I CARBOHYDRATES, CHITIN AND CUTIN 295 



phosphatides in the membranes of Avena coleoptile, the question 

 arises as to whether the wax alcohols and fatty acids in the primary 

 walls occur in the unesterihed state, in which case their hydrophilic 

 pole would be connected with the cellulose threads. It will be evident 

 from Fig. 144 why the primary cell walls can be stained with fatty 

 acid dyes, whereas the individual cellulose strands seem to be "masked". 

 Physiologically this intercalation of wax results in the impaired 

 permeability ot the wall to water, ions and lipophobic molecules, as 

 these substances are admitted, not through the entire meshes of the 

 intermicellar spaces but only through the hydrophilic regions in the 

 vicinity of the cellulose strands. 



b. Cutini':(ed Cell Walls (Cut in) 



Mkrochemistry and optics of CMtini':(ed epidermises. The morphology of 

 the thick cuticular layers of the leaf epidermises of xerophytes (Fritz, 

 1935, 1937) is particularly interesting, in that, although optically often 

 appearing to be homogeneous, they contain at least four different 

 membranous substances, the submicroscopic arrangement of which is 

 known. Our starting point will be the optics, investigated by Am- 

 BRONN (1888), of the cuticular layers which, in the polarizing micro- 

 scope, behave in a reverse sense to the cellulose layers lying beneath 

 them. The cellulose component appears optically positive with 

 reference to the tangential direction of the cell wall, while on the 

 contrary the cuticular layer is optically negative (Fig. 145a). Extern- 

 ally, the epidermis is bounded by the almost isotropic cuticle and 

 between the cellulose and cuticular layers is interposed a fairly wide 

 isotropic layer of pectins (Anderson, 1928). Ambronn had already 

 suspected that the optically negative reaction of the cuticular layers 

 was caused by intercalated waxes, but this property was later attributed 

 to the cutin. Madeleine Meyer (1938), however, demonstrated by 

 careful micromelting tests (Fig. 145 b) that the negative birefringence 

 derives from a fusible wax, while the residual cutin proves to be almost 

 isotropic. In many cases, of which Gasteria is an example, a slightly 

 positive birefringence, due to cellulose, makes its appearance after the 

 waxes have melted out. Hence, besides the cutin, the cuticular layer 

 must also contain cellulose and even pectins, which can be identified 

 by ruthenium red. The optics of the longitudinal section discloses the 

 fact that these four cell wall substances (Table XXVI) are not 



