66 



PLANT CELLS 



I00-I20 and possibly many times this number of (3 d-glucose molecules^ 

 (Chap. XXII). 



Studies of cellulose walls with the X-ray and polarized light have fur- 

 nished evidence that the "molecules" of cellulose are aggregated into bundles 

 known as micelles. The micelles have been estimated upon the basis of X-ray 

 photographs to have a diameter of approximately 5 or 6 m/x and a length of 

 about 6o mix. A unit of this size would consist of about sixty parallel cellu- 

 lose chains each being made up of about I20 glucose units (Fig. 13). It is 

 probable that in many cell walls long chain-like "molecules" formed by the 

 condensation of other sugars are associated with the cellulose chains in the 

 micelles (Norman, 1938). 



Originally the micelles were believed to be well defined units which were 

 cemented together by some non-crystalline material. Recent work indicates, 



Fig. 13. Diagram illustrating structure of a small portion of a cellulose micelle. Parts 

 of only a few of the constituent cellulose molecules are shown. 



however, that many of the cellulose chains are much longer than the micellar 

 aggregates and extend from one micelle to another, thus welding the micelles 

 into a coherent anastomosing system. It is also probable that the micellar 

 aggregates vary in size. Cellulose walls are no longer regarded, therefore, 

 as a system of discrete crystalline units bound together by some cementing 

 substance but rather as a structure composed of molecular aggregates (mi- 

 celles) which are welded together by interlocking cellulose molecules. 



There seems to be little doubt regarding the presence of intermicellar 

 spaces. The spaces, indicated in black in Fig. 14, form an interconnecting 

 system between the anastomosing micelles. In most primary cell walls the 



^ Using the ultracentrifuge method Stamm (1930) reports that cellulose mole- 

 cules contain some 250 glucose residues while Kraemer and Lansing (i935). using 

 the viscosity method, estimate that native plant cellulose contains approximately 

 1300 glucose residues. 



