156 PLANT PHYSIOLOGY 



enough fat to yield 100 kg. cal. of heat can be placed in 12 c. c, 

 at least twice that much space would be needed to contain starch 

 for the same amount of energy when oxidized. This gives an idea 

 of the advantage of storing energy in the form of fats when there 

 is not much available room, and it is, therefore, not surprising to 

 learn that more than 75% of all plants store reserves in the seed 

 in the form of fats or oils, where they compose at least a third of 

 the dry weight. As stated above, just how much of the fat is oxi- 

 dized as fat and how much is oxidized as carbohydrate, after first 

 being transformed into the sugars, is a disputed question. Accord- 

 ing to Maquenne and Ivanov, the saturated fatty acids are oxidized 

 directly, while the unsaturated ones are first changed to sugars. 



Appearance of Fats. — The fats first appear in the protoplasm as 

 small vacuoles, which later run together to form larger drops. 

 In some cases the oil has been described as originating from the 

 activity of small plastids called elaioplasts, which are supposed to 

 bear the same relation to oil as leucoplasts in the potato bear to 

 the formation of the starch grains. These plastids have been 

 observed in Vanilla, Ornithogalum, Gaillardia, and other plants. 

 Beer (1909) thought that the elaioplasts were formed from de- 

 generate chloroplasts, but more recent workers have connected the 

 elaioplasts with special chondriosomes, from which they originate 

 in much the same manner as the chloroplasts. 



The Waxes. —The waxes differ from fats in that they are esters 

 of fatty acids with monohydric alcohols of high atomic weight, 

 e. g., ceryl alcohol (C 26 H 5 30H), instead of with glycerol, a tri- 

 hydric alcohol. In common use the word wax is used to mean a 

 harder substance than a fat, but these physical differences are 

 not the correct criteria of differentiation. Thus what is commonly 

 called " wool-fat" is a wax, and " Japan-wax" is a fat. The waxes, 

 in addition to being compounds of monohydric alcohols, differ 

 from the fats in being less easily hydrolyzed and less soluble. 



The better known animal waxes are beeswax, spermaceti, and 

 wool wax. Carnauba wax from the leaves of the wax palm (Co- 

 pernicia) is the best known of the vegetable waxes. Poppy wax 

 is an ester of ceryl alcohol and palmitic acid. 



Because of their imperviousness to water, waxes are commonly 

 found on leaves and fruits where there is need to check excessive 

 evaporation. They are also often found on fruits such as plums 

 and cherries, where they form what is called the bloom. Their 



