34 THE PLANT CELL WALL 



yacids, suggests that suberin, like the estolids, contains poly- 

 ester. In addition ether linkages have been indicated. Some of 

 the acid groups are, apparently free. This is shown in cutin by 

 anionic character and an affinity for basic dyes. The polyfunction- 

 ality of several suberin derivatives could lead to a cross-linked tri- 

 -dimensional reticular structure. Such a proposal structure is 

 borne out by optical analysis, which shows that the highly polmerized 

 cuticular network is isotropic. 



III. The Wall as a Unit 



The properties of cellulose have been discussed from the view- 

 points of polymer chemistry and crystallography. Further X-ray 

 examination of fiber walls indicates that they are not uniformly 

 crystalline in character. Rather, the wall seems to contain cellulose 

 blocks measuring 55 A in cross-sections by 600 A in length. The 

 indicated length corresponds to 600/10 = 60 cellobiose units, 

 approximately, or 120 anhydroglucose residues. In the wall there- 

 fore, the DP of 10 2 indicated above must be compared with mole- 

 cular weight determinations yielding DP = 10 3 -10 5 , with the most 

 careful measurements favoring the range 10 4 — 10 5 . From these 

 comparisons it is evident that individual, extended molecular chains 

 are so organized that they participate in many crystallites, fray- 

 ing out in the intercrystalline regions. 



From cellobiose to the organized wall glucopyranose chains 

 are associated with one another in definite and orderly patterns. 

 Individual chain molecules measuring 33 A 2 in cross-section 

 aggregate into elementary fibrils, 3000 A 2 in area, each such fibril 

 containing 100 glucopyranose chains. Twenty elementary fibrils 

 form a microfibril which is 250 A on a side and 62,500 A 2 in area. 

 Individual chains are amicroscopic whereas elementary and micro- 

 fibrillar structures are submicroscopic. The aggregation of some 

 250 microfibrils forms the microscopic fibril 0.16 \i 2 in cross- 

 -section. Finally, some 1500 fibrils are organized into macroscopic 

 fiber such as the cotton hair over 300 A 2 in diameter. Thus, a single 

 fiber must contain in cross-section some 7.5 xlO 8 glucopyranose 

 chains. 



