90 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



fibrils there tend to range around 100-150 A. in diameter. Frey- 

 Wyssling and Mtihlethaler (41(b)), however, find that celluloses from 

 many sources including the tunicates contain microfibrils about 250 A. 

 in diameter. 



As remarked elsewhere (p. 81) Ranby finds further that the micro- 

 fibrils of cotton can be degenerated in sulphuric acid to give particles 

 some 600-1000 A. long and 50-100 A. wide. The same is true of 

 Valonia cellulose, though the conditions required for breakdown are 

 much more severe than they are in cotton. It seems, however, a little 

 uncertain whether these "micelles" are pre-existent in the microfibrils 

 in any real sense. The fact, for instance, that microfibrils of the same 

 diameter are found in primary wall cellulose, where the X-ray diagram 

 is blurred, and in Valonia, where the X-ray diagram is very sharp, makes 

 it difiicult to picture the relationship between "micelles" and micro- 

 fibrils. The general appearance of the wall seen in this way recalls very 

 strongly the structure suggested by Meyer and illustrated in Fig. 25, 

 though, of course, on a much higher scale of magnitude, and the photo- 

 graphs presented by the second group of workers of a variety of other 

 cell walls present substantially the same picture.* Such photographs 

 therefore leave open the possibility that the bulk of the cellulose chains, 

 even in the non-crystalline regions, still lie roughly parallel to each 

 other, since there is no suggestion of a "woolliness" at the edges of the 

 threads which randomly oriented chains would tend to form. The 

 broadening of the arcs in X-ray diagrams of cellulose would then be 

 due to lattice defects and not to particle size. The question of the rela- 

 tion between "micelles" and microfibrils has been the subject of a recent 

 discussion (76). 



Lastly, we may perhaps notice that these photographs are in the most 

 complete disagreement with the conclusions reached by Farr and her 

 co-workers that cellulose is composed of spherical or slightly ellip- 

 soidal particles visible under the light microscope. The threads ob- 

 servable in these electron micrographs are demonstrably longer, 

 probably very much longer, than 10^ and are therefore longer than the 

 hypothetical particles are wide by a factor of about 10 at least. 



* See, however, footnote, p. 88. 



