ROLES OF THE LIPIDS IN PLANTS 397 



This reaction is called saponification and the resulting salt of the fatty acid, 

 in this example sodium palmitate, a soap. Soaps are of common occurrence 

 in plant cells. They are excellent emulsifiers and probably serve in this role 

 in the protoplasm. 



Roles of the Lipids in Plants. — Protoplasm always contains lipids in 

 a finely emulsified form. They are especially abundant in the protoplasm of 

 meristematic cells. It is impossible to determine just what proportion of the 

 lipid material dispersed in this way represents storage food which may ulti- 

 mately be used as such, and how much represents indispensable constituents 

 of the protoplasm. In some cells, particularly those of seeds which are rich 

 in stored fats, relatively large droplets of oil may occur as inclusions in the 

 protoplasm. In the cells of some species, particularly monocots, oil deposi- 

 tion occurs in definitely organized bodies known as elaioplasts (Chap. VI). 



Fats serve as storage forms of food in plants and this is undoubtedly one 

 of their principal roles. The fats which accumulate in most plants are 

 liquid within the usually prevailing range of temperatures in temperate zones. 

 In many species, especially conifers, fats may remain in a liquid condition 

 at temperatures as low as — 30° C. 



Fats are especially abundant as reserve foods in the seeds of many species. 

 Those varieties of seeds in which oils occur in abundance usually contain 

 relatively small quantities of carbohydrates and vice versa. Species which 

 produce seeds rich in fats include cotton, corn, peanut, sunflower, rape, 

 flax, and castor bean. All of these species are important commercial sources 

 of vegetable oils. Olive oil, a staple food product in many countries, is 

 extracted from the fruits of the olive. Oils frequently occur in abundance 

 in many other plant organs, as for example in the rhizomes of potato and iris, 

 and in the aerial organs of many woody species, especially during the winter 

 months. 



Fats contain larger quantities of energy per unit of dry weight than 

 carbohydrates or proteins, and hence are efficient storage foods. Average 

 values for heats of combustion of i g. of dry weight are: fats 9.3 kg.-cal., 

 proteins 5.7 kg.-cal., and carbohydrates 4.1 kg.-cal. The high energy con- 

 tent of the fats is a corollary of the highly reduced state of the carbon in 

 such compounds. 



During the germination of fatty seeds the oils present gradually dis- 

 appear. This is shown for sunflower seeds in Table 37, in which 

 the "ether extract" is taken as a composite measure of the oily constituents 

 present. Concurrently with the disappearance of fats there is a temporary 

 increase in the quantity of soluble carbohydrates present as well as a progres- 

 sive increase in the amount of cellulose. The carbohydrates are undoubtedly 

 formed from fatty acids and glycerol resulting from the digestion of fats. 



