138 CHEMISTRY OF PLANT LIFE 



index, etc., and by certain special color reactions for particular 

 oils; or by measurements of certain chemical constants, such as 

 the percentage of free fatty acids which they contain, the sapon- 

 ification value (i.e., the number of milligrams of KOH required to 

 completely saponify one gram of the fat), the iodine number (per- 

 centage by weight of iodine which is absorbed by the unsaturated 

 fatty acids present in the fat), percentage of water-insoluble fatty 

 acids obtained after saponification and acidifying the resultant 

 soap, etc., etc. Most of these tests must be carried out under 

 carefully controlled conditions in order to insure reliable identi- 

 fications, and need not be discussed in detail here. Full directions 

 for making such tests, together with tables of standard values for 

 all common fats and oils, may be found in any reference book on 

 oil analysis. 



PHYSIOLOGICAL USE OF FATS AND OILS 



In animal organisms, fats are the one important form of 

 energy storage. They also form one of the most important sup- 

 plies of energy reserve material -in plants. Carbohydrates com- 

 monly serve this purpose in those plants whose storage reservoirs 

 are in the stems, tubers, etc. ; but in most small seeds the reserve 

 supply of energy is largely in the form of oil, and even in those 

 seeds which have large endosperm storage of starch, the embryo 

 is always supplied with oil which seems to furnish the energy 

 necessary for the first germinative processes. 



Fats are the most concentrated form of potential energy of all 

 the different types of organic compounds which are elaborated by 

 plants. This is because they contain more carbon and hydrogen 

 and less oxygen in the molecule than any other group of sub- 

 stances of vegetable (or animal) origin. It has been pointed out 

 that a quantity of fat capable of yielding 100 large calories of heat 

 will occupy only about 12 cc. of space, whereas from 125 to 225 

 cc. of space in the same tissue would be required for the amount of 

 starch of glycogen necessary to yield the same amount of heat, or 

 energy, when oxidized. 



The fats undoubtedly catabolize first by hydrolysis into giycerol 

 and fatty acids, and then by oxidation possibly first into carbo- 

 hydrates and then finally into the end-products of oxidation, 

 namely, carbon dioxide and water. The following hypothetical 



