[Chap. XVm RESPIRATION AND PLANT DEVELOPMENT 167 



the energy of the corn. If fed to pigs, the yield of pork is 273 pounds, 

 or sufficient food for 220 men for one day. Dairy cattle are second only 

 to pigs as transformers of food in proportion to the energy lost. A great 

 loss of energy always results when the food in plants is converted into 

 meat before it reaches the human consumer. It is evident that as the 

 population of a country increases beyond a certain point, unless it can 

 afford to buy meat from foreign markets it must depend more and more 

 directly upon the food in plants. 



There are, however, certain animals that feed, either directly or indi- 

 rectly, on plants in which the food is physically unavailable to man. 

 All our sea-food animals, such as fish, clams, and oysters, convert large 

 quantities of the food in aquatic plants into food that is available to 

 man. The sheep and cattle grazing on the open range and forest reserves 

 in the Western States and on similar areas in other countries may be 

 looked upon as gatherers and converters of the food in plants that is not 

 otherwise available to man. 



