



Fig. 21. Two unit cells of cellulose, as suggested by Meyer and Mark, with the 

 cellobiose residues in position. Note that the chain lying along the central line of 

 each unit cell is oriented in the opposite sense to those at the corners. The reasons 

 for this cannot be discussed here, but it should be mentioned that the evidence for 

 this regular alternation is unconvincing. 



indirect methods. The significant fact to be noted is that the length of 

 the b axis, 10-3 A., is exactly the length of the cellobiose molecule sug- 

 gested by Haworth, and it seems therefore very reasonable to orient the 

 cellobiose residues in the unit cell as in Fig. 21, by placing one such 

 residue along one side of the unit cell and remembering that by 

 definition all four sides, and the central line, are identical. Further, by 

 remembering that the whole structure of the space lattice of cellulose 

 can be built up by superposition of such unit cells, it may be seen that 

 the cellobiose residues are joined up end to end into long molecular 

 chains as in Fig. 22. The conception is thus immediately reached of 

 long molecular chains of cellobiose units as pre-existing in cellulose. 

 Further, the mere existence of an X-ray diagram thus implies that in at 

 least some regions in the cellulose structure the chains lie strictly parallel 

 to each other and spaced regularly the same distance apart. That this 



