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. 10. A characteristic cactus plant (Echinocactus Decaisneanus) , 

 having the cells of the trunk rilled 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 



