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, 68 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 
