FATS, WAXES, AND LIPOIDS 157 



efficiency may be easily determined by comparing the water loss 

 from a fruit from which this glaucous covering has been removed 

 with that from one on which it has been left intact. They are 

 also very resistant to the attacks of fungi; and strawberries picked 

 in the morning when the wax is firm have been found to rot much 

 less than those picked later in the day when the wax was softened. 



Sterols. — The monohydric alcohols of high atomic weight men- 

 tioned in the preceding section are straight-chain alcohols. Those 

 of high atomic weight that are cyclic (aromatic series) are com- 

 monly called sterols. While the sterols themselves are not fats, 

 they are frequently associated with the fats and produce their 

 predominant physiological properties. For these reasons they 

 may be discussed at this point. Cholesterol (C27H45OH) is the 

 most common sterol in animals but has not yet been found in 

 plants. Among the plant sterols (phy tosterols) , sitosterol, in the 

 embryo of the cereals; stigmasterol, from various plant oils; and 

 ergosterol, first found in ergot and later throughout the plant 

 kingdom, are best known. Ergosterol has been publicized much 

 more than any of the others because it was found (1926) to be the 

 precursor of vitamin D (Chap. XVII). When irradiated with 

 ultra-violet light, vitamin D is formed from this sterol, which is 

 considered by Windaus to be an isomer of the vitamin. The 

 irradiation causes a rearrangement of the molecule and the result- 

 ant vitamin properties. 



Lipoids. — The name lipoid has been given to substances of a 

 fatlike nature found in small quantities widely distributed in 

 plants and animals but in rather larger amounts in special places 

 such as nerve and brain tissue, egg yolk, and the seeds of plants. 

 The lipoids are extracted from tissues by the same solvents as 

 fats and, when hydrolyzed, yield fatty acid and a nitrogen com- 

 pound. One group, in addition, yields phosphoric acid. The name 

 lipoid means "fatlike" and was given because of the above- 

 mentioned similarities to the fats. 



Lipins. — This group of lipoids includes those which yield fatty 

 acid and a nitrogenous compound but no phosphoric acid. Those 

 found in the brain have been called cerebrosides but others are 

 also found in plants. Galactose has replaced a part of the glycerol 

 as the alcohol part of the ester in all the cases thus far reported. 



The Phosphatids. — The phospholipins or phosphatids are the 

 more common lipoids. They yield phosphoric acid in addition to 



