252 



FROTOFLASM 



In the case of common salt, there is a repetition of the unit 

 cell in all directions, giving a homogeneous distribution through- 

 out the crystal. But in cellulose, as is likely to be true in 

 colloidal material generally, the distribution, of unit cells and 

 of molecules, is not homogeneous; instead, the long chains are 



Fig. 132. — Orientation of micelles in a block of cellulose. 



aggregated into bundles of some sixty chains each. • These 

 bundles, being molecular aggregates, satisfy the botanist Nageli's 

 definition of a micelle. We shall recall (page 118) that Nageli 

 postulated a so-called micellar structure for all gels, including 



protoplasm, the unit of the structure 

 being a micelle, or aggregate of mole- 

 cules, i.e., a colloidal particle. As the 

 cellulose micelle is symmetrical in 

 structure and therefore crystalline, 

 it has received the name of crystallite. 

 An association of cellulose crystallites, 

 oriented much as are bricks in a wall, 

 presumably constitutes the colloidal 

 structure of cellulose (Fig. 132). 



The precise orientation of the 

 micelles is of significance in such 

 (B) cellophane. (After H. H. properties as conductance and tensile 

 ^"'■^•^ strength. H. H. Mark depicts two 



extremes — one in which there is perfect parallelism, and 

 the other in which there is a random or brush-heap dis- 

 tribution of the micelles, the former represented by native 

 ramie, and the latter by cellophane (Fig. 133). The cell- 

 ulose of flax displays an excellent orientation of micelles 

 parallel to the fiber axis and has a tensile strength comparable 

 to that of the best steel. The following table (from Mark) 



A B 



Fig. 133. — Orientation of 

 micelles in {A) ramie and 



