364 MICROCHEMISTRY OF PLANT PRODUCTS 



solution of potassium iodide-iodine, and chloroiodide of zinc 

 colors suberized membranes from yellow to brown. After 

 long treatment with a dilute solution of potassium hydrate, 

 suberized membranes may be stained violet with chloroiodide 

 of zinc. Alcannin stains suberized walls red. Under the polari- 

 scope suberized walls are seen to be doubly refractive. They 

 lose this property on heating and regain it on cooling. It may 

 be deduced from this that the constituents of the walls are in 

 part, at least, in crystals which are melted by heat, but reappear 

 on cooling. See under Methyl-blue and Cyanin. 



Syringin. — Syringin is a glucoside occurring in the cortex, 

 and to a certain extent in the xylem and medullary rays of 

 Syringa vulgaris, Robinia pseudacacia and species of Ligus- 

 trum. It is especially abundant in early spring. Sections con- 

 taining syringin, when treated with concentrated sulphuric 

 acid, acquire a dark blue color, which changes to violet. Nitric 

 acid dissolves syringin with a blood-red color. Syringin crys- 

 tallizes from aqueous solutions in the form of colorless, needle- 

 like crystals which are grouped in the form of a star. The 

 crystals are dissolved with difficulty in cold water, but readily 

 in boiling water or in alcohol. 



Tannins. — ^Various substances occurring in plants having 

 an astringent taste, and turning dark blue or green with salts 

 of iron, are termed tannins or tannic acid. Tannins occur in 

 greatest abundance in the bark and in pathological gall forma- 

 tions. Oak-galls furnish excellent material for the demonstra- 

 tion of tannins. When sections are treated with an aqueous 

 solution of ferric chloride, they take on a deep blue color, due 

 to the presence of tannin. Aqueous solutions of ferrous sul- 

 phate give the same result. If the reaction is watched under 

 the microscope, it is noticed that at first a deep blue precipitate 

 is formed, which soon dissolves and imparts its color to the 

 surrounding fluid. When sections are placed in a 10 per cent, 

 aqueous solution of potassium bichromate, a flocculent reddish- 

 brown precipitate is formed in the tannin-bearing cells. When 

 sections are placed in a concentrated solution of ammonium 



