BACTERIA 341 
of the organism, develop abnormally, producing scabby formations 
which constitute the scabby areas on the tuber or root. The 
plasmodia are finally transformed into spores which are liberated 
as powdery masses as the infected tissues die and the spore masses 
break open. It has been 
found that the spores can 
live in the ground fora num- 
ber of years and may also 
live adhering to the rind of 
the Potato. Treating seed 
Potatoes with weak solu- 
tions of formaldehyde or 
corrosive sublimate to kill 
the spores adhering to the 
tubers, and rotating crops, 
so that the Potatoes are 
not planted in infected soil 
are means of controlling the Fic. 294.—An Irish Potato attacked by 
disease. a Myxomycete, Spongospora subterranea. 
Bacteria 
Bacteria, of which there are 1400 or more species, are the 
smallest of plants, and their study requires microscopes of a very 
high power of magnification. Some spherical forms, visible only 
through the best microscopes, are less than 0.0005 of a millimeter 
in diameter, and some Bacteria are known to exist that are 
ultramicroscopic, that is, too small to be seen with the best 
microscopes. They are present almost everywhere, occurring 
in the soil, in water, in the air, and in all organic bodies living or 
dead. Although so insignificant in size, they are of great im- 
portance, because the service of some forms is indispensable to 
our welfare, while the forms which cause diseases are destructive 
to both plants and animals. The disease-producing forms are 
commonly known as germs or microbes. So numerous and 
important are these simple plants that their study now forms a 
special subject called Bacteriology. 
The plant body of the Bacteria consists of a single cell. Bac- 
teria are of three general forms: coccus forms, which are globular; 
bacillus forms, in which the shape is rod-like; and spirillum 
forms, in which the plant body is a curved rod (Fig. 295). 
