CHEMICAL REACTIONS OF PLANT TISSUES 65 



upon the resins, there are no general microchemical 

 tests for this group. Resins are generally soluble in 

 volatile solvents and, like the fats, give a red color- 

 ation with alkannin solution. 



Silica. — Silica, silicic acid or silicon dioxide may 

 occur as deposits upon the walls of cells or in the form 

 of rounded masses. This substance is only soluble 

 in hydrofluoric acid and is not destroyed by heating 

 at high temperatures. 



Calcium Carbonate. — Calcium carbonate occurs 

 free in the plant cells, as deposits upon the cell walls, 

 or in the form of irregular masses attached to the cell 

 wall by a slender filament of cellulose. 



1. Addition of concentrated hydrochloric acid dissolves deposits 

 of calcium carbonate with the evolution of gas bubbles 

 (carbon dioxide) . 



Fats. — The fats and fixed oils are organic salts of 

 the so-called fatty acids, chiefly palmitic, stearic and 

 oleic acids. They usually occur in the form of globules 

 within the living cells of seeds and fruits. They are 

 rarely associated with starch but often occur in seeds 

 containing aleurone. The microchemical tests for 

 oils and fats are as follows: 



1. Addition of alkannin solution colors fat and oil globules 



bright red. Suberized walls are also colored red by this 

 reagent. 



2. Boiling with concentrated potassium hydroxide solution or 



strong ammonium hydroxide causes saponification which 

 is evidenced by a loss of refractive power in the globules. 

 Upon cooling, the globules are found to be covered with 

 small needle crystals of soap. 



