298 THE PARASITIC DISEASES OF PLANTS. 



The above list contains only a few of the very large number of 

 known fungoid diseases of plants, but will serve the purpose of 

 showing their variety. The fungi that produce these diseases are 

 by no means closely related to each other. The higher fungi are 

 divided into many classes and the disease-producing parasites are 

 distributed among them all. For these distinctions, however, the 

 student must be referred to books upon botany. 



THE BACTERIAL DISEASES OF PLANTS. 



Only within recent years has it been appreciated that bacteria 

 are important agents in producing plant diseases. Even after 

 their agency in causing diseases in animals had been fully recog- 

 nized it was denied that they could produce troubles of this sort 

 in plants. Up to very recent date, it was claimed that it was an 

 impossibility for bacteria to penetrate plant tissue so as to produce 

 trouble. Plant cells are provided with hard cell walls of cellulose 

 and wood, which protect the living protoplasm within; and, since 

 these cells form the bulk of the plant and are adherent to each 

 other, it was difficult to see how bacteria could penetrate into the 

 plant at all. The mycelium of the higher fungi can do this readily 

 since it can thrust itself between the cells, and thus grow easily 

 within the solid tissues; but it seemed impossible to believe that 

 bacteria could penetrate the hard tissues. Within recent years, 

 however, it has been demonstrated that this is possible and the 

 last ten years have disclosed many bacterial diseases of plants, 

 until to-day we know of more bacterial diseases of plants than of 

 animals. 



The Black Rot of Cabbage (Pseudomonas campestris). An 

 illustration will best show the general course of such a disease and at 

 the same time indicate how conclusive is the proof of the agency of 

 bacteria. For this purpose will be chosen the black rot of the 

 cabbage, cauliflower, turnip, and several other members of the family 

 Cruciferce. The disease appears first, as a rule, upon the edges of 

 the leaves, as brown spots, that spread down the leaves following the 

 veins to the midrib and petiole and finally into the main stem of the 



