254 



Science of Plant Life 



U. S. Dept. of Agriculture 



FIG. 149. A field of upland cotton, in South Carolina, attacked by the wilt disease. 

 The fungus that causes the wilt remains in the soil for many years, 



pneumonia, grip, diphtheria, typhoid fever, colds, lockjaw, 

 and blood poisoning. With the exception of lockjaw and 

 other infections through wounds, these diseases are largely 

 communicated by one person to another. Typhoid is carried 

 also by means of water and milk supplies which have been 

 contaminated, and by flies that have visited infected matter. 



A fundamental fact that should be learned in this connec- 

 tion is that no one can contract a bacterial disease unless he 

 comes in contact with the particular bacterium which causes 

 that disease. 



The natural means of defense against disease are some- 

 what similar in the higher plants and in animals. The plant, 

 in addition to protective chemical substances within its cells, 

 has an epidermis which renders the entrance of bacteria dif- 

 ficult. Bacteria are able to enter, however, if the epidermis 

 is bruised or broken. Plants probably suffer from bacterial 

 diseases as much as do animals. Most of the well-known 



