992 The American Naturalist. (December, 
continued long enough, the fertilizer is in a condition fit for 
for the fields. Now, when put upon the fields, the plants can 
get hold of the material. You will see now what I meant 
when I stated at the beginning of my lecture that in spite of | 
all the cultivating that you and your horses might do in the 
fields, it would be useless without the agency of these organ- 
isms. You might put on your fertilizer; but, if that fertilizer 
be not acted upon by bacteria, it will be of no use, and thus 
the bacteria come in to complete the operation which you 
began. You do your duty and the bacteria do theirs, and the 
consequence is, the fertilizers which you are using are brought 
into a condition in which the plants can get hold of them, 
and thus the food of plants is produced. You see, then, that 
in this way plants and animals are able to use over and over 
again the same material. The plant gets this material out of | 
the soil and out of the air; the animal comes along then and 
feeds upon the plant; then the animal dies, and the plant 
dies, and the bacteria get into the body of the animal or 
plant, pull it to pieces and produce from it decomposition 
products, and they get into the soil in the form of nitrates and 
nitric acid compounds; or they go off into the air in the form 
of ammonia and carbonic acid. The bodies of these animals 
and plants are thus reduced to simple conditions and now the — : 
plants once more get hold of them, and use as food the same 
material that previous generations used. Thus over and over 
again the same material is used, and thus nature is kept per 
petual. This is the explanation of the constant, perpetual 
growth in nature. This is the reason that nature does not 
exhaust itself. This is the reason that animals and plants 
have been enabled to grow upon the surface of the earth for- 
the past hundreds and hundreds of centuries. : , 
But this is not the end of the agency of bacteria in plant : 
life. They are not only of value in ripening your fertilizers — 
and in keeping up this constant growth of nature, but " 
have learned within the last two or three years that at the 
very foundation the growth of plants is absolutely dependent 
upon these organisms, and similarly in the future the contint: 
ance of the vegetable world must be also dependent up? 
