SUMMARY 129 



ammonia, carbon dioxide and water. Other bacteria are able 

 to reduce nitrates and nitrites, and thus to liberate free nitro- 

 gen. These are relatively few in number, including, however, 

 the well-known species Bacillus coli, B. typhosus, B. fluorescens 

 and B. pyocyaneus. This action probably follows a condition 

 of oxygen hunger on the part of the bacteria when oxygen is 

 wrested from the nitrogen, leaving the latter as a free gas. 

 Nitrogen, thus liberated, cannot be utilized by the higher plants, 

 and were it not for the activity of other specially adapted 

 organisms there might be a leak in the current of matter and 

 energy. It is still a disputed question whether some of the 

 lower algae and some of the common molds have the power to 

 "fix" this free nitrogen and so bring it back into the cycle of 

 living things. This function is performed, however, by certain 

 groups of bacteria, some of which are able to bring about the 

 oxidation of free ammonia (NH 3 ) to nitrates, while still other 

 bacteria can fix free nitrogen of the air (Clostridium pastorian- 

 ium, two species of Azotobacter, and various species of Bacillus). 

 All products of vital activity are thus, sooner or later, returned 

 to the earth or atmosphere to be again taken up by the green 

 plants and animals, and drawn once again through the living 

 vortex of matter and energy. 



The food of Hydra thus is only a link in the endless chain 

 of matter- and energy-transformations shown in the diagram 

 (p. 121). Above and throughout the entire marvelous chain of 

 activities is the silent penetrating agency of the sun, the source 

 of all energy. 



