70 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



under the microscope over a polarizing prism, the analyser being 

 removed. The wall is blue only in certain azimuths. When the wall is 

 arranged to He parallel to the direction of vibration of the light then it 

 is colourless; when at right angles to this it is blue. This again, there- 

 fore, gives a ready method for the determination of chain direction. In 

 general, for light vibrating in any direction in the plane including the 

 major axis of the index ellipsoid. Fig. 27(a), the wall is colourless. More 

 than this, however, for if the dichroism is not blue-colourless but blue- 

 yellow then this means that the cellulose preparation is impure; and if 

 it is dark blue-light blue then this means that the cellulose micelles are 

 considerably displaced from their parallel position, i.e. they possess 

 a considerable degree of angular dispersion. Measurement of the 

 intensity of the light transmitted along the two directions can be used 

 to give a semi-quantitative measure of this dispersion (26). It should, 

 however, always be remembered that iodine staining here involves a 

 swelling of the wall in concentrated sulphuric acid. This itself will tend 

 to disturb the arrangement of the micelles. Similarly in some dyes 

 which show polarized fluorescence the angular dispersion can be 

 estimated (27). 



This dichroism resulting from iodine staining can be traced to the 

 similar dichroism of iodine crystals. It seems that the staining involves 

 the growth, in the "intermicellar spaces", of iodine crystals of sub- 

 microscopic dimensions which he parallel to the micellar direction. 



