92 EVOLUTION AND GENETICS 



ment of component elements produces fats, and the addition of 

 elements derived from inorganic salts forms proteins. While the 

 complexity of the processes involved is infinitely greater than this, 

 the essential result is the formation of these three fundamental 

 compounds of living matter through synthesis of inorganic sub- 

 stances and the addition of energy derived from sunlight, the whole 

 depending on the plant's possession of chlorophyll. 



The plant's activity is thus chiefly constructive from the point 

 of view of living things. Careful study has shown that this is not 

 its entire activity, however, for while it uses carbon dioxide in the 

 synthesis of carbohydrates, and liberates oxygen as a result, 

 exactly the converse of the process which liberates energy by 

 oxidation in the animal body, it also oxidizes some of the substances 

 which it has elaborated, for the lil^eration of energy. The skunk 

 cabbage, familiar to anyone who has studied nature in the eastern 

 part of the United States, furnishes an excellent example. It 

 blooms in swampy places as early as February, sometimes before 

 the ice has entirely disappeared, and its release of energy in these 

 frigid surroundings is so great that a temperature difference of six 

 degrees has been recorded between the inside of its curious spathe 

 and the cold outside air. 



Animal Metabolism. The animal is in general a much more 

 dynamic organism, for it takes the substances synthesized by the 

 plants, with their abundant potential energy, and carries on its 

 own activities solely by releasing this energy by oxidation, after 

 breaking down the substances into their simpler components and 

 resynthesizing these into the similar compounds of its own body. 

 The animal is thus dependent either directly or indirectly upon 

 the plant for its existence. It is wholly unable to build up the 

 substances that it needs from inorganic materials. 



In four of the five physiological properties then, as well as in 

 morphological characteristics, the plants and animals are defi- 

 nitely related. In the fifth, metabolism, we see that there is funda- 

 mental difference due to the ability of green plants to carry on 

 photosynthesis. However there is also a degree of similarity that 

 is even more striking in parasitic plants like the fungi, which are 

 as devoid of ability to utilize inorganic compounds, as dependent 

 on the green plants, as are the animals. 



Plant- Animals. In a few organisms both of these powers are 

 resident. These are called plants and animals, and as a matter of 



