CHAP. I.] 



PLANT ARCHITECTURE. 



37 



the plant itself ; oxalic acid is one of the most general of 

 organic acids produced by plants, and the lime finds its 

 way into the plant as a carbonate or phosphate of lime ; 

 these eventually combine to form oxalate of lime, which 



Fig. lo. A characteristic cactus plant [Echinocactus Decaisnearms), 

 having the cells of the trunk filled with crystals when old. (Reduced.) 



may be considered as a by-product, the result of certain 

 chemical changes taking place in the contents of the 

 cell- sap. As a rule, crystals are deposited in those parts 

 of plants that fall away annually, more especially the 



