4 Diseases of Truck Crops 



able medium for the plants. On the contrary, we 

 call it sick or poor when it teems with bacteria and 

 fungi which act as parasites on plants, or when the 

 beneficial ones are absent or perform their duties 

 imperfectly. 



STRUCTURE AND LIFE HISTORY OF BACTERIA 



The term bacteria (singular bacterium), or microbe, 

 or germ, refers to the smallest microscopical form of 

 plant life. As we shall see later, bacteria are but one 

 of the many forms of life in the soil. The first man 

 to recognize bacteria was Anton van Leeuwenhoek, 

 a native of Holland, and a lens maker by trade. He 

 made use of the microscope in testing materials for 

 lens making. In 1675 he happened to mount in a 

 drop of water some tartar which he scraped off from 

 his teeth. To his great surprise he discovered mi- 

 nute little "animals" which moved about in curious 

 fashions. In 1882, Robert Koch succeeded in grow- 

 ing bacteria artificially and outside their natural 

 environment. Thus was laid the foundation of the 

 modern science of Bacteriology. 



Bacteria are very simple in form. We recognize 

 the rod-shaped known as Bacillus (fig. i a), the 

 spherical form as Coccus (fig. i b), and the corkscrew 

 or comma form as Spirillum (fig. I c). Bacteria are 

 very minute. It would take about fifteen to twenty 

 thousand individual bacteria placed end to end to 

 make one inch in length. They occur, however, in 

 tremendous numbers and this enables them to per- 



