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 for a 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 



Fig. 294. An Irish Potato attacked by 

 a Myxomycete, Spongospora subterranea. 

 The scabby areas are pustules containing 

 powdery masses of spores. Half natural 

 size. 



tubers, and rotating crops, 

 so that the Potatoes are 

 not planted in infected soil 

 are means of controlling the 

 disease. 



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). 



