152 PLANT PHYSIOLOGY 



As will be seen from the formula of glycerol, there are three 

 hydroxyl (OH) groups which can unite with the fatty acid. All 

 of these may unite with the same acid to produce a simple fat 

 (yielding only one kind of acid) or each may be united with a 

 different fatty acid so that when broken down two or three differ- 

 ent fatty acids will result (oleostearin, stearopalmitin, etc.). In 

 this way it is seen that the number of fats produced with the 

 various kinds of fatty acids in different combinations is enormous. 

 Furthermore, only one or two of the hydroxyl groups of the glyc- 

 erol may combine with a fatty acid. 



It is interesting to note here that the fats with saturated fatty 

 acids have a higher melting point than those with unsaturated 

 acids. This means that those of the latter group will be liquid 

 at lower temperatures than those of the former. It is, therefore, 

 not surprising to find that the plants of the tropics have fats rich 

 in saturated acids, while those of the temperate and colder regions 

 of the earth contain more of the unsaturated acid groups. Thus 

 In cocoa butter, the chief fatty acid is stearic, while in olive oil 

 it is oleic. The oils of the pines are liquid at such low temperatures 

 as — 30° C, but in the peanut, the fat is in the solid condition below 

 2° C. Since materials in a solid state are difficult to transport and 

 to utilize in the plant, the importance of the type of fat from a 

 metabolic point of view is evident. 



Origin of the Fats. — The origin of the fats in the plant has 

 been the subject of much research the past few years and concern- 

 ing it there are many hypotheses but little definitely established 

 material. It is now quite generally conceded that the fats are 

 built up from the fatty acids and glycerol by means of the enzyme, 

 lipase. In the laboratory this synthesis has been accomplished 

 by heating for some time under pressure at 220° C. The problem 

 then becomes one of determining how the glycerol and fatty acids 

 are made. 



Although Fleissig thinks that in Vaucheria, which contains 

 much oil, the oil arises as a direct photosynthetic product much 

 as sugars are formed in ordinary plants, the majority of workers 

 agree that the fats are secondary products formed from the car- 

 bohydrates. Since carbohydrates are the first materials made by 

 the plant, it seems reasonable to suppose that the fats come from 

 them, and experimental evidence for this view is not lacking. 

 In animals it is a common practice to " fatten" stock by feeding 



