202 THE FOOD AND NUTRITION OF PLANTS. 



plants derived them, in the very form in which tliey were taken, 

 namely, as carbonic acid and water. The portion they accumulate 

 in their tissues constitutes the food of carnivorous animals ; who 

 consume and return to the air the greater part during life, and the 

 remainder in decay after death. The atmosphere, therefore, out of 

 which plants create nourishment, and to which animals as they con- 

 sume return it, forms the necessary link between the animal and 

 vegetable kingdoms, and completes the great cycle of organic exist- 

 ence. Organized matter passes through various stages in vege- 

 tables, through others in the herbivorous animals, and undergoes its 

 final transformations in the carnivorous animals. Portions are con- 

 sumed at every stage, and restored to the mineral kingdom, to which 

 the whole, having accomplished its revolution, finally returns. 



3G4. Moreover, plants not only furnish all the materials of the 

 animal fabric, but furnish each principal constituent ready formed, 

 so that the animal has only to appropriate it. The food of animals 

 is of two kinds ; — 1. that which serves to support respiration and 

 maintain the animal heat; 2. that which is capable of forming a 

 portion of the animal fabric, of its flesh and bones. The ternary 

 vegetable products furnish the first, in the form of sugar, vegetable 

 jelly, starch, oil, &c., and even cellulose ; substances which, contain- 

 ing no nitrogen, cannot form an integral part of the animal frame, 

 but, conveyed into the blood, are decomposed in respiration ; the 

 cai'bon and the excess of hydrogen combining with the oxygen of 

 the air, to which they are restored in the form of carbonic acid and 

 water. Any portion not required by the immediate demands of res- 

 piration is stored in the tissues in the form of fat, (which the animal 

 may either accumulate directly from the oily and waxy matters in 

 its vegetable food, or produce by an alteration of the starch and 

 sugar,) as a provision for future use : a deficiency of such materials 

 subjects the tissues themselves, or the proper supporting food, to im- 

 mediate decomposition in respiration. The quaternary or azotized 

 products furnish the proper materials of the animal frame, the 

 fibrine, caseine, albumen, &c. being directly appropriated from the 

 vegetable food to form the blood, muscles, &c. ; while a slight trans- 

 formation of them gives origin to gelatine, of which the sinews, carti- 

 lages, and the organic part of the bones, consist. The earthy poi'- 

 tion of the bones, the iron in the blood, and the saline ingredients 

 of the animal body, are drawn from the earthy constituents (33G) of 

 the plants upon which the animal feeds. The animal merely ap- 



