344 THALLOPHYTES 
But a Bacterium commonly forms only one spore, and the 
formation of spores, therefore, does not generally result in the 
multiplication of individuals. The spore stage is apparently for 
protection rather than for multiplication. In the spore stage 
Bacteria can live where there is no food and when heat and 
Fic. 297. — A petri dish containing agar upon which are colonies of the 
Bacterium (Actinomyces chromogenus) which attacks Irish Potatoes, causing 
scabby areas. The white spots are the colonies. From Bulletin 184, Ver- 
mont Agr. Exp. Sta. 
cold are much too extreme for ordinary life. In this condition 
some Bacteria can endure boiling water for an hour or longer as 
well as extremely low temperatures. It is the spores of Bacteria 
that are hard to kill in sterilizing media and other substances. 
In the spore stage Bacteria can retain their vitality for months 
or years and, floating about with the dust in the air, reach all 
kinds of situations. 
Some Bacteria of Economic Importance 
Bacteria of Decay.— The Bacteria of decay live on dead 
organisms. By their activity dead organisms, such as straw, 
weeds, corn stalks, logs, and carcasses of animals, are decomposed 
into simpler and soluble compounds, in which form they return 
to the earth and become available nutrients for plants. Their 
activity helps to rid the earth’s surface of débris and to pre- 
