CHAPTER I. 



Definition of Bacteria — Differences Between Parasites and Saprophytes 

 — Their Place in Nature — Bacterial Enzymes — Products of Bacteria 

 — Nutrition of Bacteria — Their Relation to Oxygen — Influence of 

 Temperature Upon Their Growth — Chemotaxis. 



Bacteria (more properly bacteriacese or schizomycetes) 

 were regarded by the older writers as infusoria. This was 

 because of their capacity for developing in infusions, their 

 property of spore-formation, their resistance to drying, 

 their power of independent motion, and the absence of 

 chlofophyl from their tissues. In the modern conception, 

 however, this classification is untenable, and bacteria, by 

 virtue of their distinguishing peculiarities, are now treated 

 as a group by themselves that may briefly be defined as 

 comprising microscopic, unicellular, vegetable organisms 

 that multiply by the process of transverse division. 



Inasmuch as bacteria are not possessed of chlorophyl, 

 their ■ metabolic processes are fundamentally different from 

 those of the higher plants in which it is present. They 

 cannot, as in the case of the green plants, obtain carbon 

 and nitrogen from such simple bodies as carbon dioxide and 

 ammonia, but are forced to secure these essential elements 

 from organic matter as such. This power to decompose and 

 assimilate organic matters is signally different in different 

 species of bacteria, and, singular to say, there is a small 

 group (to be described later) from which this function is 

 apparently absent, in spite of the fact that no compensatory 

 chlorophyl is discernible in their tissues. 



(31) 



