I CARBOHYDRATES, CHITIN AND CUTIN 299 



birefringence apparatus of the greatest precision, in which, in spite 

 of their Brownian movement, comparatively short rod-molecules can 

 be orientated. It was with the aid of this apparatus that Weber (1942) 

 determined the optical nature of wax molecules. The experimental 

 evidence points to optically positive rod-molecules. Thus the molecules 

 of the membrane waxes, like those of paraffin, fats and other lipids, are 

 optically positive rodlets. 



Since the waxes, referred to the tangents of the cuticular layers, 

 produce negative birefringence, their molecules must stand perpen- 

 dicular to the surface of the membrane. So perfect is the orientation 

 of the rod-molecules, that the outside layer of the epidermis of Clivia, 

 seen from above after the removal of the cellulose layer underneath 

 it, appears optically isotropic. Hence the cuticular layer possesses a 

 radial optical axis. 



, After extraction of the wax, form birefringence is exhibited (form 

 birefringence curves in M. Meyer, 1938), this, referred to the 

 optical axis of the cuticular layer, being negative. This means that we 

 have to do with lamellar birefringence; hence the wall layer consists 

 of submicroscopic lamellae, in the texture of which, judging by all 

 previous experience, the cellulose of the cutin layer must be involved. 

 The optical analysis therefore suggests the presence of submicroscopic 

 cellulose lamellae with exceedingly thin platelets of wax interposed, 

 the wax molecules being orientated perpendicular to the cellulose 

 chains (see Fig. 146 d). 



Now, in the presence of the water, present not only in cellulose, 

 but also in cutinized cell walls, the hydrophobic wax molecules cannot 

 come into contact with the hydrophilic cellulose chains. Thus there 

 must be some intermediate polar substance, and that is the cutin. This 

 wall material contains both hydrophilic (-OH, -COOH) and hydro- 

 phobic (-CH3) groups and it may be assumed that the former incline 

 more towards the cellulose, whereas the latter tend more towards the 

 wax. We then have a scheme such as that represented in Fig. i46d. 



It can be seen in this model how the cell wall substances in the 

 cuticular layers are placed one relatively to another: hydrophilic 

 lamellae consisting of cellulose and probably also of pectins, layers of 

 wax molecules in radial arrangement and, in between them, amorphous 

 cutin in random orientation. Apart from the interposition of the wax, 

 the morphological conditions are similar to those in lignification, where 



