HOW PLANTS AND AXIMALS GROW. 513
animals, arising from individual peculiarities and inherited an- cestral habits. Experiments have not as yet been made with a suflScient number of individuals or with a sufficient variety of foods to warrant any generalizations as to what constitutes a normal diet for either man or beast under even average condi- tions.
The recent recognition of energy as one of the most important factors in physiology lias led to the rejection of the purely chem- ical theories that were formerly quite generally accepted in regard to the role of particular food constituents in the processes of nu- trition. An assumed combustion of food constituents is no longer required to explain the phenomena of animal heat, which is now known to be but a phase in the transformations of energy in the processes of nutrition.
Energy is expended in building organic substance, or, in other words, in converting food-stuffs of any kind into protoplasm, the summit of the double stair of life, and its potential energy is the transformed or stored energy of the constructive process. This combined energy, in accordance with the law of conservation, may be liberated in the form of heat to a greater or less extent in various ways by the more or less complete disintegration of the organic substance in which it is stored. If the process of disin- tegration is carried on until the organic substance is resolved into its original elements, the heat liberated is the exact equivalent of the energy expended in its construction.
In living organisms the descending steps of metabolism are but successive phases of normal vital activities, resulting in the formation of a definite series of organic substances which contain less potential energy than the protoplasm from which they are formed, and heat must therefore be liberated as they are elabo- rated. Dead organic matters may be torn apart by microbes and resolved by a widely different series of descending steps into their original elements, as in the processes of fermentation and putre- faction, with a complete transformation of their potential energy into heat. The same ultimate result may likewise be obtained by burning organic substances, but the intervening steps and prod- ucts of the destructive process are less numerous and of a dif- ferent character than those produced by vital activities, while the heat liberated is still the transformed energy of the constructive process.
Plants derive the energy required to convert simple chemical elements into the complex molecules of protoplasm from the heat and light of the sun, while on the other hand the energy expend- ed in the constructive processes of animals is exclusively derived from the potential energy of their food (stored energy of plants), and a disintegration and apparent waste of the material, or chem-
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