THE FLOWER GARDEN 317 



Various bacterial and fungous diseases also 

 attack plants that are grown in one place year 

 after year. These organisms, although they may 

 not be entirely destructive the first year or two, 

 gradually multiply and become a greater pest 

 from year to year. 



When trouble arises from this source the 

 remedy is to rotate the crops or, in other words, 

 move the crop infested to another location. 



Fungous diseases are especially destructive in 

 potato fields. The potato scale, blight, and wart 

 are well-known diseases which can often be 

 wholly or partially controlled by the proper 

 rotation and the planting of uninfected seed. 



The third cause for failures is the unfavorable 

 condition of the soil produced by the toxic sub- 

 stances thrown off from the growing plants. 

 Plants, like animals, give off waste matter which 

 is not only useless but poisonous to the plant 

 itself, and often to other plants of similar 

 nature. 



These toxic substances are often less poison- 

 ous, and in some cases are beneficial, to other 

 crops. It is obvious that when waste products 

 from a certain crop have accumulated in the soil 

 for a number of years, that soil is not as well 

 suited to the crop as formerly. A change of 

 crops practically always results in a more profit- 



