116 THE PESTS OF CULTIVATED PLANTS 



The most important factor connected with the life of 

 bacteria is their exceptionally rapid power of multiplication. 

 The division of the bacteria which results in two plants 

 often takes place in less than half an hour, and in less than 

 half that time each division is again ready to reproduce. 

 The reproduction in this ratio results in an increase in 

 number which is almost inconceivable. 



Bacteria are also divided into two kinds, with regard to 

 their needs for oxygen. Most bacteria demand oxygen to 

 enable them to grow, but there are some species, and these 

 are quite numerous, that can live without a supply of oxygen, 

 and in fact can only grow and multiply when in an atmosphere 

 devoid of oxygen. 



Bacteria bear the same relation to temperature as do other 

 ordinary living organisms. If the temperature is high the 

 multiplication of the bacteria is more rapid, and as the 

 temperature approaches the freezing-point, the activity of 

 the bacteria ceases. The temperature at which the maximum 

 growth of bacteria occurs is quite variable and varies from 

 70° F. to about 95° F., while other forms will grow well at 

 still higher temperatures. 



Environment and Soil Sanitation. — The environment in 

 which a plant is grown regulates to a large degree the preva- 

 lence of certain diseases. The " damping off " of seedlings is 

 commonly due to poor environment. While it is known there 

 is a definite cause for the trouble in the form of a fungus, 

 it is conceded that if the proper growing conditions are 

 maintained, the plants will not be attacked by this fungus. 

 It has been demonstrated conclusively that if good fresh 

 air is given, the proper temperature is maintained, and the 

 correct amount of water is given at the right time, the 

 seedlings will not be attacked by the fungi that cause this 

 trouble. An oversupply of water accompanied by excessive 

 heat affords ideal conditions for the growth of fungus troubles 

 which often attack the plant and cause serious injury. The 

 damping off of seedlings, while due primarily to a parasitic 

 growth at the crown of the plant, is brought about by unfa- 

 vorable conditions of growth of the parent plant and can 

 only be controlled by regulating the conditions under which 



