Bacteria as agents in nature’s food cycle.

But the matter by no means ends here.  When we come to think of it, it is a matter of considerable surprise that the surface of the earth has been able to continue producing animals and plants for the many millions of years during which life has been in existence.  Plants and animals both require food, animals depending wholly upon plants therefor.  Plants, however, equally with animals, require food, and although they obtain a considerable portion of their food from the air, yet no inconsiderable part of it is obtained from the soil.  The question is forced upon us, therefore, as to why the soil has not long since become exhausted of food.  How could the soil continue to support plants year after year for millions of years, and yet remain as fertile as ever?

The explanation of this phenomenon is in the simple fact that the processes of Nature are such that the same food is used over and over again, first by the plant, then by the animal, and then again by the plant, and there is no necessity for any end of the process so long as the sun furnishes energy to keep the circulation continuous.  One phase of this transference of food from animal to plant and from plant to animal is familiar to nearly every one.  It is a well-known fact that animals in their respiration consume oxygen, but exhale it again in combination with carbon as carbonic dioxide.  On the other hand, 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 oxygen 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 discoveries 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 understand it, an explanation of the various steps in this cycle is necessary.