CH. V] POLARISCOPE. 121 



The arrangement of the markings is constant with 

 given objects. Starch grains exhibit on a black field a 

 large black and white Maltese cross with its centre at 

 the hilum of the grain. To see this well, starch grains 

 of potato should be mounted in Canada balsam so 

 as to be transparent and when examined in ordinary 

 light quite invisible. Advantage may be taken of this 

 method for detecting small doubly refractive bodies, other- 

 wise difficult to perceive. Thus the very minute crystals 

 of calcium oxalate occurring in many leaves are often 

 difficult to make out, but if the leaf be decolorised and 

 rendered quite transparent by soaking it in strong 

 chloral-hydrate solution, the distribution of these crystals 

 may be easily observed by their light crossed markings on 

 a black field. This method was employed by Scbimper^ 

 in his interesting researches on the physiology of calcium 

 oxalate. An example of its use has been given in experi- 

 ment 78 (p. 66) where the amount of oxalate occurring in 

 leaves under various conditions is studied. 



(141) Tension. 



Crystalline bodies are anisotropic, and it is one view of 

 the significance of the anisotropism of organised bodies, 

 cell-walls, starch grains, etc., to attribute this to a 

 crystalline structure of the ultimate particles — micellae of 

 Nageli — of which these bodies are held to be built up. 



Another view denies this crystalline structure and 

 attributes the anisotropism to the tensions or strains 



Schimper, Bot. Zeitiing, 1888, p. 81. 



