THE CELL 43 



trast, animals in their metabolic processes are constantly liberating 

 the stored-up energy from the compounds formed by plants and 

 deriving their peculiar proteins from the proteins that plants have 

 synthesized. The nutrition of animals, holozooic nutritiox, is 

 therefore essentially destructive. Here, then, is a most important 

 difference between typical animals and typical plants, and these 

 facts constitute the most fundamental relation in the science of 

 Biology. 



Some types of plant cells, for example, molds, are able to extract 

 what energy remains after animals and other plants have utilized 

 food, and can absorb substances directly from living or dead bodies. 

 This type of nutrition is known as saprophytic. Animals which 

 carry on their subsistence in this fashion are known as saprozooic. 



The Nitrogen Cycle. It is now clear to the reader that nitro- 

 gen is necessary for the formation of proteins of all sorts, both plant 

 and animal, and that the source of nitrogen is the nitrogen-contain- 

 ing salts in the soil. The relation between plant and animal nutri- 

 tion and their respective places in Nature are best shown by 

 following the history of nitrogen from the air, through plant and 

 animal proteins, and back again to its original gaseous condition. A 

 diagram (Fig. 9) shows the essentials of the cycle of nitrogen in 

 living organisms. 



Although green plants are capable of synthesizing carbohydrates 

 from inorganic materials and upon carbohydrate bases construct 

 proteins, they, like animals, are unable to employ atmospheric nitro- 

 gen and depend upon soil nitrates. For the replacement of the soil 

 supply of inorganic nitrogen compounds green plants are dependent 

 on the peculiar properties of the metabolism of minute plants, 

 NITROGEN-FIXING BACTERIA. Thcse microscopic organisms live in the 

 soil associated with the roots of certain green plants, for example, 

 the clover. In their life processes they normally utilize the nitrogen 

 of the air, a characteristic that occurs in no other living organism. 

 The nitrogen is combined with potassium and oxygen to form po- 



