LIFE PROCESSES 
311 
cases they remain connected, forming a chain. Under 
favorable conditions each cell can grow to full size in half 
an hour and be ready to divide again. It is this ability 
to multiply rapidly which makes them of so great impor¬ 
tance, for a few hundred bacteria, even of the harmful 
ones, could produce little effect. 
In the process of growth, bacteria produce two sub¬ 
stances, enzyme (see page 14) and toxin (tox'in: Greek, 
toxicon, poison). Enzymes produce fermentation, a break- 
ing-up process of which man makes use to secure certain 
flavors and odors, as well as to soften hard materials. 
Toxins are usually poisonous to living organisms, including 
the bacteria which produce them. 
Enzymes cause the pleasant flavor of such articles of 
food as cheese and butter. The quality of tobacco depends 
largely upon the kind of bacteria which have been at work 
upon it. Such bacteria are classed as helpful, as are those 
which gather nitrogen for the plants of the bean family. 
Other helpful bacteria are those which make it possible 
for man to use sponges by ridding them of the soft, slimy 
substance with which they are filled when alive, as well 
as the bacteria which soften the useless parts of the flax 
plant so that the rest of it may be separated and made into 
linen. 
When food, air, warmth, or moisture is not sufficient, 
bacteria cease to grow and go into a resting state. That 
is, they change their form, and surround themselves with 
a substance which protects the soft protoplasm from being 
harmed by freezing, heating, or drying. The simple 
plants all do this, but the simpler the plant, the more 
easily does it resist. It is this ability to withstand un¬ 
favorable conditions and to resume growth when condi¬ 
tions change for the better that makes bacteria “ such good 
friends and such bad foes.” Our ability to control them is 
due largely to a knowledge of their habits. 
