GENERAL CHARACTERS OF BACTERIA. 15 
tions, but ever ready to exert their influence when the conditions are 
favorable. Since they are feeding during their growth, they must 
produce profound changes in the material upon which they feed. 
It is this reserve force, possessed in greater or less degree by all 
bacteria, which makes them such wonderful and powerful agents 
in producing the great changes in nature which we are now forced to 
attribute to them. 
Production of Spores. There is another method of produc- 
ing new individuals, an understanding of which is necessary to a 
knowledge of bacteria. This is the production of spores, and is 
illustrated in Fig. n, a-e. The bacterium there figured consists of 
a rod. The contents of one of the rods 
collects itself in a spherical or oval body in 
the center. This later breaks out of the 
rod, the rest of the individual then dying 
and disappearing. The oval body itself is 
a spore, and is capable, when placed under 
proper conditions, of developing into a new 
rod, e. Inasmuch as only a single spore 
. FIG. ii. Spore formation. 
arises from a single bacterium, it is not a a to e, stages in spore for- 
multiplication. Its purpose is not so much mation and gemination. 
to increase the number of individuals as to enable the bacteria to 
endure adverse conditions without being killed. The ordinary bac- 
teria are likely to be killed by being dried, and will readily succumb 
to moderate heat, a temperature of 165 F.* being sufficient to kill 
almost any of them. But these spores are covered with a hard case 
which enables them to resist the conditions which the active, growing, 
and multiplying forms cannot resist. They may be completely dried 
for months, and even years, and still retain their vitality. They may 
be heated very much hotter than the active forms without injury; in- 
deed, some of these spores may be in boiling water for many minutes 
an hour or longer without having their vitality destroyed, since, 
if the spores are subsequently cooled, they are capable of germinating 
and growing into new bacteria. As a result of this it will follow 
that, while it is very easy to kill ordinary bacteria by heat, it is 
* Temperatures used in this book always refer to the Fahrenheit scale. 
