44 FATS, OILS, AND WAXES 



walls of the endodermis, exodermis, or cork, depend upon a 

 variety of factors amongst which the ratio between calcium 

 and potassium and sodium would appear to be important. 

 Their fate, however, is the same ; the unsaturated fatty acids 

 undergo oxidation and condensation resulting in a waterproof 

 layer, the fat constituents of which are no longer soluble in 

 fat solvents. 



MICROCHEMICAL REACTIONS. 



1. The microscopical appearance of oil when mixed with 

 water is characteristic owing to its immiscibility with water 

 and its different refractive index. 



2. Its solubility in ether, chloroform, benzene, or other fat 

 solvents is easily noted. 



3. If oil be present in the preparation it will fairly rapidly 

 turn brown and then black when treated with a I per cent 

 solution of osmic acid. This is not absolutely conclusive since 

 osmic acid stains proteins brown. 



4. Tincture of alkannin, or a saturated solution of Scharlach 

 R in 75 per cent alcohol, colours oil globules red or pink. 



The reaction with the first-named reagent is often ill-defined 

 and frequently fails when the alkanna used has been extracted 

 from the root some time. The test is more satisfactory when 

 freshly prepared tincture is used. 



A similar reaction is given by Sudan III. 



It is important to note that these and similar reactions are 

 not conclusive of the chemical nature of the substances acted 

 upon. For example, Sudan III not only stains oils red but 

 also resins, latex, wax, and cuticle ; chloroplasts are stained 

 a pale red ; cellulose, lignified walls, gelatinized membranes, 

 starch, and tannin are unstained. 



The staining tests mentioned above may be employed after 

 extracting the oil with ether or other solvent. 



WAXES. 



The chief function of waxes in plants is to form a protec- 

 tive covering against undue evaporation of water. They are 

 found most commonly in or on the cuticle of leaves and fruits 

 where they give rise to the glaucous effect. 



