Introduction 5 



High tensile strength was particularly well explained by this 

 arrangement. But there was still reason to believe that some dis- 

 continuity is present in cellulose and like material. 



An examination of natural cellulose with dark-field illumination 



Fig. 3 



reveals the presence of short rods, 1.5 to 2|.i in length, regularly 

 spaced in parallel rows (Fig. 4) . 



Preston' compares the actual and theoretical tensile strengths of 

 cellulose and finds that the former is 20 kg/mm- for cotton and 100 

 kg/mm- for flax, whereas the theoretical tensile strength for cellu- 

 lose, based on the known chemical energy of the C-C link, is 900 

 kg/mm-. Discontinuity in the structural arrangement of parts would 

 account for the lowered tensile strength. The micellar hypothesis 

 appears, therefore, to still hold. 



The demands made by elasticity indicate that jellies are built of 

 fibers. Low tensile strength, as in cellophane, suggests a scattered 

 distribution (Fig. 2) . High tensile strength, as in ramie fiber, 

 requires overlapping (Fig. 3) . And certain optical properties indi- 

 cate discontinuity (Fig. 4) . 



The best interpretation, so it seems to me, of these somewhat 

 contradictory requirements is that indicated in Figure 5 where sym- 

 metrically arranged over- 



trated at regular intervals ^~~ "^^ "" *""" """ '~~~ """" *""" 



(See also Fig. 1, page 268) . Fig. 4 



Out of the numerous facts 

 and suggestions so far mentioned, the most significant is the recogni- 

 tion that the structural unit of elastic jellies is a fiber. This is a great 

 gain over older concepts in which granules and globules were the 

 structural units of protoplasm. 



The foregoing is the background of our subject. The present will 

 be dealt with by the contributors to this Monograph. There remains 

 the future. 



The final word on the full meaning of protoplasmic structure will 

 rest with the biologist, for he alone knows the conditions to be met, 



Biol. Revs., 14:281, 1939. 



