40 .., PRACTICAL BOTANY. 



The examination is to be commenced by rotating the 

 analyser, 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 analyser 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 teaches that the cell-walls, but not 

 the protoplasmic 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 sec- 

 tion of a much thickened cell-wall (that of a bast-fibre, 

 for instance), presents, when the field is dark, a dark 

 cross : when the analyser is rotated through an angle of 

 90, the dark cross is replaced by a bright one the field 

 being also bright. For the explanation of this phenome- 

 non reference should be made to textbooks of Physics. 



Mount some starch-grains (potato) in water ; examine 

 as described above. It will be seen that when the field 

 is dark the grain is bright and presents a well-marked 

 dark cross ; when the field is bright, the dark cross is 

 replaced by a bright cross. 



It will be observed that in examining sections in 

 polarised light thick stratified coil-walls (particularly 

 sclerenchymatous cells) are coloured ; this is most 

 apparent when the field is dark. This coloration is 

 due to interference of light. 



