64 THE ELEMENTS OF VEGETABLE HISTOLOGY 



Aleurone. — Aleurone is a protein or nitrogenized 

 principle found in many seeds, especially those rich 

 in fixed oil. Except in the cereals it seldom occurs 

 associated with starch. Aleurone is present in the 

 form of granules surrounded by a membrane enclos- 

 ing a protein substance, in which are embedded glob- 

 ular particles (globulins) and calcium oxalate crys- 

 tals (crystalloids). Very little is known regarding 

 the exact composition of these proteins and their 

 formation in the plant. The general tests for aleurone 

 and other plant proteins are as follows : 



1. Iodine-potassium iodide solution colors proteins yellow to 



brown. The iodine solution should be more concentrated 

 than that used in the test for starch. 



2. Concentrated nitric acid colors proteins bright yellow (xan- 



thoproteic reaction). 



3. Millon's reagent gives a bright red color upon warming the 



material to which the reagent has been applied. 



Tannins. — The tannins are a group of chemically 

 allied substances chief among which are tannic and 

 gallic acids. In the living plant they occur in solution 

 or as amorphous deposits. Tests for tannins in veg- 

 etable materials are as follows: 



1. Ferric chloride solution gives a bluish or greenish black 



coloration, depending upon the particular tannin com- 

 pound present. 



2. Cupric acetate solution produces a reddish brown precipitate 



with tannins. 



Resins.— The resins include a complex group of 

 substances which are found in many plants. In a 

 number of instances volatile oils or gums are associ- 

 ated with these resins thus forming oleoresins and 

 gum resins. Owing to the great variation in chemical 

 composition and the scarcity of definite information 



