CALCIUM CALCIUM PHOSPHATE 337 



yellowish, silken crystals of a double chloride of gold and caffeine 

 begin to be formed on the evaporation of the reagent. However, 

 theobromine forms quite similar crystals when treated as above. 

 Another method for the detection of caffeine is to place sections 

 in a few drops of water and heat to boiling; then to allow the 

 water to evaporate slowly and to treat the residue with a drop 

 of benzol. On the evaporation of the benzol, caffeine appears 

 in the form of fine needle-crystals. 



Calcium. When the ash of plants is treated with sulphuric 

 acid, this unites with the calcium present to form crystals of 

 gypsum. If calcium sulphate is already present in the ash, its 

 characteristic crystals may be detected when an aqueous solu- 

 tion of the ash is allowed to dry slowly. If calcium is present 

 in sections, it may be deposited in the form of crystals of cal- 

 cium oxalate if the sections are treated with a solution of ammo- 

 nium oxalate. 



Calcium Carbonate, CaCO 3 . This rarely occurs in the 

 crystalline form within the cells. It may, however, be found 

 imbedded in, or incrusted on, the cell-walls. Calcium carbonate 

 dissolves with effervescence when treated with dilute acetic 

 acid. When treated with concentrated hydrochloric acid, it 

 dissolves with the evolution of carbon dioxide gas. The in- 

 growths from the walls of certain cells of the leaves of Ficus 

 elastica, known as cystoliths, are thickly incrusted with calcium 

 carbonate and afford excellent material for the demonstration 

 of this salt within plant tissues. 



Calcium Phosphate, Ca 3 (PO 4 ) 2 . This salt of calcium 

 occurs usually, if not always, in solution in the cell-sap. It may 

 be deposited in the form of sphaerocrystals when plant tissues 

 containing it are kept for a long time in strong alcohol. When 

 treated with sulphuric acid, the sphaerocrystals are dissolved 

 and crystals of calcium sulphate are formed in their stead. 

 When sections containing calcium phosphate are heated on a 

 slide in a drop of ammonium molybdate acidulated with nitric 

 acid, a yellow precipitate is produced. This reaction may be 

 hindered by the presence of certain organic compounds, such 



