CHAPTER II 



MICROORGANISMS AND THEIR RELATION TO HUMAN 

 WELFARE 



I. Structure and Functions of Bacteria 



9. Bacteria: 1 their microscopical appearance and size. — 



In the preceding chapter we considered to some extent the 

 organs, tissues, and cells of the human body. However, 

 before we discuss further the structure and functions of 

 these various parts of our bodies, we shall study in some 

 detail certain microscopic plants which have a most intimate 

 relation to human welfare. Chief among these are the tiny 

 organisms known as bacteria. 



Every one is familiar with the fact that if a bouquet 

 of flowers is left for some time in a vase of water, the stems 

 decay and disagreeable odors are given off. This is a com- 

 mon example of the action of bacteria, for all decay is due 

 to the work of these organisms. When we come to examine 

 the flower stems or the putrid water, we find a slimy scum. 

 If we put a drop of this scum on a slide, cover with a cover 

 glass, and examine with the highest powers of the microscope, 

 we usually see many different forms of living things. Some 

 of them appear relatively large, and these, as we have already 

 seen (A. B., Chapter VI), are single-celled animals. A closer 

 examination will disclose countless numbers of very minute, 



' The substance of this section, and several of those that follow, 

 appear in Part I, "Plant Biology." Many teachers, however, find 

 it impracticable to discuss bacteria until the work in human biol- 

 ogy is taken up ; hence the repetition of this material in this volume. 



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