60 BOTANICAL MICROTECHNIQUE. 



cium oxide. The crystals preserve their original form, but 

 become opaque and therefore appear black by transmitted 

 light, but pure white by reflected light (or dark-field illumi- 

 nation). If the crystals dissolve, after the burning, in dilute 

 acetic acid or concentrated hydrochloric acid, without the 

 formation of gas-bubbles, this shows that an oxalate has 

 been changed to the oxide ; while the carbonate dissolves in 

 hydrochloric acid with the liberation of carbonic acid. 



89. The finding of calcium oxalate crystals can be made 

 much easier by examination by polarized light. They are 

 distinguished in general by their strong double refraction, 

 which is, however, much greater in those of the monoclinic 

 system than in those of the tetragonal system. The latter, 

 naturally, cannot glisten in the polarizing microscope with 

 crossed nicols, when their optical axes stand vertical. 



To make visible the crystals of calcium oxalate within 

 large organs, for example whole leaves, without cutting 

 them into sections, these may be made quite transparent. 

 For this purpose chloral hydrate, which does not attack 

 calcium oxalate, has been used ; and phenol can also be em- 

 ployed. If the pieces are heated to boiling in one of these 

 fluids, they usually become wholly cleared in a short time. 

 The alcoholic solution of sulphurous acid used by Wehmer 

 (I, 218) for decolorizing leaves will certainly be of much 

 service in many cases. 



For the preservation of such preparations Canada balsam 

 is best adapted. They may be transferred directly from 

 phenol to xylol and xylol-Canada balsam. The study of 

 these cleared preparations is best conducted by polarized 

 light. 



b. Calcium Carbonate, CaCO,. 



'pO. Calcium carbonate rarely occurs in the interior of 

 cells, but is usually deposited in or upon the cell-wall (cf. 

 Zimmermann I, 104). 



For the recognition of the carbonic acid in calcium car- 

 bonate, acetic or hydrochloric acid may be used. After the 

 addition of one of these, the carbonic acid is set free in 



