AGRICULTURAL SUBSTANCES CARRY FORCE 21 



eaten plants. So we have the true saying, " All flesh 

 is grass." 



But the reverse, all grass is flesh, is not true, 

 for the animal is not able to use all the compounds of 

 the plant. The part used by the animal depends on 

 several things. In some plants the compounds are much 

 more easily broken up than in others, and the animal 

 secures a greater amount of available energy than from 

 those plants where a considerable portion of energy is 

 not digested or where the process of digestion consumes 

 much of the energy. Certain animals have better diges- 

 tive systems, and are capable of breaking up more of 

 these compounds than others. When food is plentiful 

 the animal may eat more than is necessary; and if food 

 is scarce, it may digest the plants more completely. In 

 either case the animal uses the compounds as fuel in 

 the body to keep up body heat, to produce energy for 

 locomotion and other vital functions, or to furnish to its 

 young as milk. The parts of the plants eaten that can- 

 not be used, are excreted as waste. 



Latent energy in plants and animals. Energy first 

 stored up from the sun by the plant is said to be in a 

 latent form. This may be illustrated by placing a par- 

 tially melted piece of ice in a kettle on a stove, and 

 beside it a kettle full of water, both being at the freez- 

 ing temperature when placed over the fire. Heat is 

 conducted into the water in both kettles, but the water 

 in the kettle with the ice remains at the same tempera- 

 ture until the ice is melted, while the water in the other 

 kettle rises in temperature. Heat is constantly going 

 into each mass of water. In one it is stored up in the 

 latent form, that is, used in merely changing the form 

 of the water from a solid to a liquid without causing a 

 rise in temperature. In the other it is stored up in the 

 active form, resulting in a rapid rise in temperature. If 

 the two kettles are now placed in air at the freezing 



