l6o PLANT LIFE. 



213. Fats. — These are likewise combinations of the same 

 three elements, but in them the hydrogen and oxygen do not 

 exist in the ratio of two to one, the oxygen being much less 

 in proportion. Some are solid at ordinary temperatures, 

 while others are fluid. They are combinations of free fatty 

 acids and glycerin. Upon the addition of an alkali, the fatty 

 acids combine with it to form soap and other compounds of 

 less amount while the glycerin is set free. Commercial ex- 

 amples of plant fats are olive oil, linseed oil, and cacao butter. 



214. Proteids are foods consisting of at least five and 

 generally of six elements, namely, carbon, hydrogen, oxygen, 

 nitrogen, sulfur, and (ordinarily) phosphorus. These elements 

 are complexly combined in varying proportions. Proteids 

 are generally recognizable by their property of coagulation 

 upon the application of heat, acids, or other agents. Well- 

 known examples are the proteids forming the "white of egg." 

 Examples from the vegetable kingdom are less familiar. 



Proteids always, and either carbohydrates or fats, or both, 

 must be available in order that a plant may be properly 

 nourished. Green plants obtain their food chiefly by manu- 

 facturing it out of inorganic materials taken into the plant 

 body from without. They are the only organisms, so far as 

 known, which have the power of building up organic material 

 from inorganic. They are, therefore, the ultimate source of 

 the food supply of the world. 



215. Metabolism. — After suitable foods become available 

 to plants, whether by manufacture or by absorption ready- 

 made, they suffer various chemical changes both before and 

 after becoming a part of the body. The changes by which 

 foods are manufactured and assimilated and those by which 

 the products of waste are gotten rid of are all comprehended 

 under the term metabolism. 



