98 THE STORY OF GERM LIFE. 



plants in their life consume the carbonic dioxide 

 and exhale the oxygen again as free oxygen. 

 Thus each of these kingdoms makes use of the 

 excreted product of the other, and this process 

 can go on indefinitely, the animals furnishing our 

 atmosphere with plenty of carbonic acid for plant 

 life, and the plants excreting into the atmosphere 

 at the same time an abundant sufficiency of oxy- 

 gen for animal life. The oxygen thus passes in 

 an endless round from animal to plant and from 

 plant to animal. 



A similar cycle is true of all the other foods 

 of animal and plant life, though in regard to the 

 others the operation is more complex and more 

 members are required to complete the chain. 

 The transference of matter through a series of 

 changes by which it is brought from a condition 

 in which it is proper food for plants back again 

 into a condition when it is once more a proper 

 food for plants, is one of the interesting dis- 

 coveries of modern science, and one in which, as 

 we shall see, bacteria play a most important part. 

 This food cycle is illustrated roughly by the 

 accompanying diagram ; but in order to under- 

 stand it, an explanation of the various steps in 

 this cycle is necessary. 



It will be noticed that at the bottom of the 

 circle represented in Fig. 25, at A, are given 

 various ingredients which are found in the soil 

 and which form plant foods. Plant foods, as 

 may be seen there, are obtained partly from the 

 air as carbonic dioxide and water ; but another 

 portion comes from the soil. Among the soil 

 ingredients the most prominent are nitrates, 

 which are the forms of nitrogen compounds 

 most easily made use of by plants as a source of 



