THE STRUCTURAL FEATURES OF CELLULOSE 89 



to a later chapter; for the moment the chief point of interest Hes in the 

 striking demonstration, in a wall as nearly normal as a wall can be 

 when looked at in this way, of the presence of extremely fine threads. 

 The scale drawn on the photograph will show that these threads are 

 about 250-300 A. wide and are astonishingly uniform. We might 

 have expected that the prehminary drying of the specimen should have 

 caused the development of cracks in the wall but it seems highly 

 probable that these would have been much more irregular than the 

 structures we see here. It seems at the moment certain that these 



Fig. 33. Cross-section of the contoured surface of a specimen to show the eflfect of 

 metal shadowing. The sides A, B, C, D of elevations toward the filament become 

 coated with metal, but the elevations protect the far sides, A\ B', C and D' from 

 metallic deposits. In the microscope, the parts A, B, C, and D are therefore opaque 

 to electrons and A', B', C, and D' are transparent. The angle of shadowing is 



exaggerated for clearness of figure. 



minute threads are a feature of the cellulose in this particular wall. This 

 refers, of course, only to the very surface of the wall, but in other 

 photographs it has sometimes happened that the stripping of the 

 replica has removed layers from the wall itself, and these layers, lying 

 more deeply in the wall, show exactly the same structure. This latter 

 type of observation constituted an accidental example of the spectacular 

 advance in electron microscopy already made by Reed and Rudall 

 (41(c)) using layer dissection to work out the detailed structure in the 

 earthworm cuticle and underlying tissues. 



More recent work, however, has shown that these microfibrils are not 

 so uniform as the earlier photographs suggested {42(b) and (d)). Material 

 teased apart, often after preliminary treatment with sulphuric acid at 

 concentrations known to have no effect on microfibril diameter, shows 

 the presence of microfibrils ranging in diameter from about 100 A. up to 

 about 450 A. They are now known to be fiattish ribbons rather than 

 cylindrical rods (42(<i)), a point to be discussed in more detail later on 

 when the other Algae showing the same type of structure are discussed. 

 The point has been made by Hodge and Wardrop (41(a)) for wood 

 cellulose and by Ranby (74) for wood and cotton cellulose that the 



