14 THE PHENOMENA OF LIFE 



dered potential in the chemical products formed. The potential energy is 

 set free, or is again made kinetic, when these products by simple combustion 

 produce heat, or when they are taken into the animal organism and used as 

 food and there later produce heat and motion. 



The influence of light is not absolutely essential to animal life; indeed, 

 it is said not to increase the metabolism of animal tissue to any great extent, 

 and the animal cell does not receive its energy directly from the sun's light 

 nor yet to any extent from the sun's heat, but from the potential energy of the 

 food stuffs. But it must be always kept in mind that anabolism is not pecu- 

 liar to vegetable, or katabolism to animal cells; both processes go on in each. 

 Some of the lowest forms of vegetable life, e.g., the bacteria, will live only in a 

 highly albuminous medium, and in fact seem to require for their growth 

 elements of food stuffs which are essential to animal life. In their metabo- 

 lism, too, they very closely approximate animal cells, not only requiring an 

 atmosphere of oxygen, but giving out carbon dioxide freely, and secreting 

 and excreting many very complicated nitrogenous bodies, as well as forming 

 protein, carbohydrates, and fat, requiring heat but not light for the due per- 

 formance of their functions. However, certain bacteria grow only in the 

 absence of oxygen. 



There is, commonly, a difference in general chemical composition be- 

 tween vegetables and animals, even in their lowest forms; for associated 

 with the protoplasm of the former is a considerable amount of cellulose, a 

 substance closely allied to starch and containing carbon, hydrogen, and 

 oxygen only. The presence of starch in vegetable cells is very character- 

 istic, though, as we have seen above, it is not distinctive, and a substance, 

 glycogen, similar in composition to starch, is very common in the organs and 

 tissues of animals. 



Inherent power of movement is a quality which we so commonly consider 

 an essential indication of animal nature that it is difficult at first to conceive 

 of its existence in any other. The capability of simple motion is now known, 

 however, to exist in so many vegetable forms that it can no longer be held 

 as an essential distinction between them and animals, and ceases to be a mark 

 by which one can be distinguished from the other. Thus the zoospores of 

 many of the Cryptogams exhibit ciliary or ameboid movements of a like 

 kind to those seen in amebae; and even among the higher orders of plants, 

 many, e.g.,Dioncca muscipula (Venus's fly-trap), and Mimosa sensitiva (Sensi- 

 tive plant) exhibit such motion, either at regular times or on the applica- 

 tion of external irritation. Were this fact taken by itself, it might lead one to 

 regard them as sensitive organisms. Inherent power of movement, then, 

 although especially characteristic of animal nature, is, when taken by itself, 

 no proof of it. 



Sources and Utilization of Physiological Material. In studying the 

 functions of the human body it is necessary first of all to know of what it is 



