8 THE PLANT CELL WALL 



aggregates, micelles, together with intermicellar substances. Phys- 

 ical methods which have been employed in cell wall studies are 

 directed toward demonstration of the fine structure in reticular 

 systems and the texture, or orientation of structural compo- 

 nents. 



In principle, the successful application of optical methods to 

 the study of micellar frameworks depends upon the relation 

 between micellar strands and intermicellar space and upon the 

 presence of some degree of spatial orientation. 



Ideally textured bodies consisting of parallel isotropic cylinders 

 or parallel isotropic planes (composite bodies), become optically 

 anisotropic if the diametres of the rods and the distances between 

 rods or layers are small compared to the wavelength of light, 

 and if there is a true phase boundary between the structural units 

 and the material in the spaces separating them. A homogeneous 

 crystalline substance which exhibits the same index of refraction 

 from all aspects (that is, crystallographic axes) is isotropic, whereas 

 such a substance which refracts light differently along different 

 axes is double-refracting, or anisotropic. If perfectly isotropic 

 rods or planes are oriented in a suitable dispersing medium, the 

 system of isotropic rods or planes as a whole will rotate plane 

 polarized light which is passed through it if the medium enclosed 

 by the particles differs from the particles in its refractive index. 

 Further, the magnitude of double refraction is a hyperbolic function 

 of the refractive index of the dispersing medium. 



As this kind of anisotropic behavior decreases to zero when 

 particle and dispersing medium have the same refractive index, 

 it depends upon the oriented condition of the particles, that is, 

 upon the texture of the composite body, hence may be termed 

 "textural double-refraction". Textural double refraction has sign 

 as well as magnitude. Thus in a fiber or thread which consists of 

 rodlets oriented parallel to the long axis of the fiber (or "fiber 

 axis") the double refraction is positive, whereas a fiber made of 

 stacked plates whose planes lie perpendicular to the fiber axis has 

 negative double refraction. 



As we have noted, homogeneous crystalline materials may 

 show double refraction. Similarly, the long chain molecules consti- 



