258 Introduction to Botany. 



duced by division assuming a spore character, or by the 

 cell contents rounding off, expelling water, and entering 

 into a condition in which great extremes of heat, cold, and 

 desiccation can be withstood (see Fig. 133, c). While in 

 the vegetative state, most bacteria are killed at a tempera- 

 ture of 65 C, their spores can stand 100, or in some 

 cases 130 C., of dry heat; but no spores can withstand an 

 air-free steam heat of 120 C. for half an hour. This high 

 temperature is achieved by generating the steam under 

 ij atmospheres' pressure in an apparatus known as an 

 autoclave. 



The spores are very resistant to cold ; freezing does 

 not affect them, and they have been known to survive 

 even after exposure for a short time to the temperature 

 of freezing oxygen, namely, 213 C. below zero. By a 

 process known as discontinuous heating they may be 

 killed at 100 C. By this method the heating is carried 

 on for half an hour on each of three consecutive days. 

 The explanation of the success of this process is that most 

 of the spores which resist the first heating will probably 

 have germinated, and so have passed into a less resistant 

 state, by the time of the second heating. The third heat- 

 ing is certain to destroy the remaining bacteria, since all of 

 the spores will have germinated by that time. 



154. Forms of Bacteria. In form, bacteria are round, 

 oblong, rod-shaped, or spiral (Figs. 133-134). But while 

 exceedingly simple in the construction and contour of their 

 bodies, the results of their activities are quite diverse, and 

 of vast importance to other plants and animals. 



155. Nutrition of Bacteria. Although bacteria may pro- 

 duce pigments of various colors, they do not form chloro- 

 phyll and cannot obtain their living by employing the 

 energy of 'the sunlight to build their food ; they must, 



