14 MICROBIOLOGY 



2. The consideration of the bacteria in milk and their 

 relations to dairy products such as pure milk, butter and 

 cheese forms a very distinct line of investigation, already 

 possessed of an extensive literature, known as dairy bac- 

 teriology. 



3. The study of the bacteria that are useful in the 

 arts, those found to be advantageous in the manufacture of, 

 or more or less destructive to, certain food preparations, 

 such for example as canned vegetables, has somewhat closely 

 circumscribed what is known as industrial bacteriology. 



4. The inquiries into the causes of plant diseases have 

 shown that many of the ills .of our flowers, fruits and vege- 

 tables are due to the invasion by certain bacteria. We have 

 thus a distinct line of bacteriology in the study of plant 

 pathology. 



5. A certain number of bacteria that are more or less 

 parasitic for animals and man and which when multiplying 

 within or upon the tissues of the body produce serious physi- 

 ological disturbances or morbid changes in the tissues con- 

 stitute another very important part of bacteriology. As it 

 was the discovery of the bacteria capable of producing dis- 

 ease in man and animals that first brought bacteriology 

 into general recognition, the term bacteriology usually sug- 



physiologists had come to the conclusion that in nature the nitrates 

 of the soil were the one and only form in which nitrogen was taken 

 tip by plants. For although it was known that experimental plants 

 could be induced to take up ammonia salts and even gaseous am- 

 monia, it was evident that ammonia was not a common source of 

 nitrogen in nature. And, finally, the atmosphere, the greatest store- 

 house of nitrogen, seemed closed to plants. These views, however, 

 were overturned by detailed investigations into the nutrition of the 

 Leguminosae. That the Leguminosae could grow in soil poor in 

 nitrogen and thrive thereon, even without nitrogenous manuring, 

 had long been known, and it has now been demonstrated that they 

 take up nitrogen from the air and convey it to the soil. This 

 enrichment is particularly evident when the plants are ploughed 

 in. All other plants, all our cereals and food crops, are, as regards 

 the soil, merely consumers of nitrogen, since they are unable to take 

 it up in any form but that of the nitrates of the soil." 



