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BIOLOGY OF SPECIALIZED GROUPS 



Method of Infection. In plants, as in animals, bacteria 

 usually cannot enter the body unless it has been injured in 

 some way and an avenue of infection thus provided. The 

 healthy uninjured animal or plant body is usually impreg- 

 nable as far as bacteria are concerned. The resistance of the 



FIG. 55. Bacterium pruni in green plum. (Cavity formation.) After Smith. 



body must be lowered before infection is possible. In the 

 case of plants, the resistance or vitality of the plant is usually 

 lowered and the avenue of infection provided by the sting of 

 an insect, a frost bite, or a bruise. This is particularly notice- 

 able in the case of the fruits of plants which have become 

 infected. A few cases of bacterial disease are on record where 

 no lesion of the plant could be found. 



