1 CARBOHYDRATES, CHITIN AND CUTIN 295 



the retardation is at its most negative (Frey, 1926b). The wax content 

 diminishes outside and the cuticle contains no wax at all, consisting 

 •of pure cutin (Fig. 145a). 



In hydrophytes the cutinization of the epidermis is confined to a 

 thin, optically isotropic cuticle. It is probable that all cell walls that 

 are in contact with air are superficially cutinized, since Elsa Hauser- 

 MANN states (1944) that the cells of mesophylls, which serve to 

 ventilate the leaf, are covered with a submicroscopical film of cutin. 



Molecular stridcture of lipophilic cell wall substances. To understand the 

 submicroscopical arrangement of the four cell wall substances in 

 cuticular layers it is necessary to know the morphology of their mole- 

 cules. We shall therefore have to consider briefly the chemistry of the 

 waxes and of the very imperfectly known cutin. Unlike the enormously 

 long cellulose chain molecules and the very long pectin chains, the 

 waxes are, as already mentioned, short rod molecules of less than 

 100 A length. In the simplest case they consist of higher aliphatic 

 alcohols, the corresponding fatty acids and higher paraffins. According 

 to Kreger (1949), there is no stoichiometrical relation between 

 alcohols and acids. Therefore, the plant waxes are only partly esters, 

 the rest being mixtures of higher alcohols, paraffins and fatty acids, 

 with a predominance of the first two. The alcohols and fatty acids 

 have even-numbered chains between C24 and C34 (Chibnall, Piper 

 and their collaborators, 1934); for instance, myricyl alcohol CgoHgjOH 

 or cerotic acid C25H51COOH. If esters occur they have the same 

 overall formula as fatty acids C^HanOg. On the other hand, the paraf- 

 fins have odd-numbered chains between C27 and C31 (Kreger, 1949); 

 e.g., n-noneicosane CogHgo. As indicated in the last formula, the mole- 

 cules of plant waxes are always unbranched chains. 



Besides the aliphatic waxes the cutinized, and especially the 

 suberized, membranes contain the waxes cerin and friedelin, which 

 have a substantially lower hydrogen content. The inference therefore 

 is that they contain aromatic rings and thus approximate the sterols, 

 which represent the cycHc alcohols. Luscher (1956) states that 

 friedelin and cerin contain an alcoholic OH group which can be 

 acetylated or otherwise esterified, while the second constituent O 

 atom is masked, presumably as a cyclic ether bridge. Thus friedelin 

 and cerin are alcohols, not esters. On the other hand, they may 

 possibly be esterified with other molecules in the membrane. Other- 



