MICRO-PHYSICS OF THE CELL. 41 



The phenomena of interference can be best studied 

 by introducing a plate of selenite between the polariser 

 and the analyser; it is to be placed on the stage of 

 the microscope beneath the object. Various kinds of 

 selenite-plates may be used ; it is assumed here that 

 the plate shows red and green tints. 



Mount a section of a twig or of a leaf-stalk ; rotate 

 the analyser so that the field is red or green. The 

 interference colours will not be well seen in the thin 

 cell-walls ; they will appear merely red or green. The 

 thickened cell-walls will exhibit a play of colours which 

 differs in different cases. 



Mount a section of part of a succulent leaf (Aloe, Crassula, 

 Sedum, &c.). Observe that the interference colours in the ctiti- 

 cularised external layer of the outer walls of the epidermal cells 

 are complementary in position to those of the subjacent cellulose 

 layers ; this indicates differences of tension in the cuticularised 

 and uncuticularised layers. 



The relation of the interference colours can be more 

 definitely made out in starch-grains. 



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

 the analyser so that the field is red. Assuming that 

 the starch-grain under examination is so placed that its 

 long axis is directed away from the observer, it will be 

 seen that there is a red cross on the grain corresponding 

 in position to the dark cross mentioned above, that the 

 two lateral segments of the grain are coloured yellow, 

 and that the anterior and posterior segments are 

 coloured blue. 



2. Spectrum of Chlorophyll. 



In order to observe this, an alcoholic solution must 





