CHAPTER II. 

 THE SYNTHESIS OF FATS. 



IN the plant fats are commonly associated with the reserve 

 food in seeds, spores, and vegetative perennating organs. As 

 a food they have considerable value in that on physiological 

 combustion they yield more energy weight for weight than 

 either protein or carbohydrate, the relative energy producing 

 values being roughly 5:3:2 respectively. 



A consideration of the salient features regarding the physio- 

 logical significance of fats has been given in the first volume,* 

 in which place the consideration was retained for the sake of 

 a closer association of the characterization and other features 

 of fats with the physiological problems involved. For this 

 reason a general survey only, with such departures into detail 

 as necessity demands, will be given on the present occasion. 



The fat characteristic of a plant makes its appearance in 

 the storage organ as that member approaches maturity. The 

 problem immediately presented is the origin of the fat. There 

 can be but little doubt that it is formed in situ from materials 

 in the immediate neighbourhood, a fact which is emphasized 

 by many observations on the development of fats in seeds 

 isolated from the plant whilst in an immature condition and 

 originally containing little or no fat. 



Within the storage organ, the fat may have its origin in 

 the products of the hydrolysis of fat synthesized in some other 

 organ, the leaf for example, in much the same way as the 

 starch in the potato tuber has its ultimate origin in the carbo- 

 hydrate produced in the leaf. Possibly the best ascertained 

 instance of the normal occurrence of fat, or fat-like substances, 

 in green assimilating organs is that provided by Vaucheria. 



With regard to the higher plants, the facts relating to the 

 occurrence of fats and kindred substances in the leaves do not 



* Vol. I., p. 36. See also Terroine : " Ann. Sci. Nat. Bot.," 1919, se"r. x. I, i. 



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