158 PLANT PHYSIOLOGY 



the fatty acid and nitrogen compound. Of these, the best known 

 and most common is lecithin, which occurs in the cereals, legumi- 

 nous seeds, the brain, the heart, and the liver of animals, and in 

 egg yolk. In fact, it is thought that every living cell contains some 

 lecithin, but it is especially abundant where fats occur. Lecithin 

 is an ester of glycerol and fatty acids in which one of the fatty 

 acids is replaced by phosphoric acid, which, in turn, has one of its 

 hydrogen atoms replaced by the nitrogen base, choline. Its com- 

 position may be illustrated thus : 



' fatty acid, 

 glycerol \ fatty acid. 



phosphoric acid 4- choline. 



The fatty acids are different in different lecithins. In some lupine 

 seeds, the fatty acids are reported as palmitic and stearic and the 

 composition of the lecithin would then be: 



glycerol 



' palmitic acid, 

 stearic acid, 

 phosphoric acid + choline. 



Other phosphatids have been found in plants besides lecithin. 

 In many of these, various carbohydrates replace all or a part of 

 the glycerol as the alcoholic part of the ester. Among the car- 

 bohydrates thus identified have been galactose, dextrose, and pec- 

 tose, which may vary from mere traces to 15% of the weight of the 

 material from which the phosphatid was prepared. Such phos- 

 phatids have been found in potatoes, whole wheat flour, and pollen. 

 Other nitrogen compounds may also be found instead of the 

 choline; and, in addition to phosphorus, other minerals such as 

 calcium and iron may be present. 



Physiological Significance. — The lipoids are so universally 

 present in tissues and so commonly found where the vital processes 

 are the most pronounced, e. g., in the brain, heart, egg yolk, and 

 seeds, that they must play some part in the activity of living proto- 

 plasm. Experimental studies have been made for the most part on 

 animal cells and as yet there is little definite knowledge concerning 

 their physiological importance. The fact that the lipoids are hard 

 to isolate and properly purify for experimental purposes makes the 

 problem all the harder. To date, only hypotheses have been ad- 

 vanced, among the more important of which are the following: 



1. Overton (1901) suggests that the surface layer of all cells 

 contains lipoids which, owing to the fact that they lower the sur- 



