CHEMICAL REACTIONS OF PLANT TISSUES 63 



alkaloids are often complicated by interfering sub- 

 stances which seriously impair the value of tests. 

 The general test for alkaloids in plant tissues are as 

 follows: 



1. Mercuric-potassium iodide solution causes the formation 



of a yellowish flocculent precipitate with most alkaloids. 



2. Iodine-potassium iodide solution yields reddish or brownish 



precipitates with nearly all alkaloids. 



3. The alkaloids of Hydrastis may be precipitated in situ by 



mounting freshly cut sections in sulphuric acid. 



Glucosides. The glucosides are plant principles 

 which, when decomposed by dilute acids or enzymes, 

 yield sugar (chiefly glucose) as one of the products 

 of the reaction. Aside from this property of yielding 

 glucose upon decomposition there are no group reac- 

 tions which may be used in their identification. A 

 few glucosides have the property of reducing copper 

 sulphate directly, or without apparent decomposition 

 and subsequent production of glucose. Many of the 

 glucosides, upon hydrolysis or decomposition, yield 

 sugars which give characteristic crystalline precipi- 

 tates with phenylhydrazine. 



Calcium Oxalate. The great majority of crystals 

 occurring naturally in plants are composed of calcium 

 oxalate. This substance crystallizes either in the 

 monoclinic or hi the tetragonal system, but the crys- 

 tals naturally occurring in plants are rarely symmetri- 

 cal. In many plants the calcium oxalate occurs in 

 small broken crystals termed crystal sand. The tests 

 for calcium oxalate are as follows : 



1. Nitric and hydrochloric acids dissolve the oxalate crystals. 



2. Concentrated sulphuric acid causes destruction of the oxalate, 



resulting in the formation of insoluble needle crystals 

 of calcium sulphate. 



