BACTERIA 343 
forms and as a result of their activity various substances are 
produced, the accumulation of which check their activity. 
Some forms, called anaérobic, get along better without air, while 
others, called aérobic, must have air. 
Their reproduction is accomplished by cell division, which is 
not so complex and takes place more rapidly than in the cells of 
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Fic. 296. — Bacillus subtilis, a Bacterium of decay. Above, the active 
form (xX 1500); at the left, below, spore stage (x 800); at the right, below, 
the zodgloea stage (x 500). 
the higher plants. Cell division is so rapid that the progeny of 
one individual often runs into many millions in twenty-four 
hours. The new individuals may separate immediately after 
division is complete or cling together in filaments. Sometimes 
in shrinking the protoplasm and enclosing it in an inner heavy 
wall in preparation for the resting stage, the protoplasm divides 
and each separate mass of protoplasm forms a spore. Since each 
spore is an individual in a dormant protected state, the formation 
of more than one spore results in the multiplication of individuals. 
