BACTERIA 15 



ditions remained favorable, to an almost inconceivable 

 number. 



It is not surprising, therefore, that bacteria are as numerous 

 as they are and that they are found practically everywhere ; 

 nor that, although each one is so extremely small, they are 

 able to accomplish tremendous results. The " scum " that 

 rises to the surface of the hay infusion contains great num- 

 bers of separate cells and rows of cells in a quiescent condi- 

 tion held together by a sticky, slimy substance which is 

 secreted by the bacteria. Although the cells in this scum are 

 not moving (because they have lost their vibrating threads), 

 they may still be actively dividing (Fig. 6, B). 



24. Spore Formation. If the liquid in which the bac- 

 teria live is allowed to dry up slowly, or if we keep it from dry- 

 ing up until the supply of food for the bacteria is exhausted, 

 a change takes place in the appearance of many of the cells. 

 The protoplasm of each cell shrinks away from the wall and 

 rounds up into a body much smaller than the original cell 

 (Fig. 6, Q, which lies at the center or toward one side or 

 one end of the old cell cavity and is closely surrounded by a 

 thick new wall. This shrinkage is at least partly due to the 

 loss of water, and the spore so formed is much less easily af- 

 fected by unfavorable conditions, such as dryness, heat, and 

 cold, than is the ordinary cell. Many kinds of bacteria 

 (though by no means all) are able to pass into the form of 

 spores, in which condition they may remain unaffected by 

 surrounding conditions for a long time. In this form they 

 may be blown about in the air, and when they land in a place 

 where conditions are favorable they can return to their origi- 

 nal form and then grow and multiply as before. This abil- 

 ity to change from one form to another is of great advantage 

 to bacteria, since it enables them to adapt themselves to 

 great changes in their surroundings. 



25. Distribution and Forms of Bacteria. More than two thousand 

 kinds or species of bacteria have been recognized, and new species are 



