I CARBOHYDRATES, CHITIN AND CUTIN 297 



The polymerization plan of the high-polymeric cell wall substances 

 cutin and suberin must be similar to that of the estolids, since their 

 hydrolytic and decomposition products ordinarily exhibit two or more 

 reactive groups capable of esterifying or etherifying (dicarboxylic 

 acids, hydroxycarboxylic acids, Table XXVII). This is the distin- 

 guishing feature between the monomeric molecular residues of cutin 

 and suberin, on the one hand, and the molecules of waxes on the 

 other (LuscHER, 1936). Seeing that suberin is more readily decom- 

 posed than the cutins (Zetzsche, 1932), it is probable that the degree 

 of polymerization or of interlinking attained within it is lower than 

 in the latter. It is presumably at its highest in sporopollenin, as this 

 wall substance is exceedingly resistant to saponification and decay, so 

 that the cell walls of fungus spores and grains of pollen are preserved 

 for thousands of years in peat deposits. 



The isolated dicarboxylic acids (Table XXVII) may possibly be 

 oxidized degradation products of higher hydroxyacids ; suberic acid, 

 COOH- (CH2)6-COOH, for instance, results from the oxidative de- 

 gradation of suberin. Probably not all the carboxyl groups of the 

 carboxylic acids in the membrane are esterified, for cutin has some 

 of the characteristics of an acid, or a high-polymeric anion (pro- 

 nounced negative charge Brauner, 1930, selective cation perme- 

 ability, staining by basic dyes). Since its behaviour is almost iso- 

 tropic, it must be presumed that the linkage of the carboxyl and 

 hydroxyl groups is not that of a linear chain scheme, but reticular in 

 aU spatial directions as in lignin. 



Suhmicroscopic structure of the cuticular layers. It now remains to build 

 up a picture of the mutual spatial relationship between the cell wall 

 substances in the cuticular layers. A possible clue is afforded by the 

 optical anisotropy of the suhmicroscopic particles of wax. If their 

 form and optics were known, the orientation of the intercalated wax 

 could be inferred from the nature of the wall birefringence. 



The wax molecules are rod-shaped and therefore, when spread on 

 a slide, might be expected to be orientated and reveal something as 

 to their intrinsic birefringence. Many waxes, like paraffin, fats, phos- 

 phatides and other lipids, produce w^hat is known as a "negative 

 streak" (Fig. 146 b), which might incline one to conclude that the 

 wax molecules are optically negative with reference to their longi- 

 tudinal axis. Such a conclusion is, however, inadmissible, since short- 



