32 



STUDIES IN PHYSIOLOGY 



process is followed by a period of assimilation in which 

 both daughter cells increase in size. 



Before we leave this discussion of living substance, it will 

 be well to consider the single-celled organisms known as 

 bac-te'ri-a or germs, since they have most important rela- 

 tions to the health of our bodies. Yeast, too, will be dis- 

 cussed, because of its interesting physiology and because of 

 its importance in connection with bread- and liquor-making. 



4. A STUDY OF BACTERIA 1 



Changes caused by Bacteria. If milk is allowed to stand 

 in a warm room, it becomes sour to the taste, and it thickens 



or curdles. Meat that 

 has been kept for a 

 considerable time de- 

 cays, giving off disagree- 

 able odors. We know, 

 too, that water in which 

 flower stems have been 

 kept, at length becomes 

 putrid. All these, and 

 countless other changes, 

 are caused by bacteria, 

 those microscopic organ- 

 isms that were hardly 

 dreamed of forty years 

 ago. 



Microscopic Appear- 

 ance of Bacteria. The 

 thin scum formed on 





FIG. 



which 



11. Rod-shaped Bacteria 

 cause Lock-jaw. 



Magnified about 800 times. Photographed 

 through the microscope. Some of the 

 bacteria have an oval light colored 

 spore near one end. 



scum 



the top of all stagnant 

 water consists of millions of bacteria. When we examine 

 with the highest powers of the microscope a bit of this 

 scum, these micro-organisms are seen to have several differ- 



1 See "Laboratory Exercises," No. 49. 



