l6o PLANT LIFE. 



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

 three elements, Imt in them the h)(lrogen and oxygen do not 

 exist in the ratio of two to one, the ox\"gen 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 latty 

 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 oli\'e oil, linseed oil, and cacao laitter. 



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 \arying jjroportions. Proteids 

 are generally recognizalile by their jiroperty 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 l)0th, 

 must be a\ailable in order that a plant may be ])ro]'erly 

 nourisherl. Green plants obtain their food chiefly by manu- 

 facturing it out of inorganic materials taken into the ]ilant 

 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 rdtimate source of 

 the food sn])[)ly of the world. 



215. Metabolism. — After suitable foods become availalde 

 to ])lants, whether by manufacture or by absor[ition rcach-- 

 made, they suffer various chemical changes both before and 

 after becoming a jiart of the body, 'i'he changes bv 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. 



