PLANT DISEASES 249 



simple in structure, consisting of a single cell only. They are 

 almost universally present. If one should wipe one's finger 

 across the desk there will be literally thousands of bacteria 

 on the finger tip. A glass of ordinary milk, even if the purest 

 certified, will probably contain more than a hundred thousand 

 bacteria. If the milk has not been handled with great care, 

 this number may be counted in the hundreds of millions. 

 The earth swarms with bacteria, many of which are benefi- 

 cial to plant life. But, on the other hand, many are injurious 

 and the total injury by them yearly is enormous. 



Reproduction takes place with such rapidity that a single 

 bacterium may in the course of a day or two increase under 

 favorable conditions to many millions of individuals. If 

 climatic or other factors are unfavorable, they have the power 

 to form hard crusts about themselves and in this state may 

 remain quiescent for months or even years, ready to grow and 

 multiply whenever the environment is favorable. 



Bacteria multiply by a process of simple division. One 

 bacterium divides into two, these into four, then eight, six- 

 teen and so on. If such division takes place every hour (as 

 is sometimes the case, when food and temperature factors 

 are favorable) it is a matter of simple arithmetical com- 

 putation to ascertain the number to which one bacterium 

 would increase in, say, twenty-four hours. 1 



Some bacteria are motile, that is, they have propelling 

 organs by means of which they are able to move through a 

 limited space. Others have no such structures. All bacteria 

 thrive best under conditions of warmth and moisture, 

 coupled with abundance of food material. 



361. Pear-blight (Fig. 127) is one of the diseases known to 

 be directly due to bacteria. They gain access usually either 

 through the flowers or through the tender growing parts of 

 the plant, and then spread, attacking the cambium layer 

 just beneath the bark. This cambium is the actively growing 



1 It is suggested that the student make the computation. 



