578 ELEMENTS OF PLANT HISTOLOGY AND MICROTECHNIC 



Syringin. Syringin is a glucoside occurring in the cortex, and to a certain 

 extent in the xylem and medullary rays of Syringa vulgaris. It is especially abun- 

 dant in early spring. Sections containing syringin, when treated with concentrated 

 sulphuric acid, acquire a dark blue colo'r, which changes to violet. Nitric acid 

 dissolves syringin with a blood-red color. Syringin crystallizes 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 blue or green with salts of iron, are termed tannins or tannic acid. 

 Oak-galls furnish excellent material to illustrate the demonstration of tannin. 

 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 

 sulphate 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 molybdate in concentrated ammonium chloride, the same 

 character of precipitate is produced as when potassium bichromate is used. Lead 

 acetate produces a white precipitate with tannins. The following method may be 

 employed; Sections are placed in a 7 per cent, solution of copper acetate for a week 

 or longer, and are then placed on a slide in a drop of a 0.5 per cent, solution of 

 ferrous sulphate. After a few minutes, and before the cell-walls begin to turn 

 brown, the sections are washed in water and transferred to a watch-glass of alcohol 

 to drive out air-bubbles and extract chlorophyll, if any is present. The sections 

 are then mounted in glycerine for examination under the microscope. By this 

 treatment an insoluble brown precipitate is produced in the presence of tannins. 

 The sections may be transferred from the glycerine to glycerine- jelly if permanent 

 mounts are desired. If the sections are taken from the alcohol in which they were 

 placed to remove the chlorophyll, etc., and placed in a solution of iron acetate, a 

 blue or a green color will be produced, according to the kind of tannin present. 

 If it is desired to fix the cell-contents while testing for tannins, the sections should 

 be placed in a concentrated alcoholic solution of iron acetate instead of in the 

 aqueous solution, as above. When living tissues are placed in a solution of i part 

 of methylene-blue in 500 parts of distilled water, those cells which contain tannins 

 take on a blue color, and later a deep blue precipitate is formed in these cells. 

 Cells containing phloroglucin act in the same way to this reagent as those contain- 

 ing tannins. 



Theobromine, Dimethyl -xanthin, C7H 8 N 4 Oj. This alkaloid occurs in the 

 cocoa-bean. Its presence may be demonstrated by the use of hydrochloric acid 

 and chloride of gold, as directed under Caffeine. The reactions for caffeine and 

 theobromine are sometimes difficult to distinguish. When sections containing 

 theobromine are heated in distilled water on the slide to the boiling-point, and the 

 sections are allowed to dry slowly, and a drop of benzol is added to the residue, 

 crystals of theobromine appear in the form of a fine powder on the evaporation of 

 the benzol; whereas, when sections containing caffeine are treated in like manner, 

 the crystals containing caffeine take on the form of needles. 



Tyrosin, C 6 H 4 OH.CH 2 .CHNH.2.COOH. Tyrosin may be demonstrated in 

 abundance in the tubers of the dahlia. When sections are mounted under a cover- 

 glass in glycerine for several days, needle-shaped crystals of tyrosin are deposited in 

 radiating groups. In an abundance of glycerine the crystals are not deposited, for 



