MICROSCOPIC OBJECTS. 90 



Fig. 179. Raphides in the Cuticle of the Onion (Allium 

 Cepa), x 120. 



The raphides in this cuticle consist of crystals of oxalate 

 of lime. They are drawn as seen under polarized light. 



On testing them with sulphuric acid, minute crystals of 

 sulphate of lime are formed (see fig. 182), the sulphuric acid 

 having a greater affinity for the lime than the oxalic acid has. 

 If the crystals had been composed of sulphate of lime, scarcely 

 any change would have been perceived. 



The tests for the carbonate and phosphate of lime are men- 

 tioned under fig. 177. 



The cuticle is best seen when mounted in balsam. 



Fig. 180. Raphides in the Sepal of a Geranium (Geranium 

 Robertianum), x 200. 



The minute stellate crystals which occur in such large 

 numbers in the sepals &c. of most of the species of this genus 

 (Geranium), are composed of oxalate of lime. The presence 

 of raphides in any plant is easily ascertained by the use of 

 the polariscope, as all the forms polarize well. 



It is not known that these vegetable crystals are of use in 

 the nutrition or structure of plants ; but they are no doubt 

 caused in an accidental manner, by the various acids meeting 

 with calcareous matter, uniting with it, and forming these 

 vegetable salts. 



The sepal may be placed in turpentine, and mounted in 

 balsam in the usual manner. 



