310 PRACTICAL COURSE IN BOTANY 



for bacterial growth, others claiming the reverse. The 

 consensus of opinion at present is toward the view that the 

 spores are a provision for tiding over periods of stress and 

 difficulty. They are capable of retaining their vitality 

 for a long time, and are much harder to kill than the bac- 

 terial cells in their ordinary vegetative state, as was seen 

 in the case of the hay bacillus. The spores of one species 

 of potato bacillus have retained their vitality after four 

 hours of boiling, and those of the typhoid bacillus after 

 continuous exposure to a freezing temperature for more 

 than three months. The majority of bacteria, in their 

 vegetative state, are, however, either killed or rendered 

 inert by temperatures ranging below 10° or above 50° cen- 

 tigrade — equivalent to about 50° and 122° Fahrenheit, 

 respectively. It is easy to see what important bearing 

 these facts have on the process of disinfection. 



351. Reproduction and multiplication. — The ordinary 

 mode of reproduction in bacteria, as in other unicellular 

 organisms, is by fission (337, 338). As each individual 

 forms but a single spore, no increase in numbers could take 

 place by this means alone. Hence, while the spores are 

 an important factor in the preservation of the species by 

 continuing its existence under conditions which the active 

 organisms could not survive, their successful propagation 

 depends on their power of rapid multiplication by division. 

 If this process were to go on unchecked, every hour, in an 

 unbroken geometrical progression, the progeny of a single 

 bacterium would, in 24 hours, number nearly 17 million; 

 in 25 hours, 34 million ; in 26 hours, G8 million, and in five 

 days they would cover the entire surface of the globe, land 

 and sea, to a depth of 3 feet ! In ordinary standard milk 

 sold by dairymen, and containing, when examined, less 

 than 10,000 microbes to the cubic centimeter, — about 

 20 drops, — the number was found to have increased after 

 24 hours to 600 million. It is comforting to know, how- 

 ever, that the majority of these are of the harmless kinds 



