CELL-WALLS 41 



Double Refraction of Cell- walls. 



In order to study this subject, apparatus for polarizing light 

 must be adapted to the microscope. This consists of two Nicol's 

 prisms, one of which is fitted into an eye-piece, the other being 

 fixed below the stage of the microscope, so that the light which is 

 reflected from the mirror must pass through it : the former 

 prism is termed the analyzer, the latter the polarizer. 



The sections to be examined may be mounted in water or in 

 glycerine, but the best results are obtained with sections mounted 

 in Canada balsam. A twig of almost any tree affords good 

 material for observation. A thin, nearly median, longitudinal 

 section is to be made and mounted : a high power must be used. 



The examination is to be commenced by rotating the analyzer, 

 so that the field of the microscope is bright : the section will 

 then appear much as it does when examined with an ordinary 

 microscope! 



The analyzer is now to.be rotated until the field is quite dark : 

 it is then seen that the outlines of the cells appear bright, 

 the thick, dense cell- walls (those of the fibres and vessels, for 

 instance) being brighter than the thin cell- walls (those of 

 parenchymatous cells). 



This observation indicates that the cell- walls, but not the pro- 

 toplasmic cell-contents or the cell-sap, are doubly refractive, 

 and that the denser the cell- wall the more highly refractive it is. 



A thin transverse section examined in the same way is seen to 

 present similar appearances. 



It will be observed, in addition, that the transverse section of 

 a much thickened cell-wall (that of a bast-fibre, for instance) 

 presents, when the field is dark, a dark cross : when the analyzer 

 is rotated through an angle of 90, the dark cross is replaced by a 

 bright one. the field being also bright. 



It will also be seen that in examining sections in polarized 

 light thick stratified cell-walls (particularly sclerenchymatous 

 cells) are coloured : this is most apparent when the field is dark. 

 This coloration is due to interference of light. 



The phenomena of interference can be best studied by intro- 

 ducing a plate of selenite between the polarizer and the analyzer : 

 it is to be placed on the stage of the microscope beneath the 



