104 A TEXTBOOK OF BOTANY [Cn. Ill, 13 



into the gums, like gum arabic, all readily digestible by plants 

 and animals. 



The Fatty CLils come ultimately from grape sugar, through 

 intermediate stages, including fatty acids. They are really 

 mixtures of true fats, which are not volatile, and thus differ 

 from the essential oils, to be considered under secretions. 

 They are found in a few fruits, such as Olive (yielding olive 

 oil), but accumulate in quantity in a good many seeds, from 

 which we obtain Castor oil, Cottonseed oil, Linseed oil, and 

 some others. They occur usually in small round globules 

 among other food substances, giving a characteristic oily 

 luster to sections through such tissues, and, while commonly 

 liquid, they form sometimes a butter-like solid, as in cocoa- 

 butter. They are insoluble in water, and hence not movable 

 through the plant until digested back to the soluble fatty 

 acids. Chemically they are rather diverse in composition (a 

 typical formula, that of tri-olein, being CBTH^OC), but are all 

 marked by this peculiarity, that their proportion of oxygen 

 is very small to that of their carbon and hydrogen. 



As with sugars and starches, the fatty oils are also good 

 food for animals. They are a valuable constituent of the 

 seeds eaten by animals, including man, who also extracts 

 and refines them for food and for diverse uses in medicine, 

 arts, and manufactures. Like the animal fats to which they 

 are so closely related, their paucity of oxygen makes neces- 

 sary a large supply of fresh air for their assimilation ; but 

 they yield a great deal of heat, which explains why fats are 

 so craved in cold climates. 



The Proteins are much more complicated substances, form- 

 ing the most important, even if not the most abundant, 

 of the reserve foods. While scattered throughout all living 

 cells, they accumulate chiefly in seeds, where they occur 

 mostly as solid grains, either scattered throughout the cells, 

 as in Peas and Beans, or in a special layer just underneath 

 the husk, as in Wheat and other grains (Fig. 65). There 

 are hundreds of kinds of named proteins, grouped under 



