CHAPTER II 



DISEASES CAUSED BY BACTERIA 



Bacteria are minute organisms, typically unicellular, but in 

 some species and under some conditions adhering in filaments 

 or massed in a colony in a gelatinous matrix. Some are so small 

 as to be close to the limit of visibility with the highest powers of 

 the microscope, and others probably exist which are beyond it. 

 The cells are enclosed by a thin membrane. Bacteria possess no 

 chlorophyll, and for the most part are either saprophytic or 

 parasitic, though there are some which are able to derive their 

 energy from inorganic compounds. In their mode of life they 

 consequently resemble fungi, but differ in their method of re- 

 production, which is by equal subdivision of the parent cell. 

 They are accordingly classed as Schizomycetes (fission fungi), 

 though there is no sound reason for regarding them as especially 

 related to fungi at all. Their possession of a cell wall is usually 

 held to justify their classification as plants. They are frequently 

 motile by means of protoplasmic threads (flagella). Some 

 species produce thick-walled resistant spores, but this is seldom 

 the case among the known parasites of plants. 



The earliest definite discoveries of bacterial disease of plants 

 were made in the years round about 1880 by several independent 

 investigators. The destructive pear-blight of the United 

 States, worked out by T. J. Burrill, was the first to be clearly 

 established as bacterial in origin. For many years the attitude 

 of academic botanists, based on preconceived opinions as to the 

 ability of bacteria to enter or to lead a parasitic existence in 

 plant tissues, was hostile or sceptical, and, although overt opposi- 

 tion to the idea was silenced iDy the proofs brought forward in 

 1901 by Erwin F. Smith, it is only within the last few years that 

 the reluctance to accept it may be said to have died down in 

 Europe. 



To the working pathologist it is now clearly evident that 

 bacterial diseases, though fewer in number and somewhat 

 restricted in range, stand in respect of specific parasitism and 

 capacity for injury on an equal footing with diseases of fungus 

 origin. There is indeed so close an analogy between the two 

 groups of causative organisms in the various degrees of parasitism, 

 in the nature of the attack and of infestation, and in the re- 

 actions of the host plant towards them, that it is unnecessary 

 to repeat at any length or much to qualify in respect of bacterial 

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