I CARBOHYDRATES, CHITIN AND CUTIN 3OI 



a slio-ht tendency to swell. As the cutin layer strongly absorbs ultra- 

 violet lio-ht (Frey, 1926 b) and retains this property even after the 

 waxes have been extracted, it impedes any intensive ultraviolet ir- 

 radiation of the mesophyll of xerophytes. As aliphatic compounds in 

 general do not absorb ultraviolet light, there must be some unknown 

 cyclic compound (cyclic waxes) in the cuticular layer. The hydrophilic 

 quality of the lamellae of cellulose and pectins is responsible for the 

 cuticular transpiration (Gaumann and Jaag, 1936) of the leaves, which 

 occurs not only in hydrophytes, but also in xerophytic evergreens. 

 The loss of water is a sign that the submicroscopic wax lamellae are 

 not continuous, but that the hydrophilic (cellulose) and semi-hydro- 

 philic (cutin) regions cohere and thus offer the water an outlet. 



c. The Chitin Frame (Cbitin) 



Chitin is a nitrogenous frame substance, primarily characteristic of 

 the animal phylum of Arthropoda (Crustacea, insects). It also forms 

 the membranous frame oi fungi (Harder, 1937; R. Frey, 1950). The 

 behaviour of vegetable and animal chitin is identical, as has been 

 proved for the sporangiophores of Phycomyces chemically, optically 

 and by X-rays (Diehl and Van Iterson, 1935 ; Van Iterson, Meyer 

 and LoTMAR, 1936). In the same way as the cellulose characteristic of 

 autotrophic plants may be built by both bacteria {Bacterium xylinum) 

 and by the animal class of the Tunicata (Fig. 86d, p. \z%), fungi are, 

 inversely, able to synthesize an animal frame substance. One cannot 

 go very far wrong by assuming that this similarity is connected with 

 the heterotrophic life of fungi, which, like animals, have so much 

 nitrogen to draw upon that some of it is deposited in the cell walls 

 and is there immobilized. As there is, on the contrary, only a minimum 

 of nitrogen in autotrophic plants, it cannot contribute to the form- 

 ation of their frame substances; otherwise chitin, which is more 

 resistant than cellulose in many respects, would certainly also occur 

 elsewhere in the vegetable kingdom. Morphologically, the two frame 

 substances are very similar in behaviour, as will be shown in what 

 follows, the micellar frame of each being composed of very long chain 

 molecules. 



Molecular structure of chitin. The structural unit of chitin is glucos- 

 amine, i.e., a pyranose ring in which an OH group has been substi- 

 tuted by an NHg group (Fig. 147a). It is not known whether the 



