THE BACTERIAL DISEASES OF PLANTS. 



299 



plant. It then travels rapidly through the whole plant causing the 

 leaves to wilt, turn yellow, dry up and become thin and parchment like. 

 The veins in the leaves and stem are particularly affected and turn 

 black, this being the characteristic feature of the .disease and the 

 source of the name black rot (Fig. 53). Sometimes the veins alone 

 are affected. Sometimes the trouble does not appear in the grow- 

 ing plant, but only in the cabbage after storing, extending through 

 them rapidly and ruining them. 



When these black veins are 

 studied with the microscope they 

 are found to be filled with bacteria 

 and it is easy by proper methods to 

 remove them and cultivate them in 

 the laboratory. Pure cultures of an 

 organism are thus obtained, Pseud, 

 campestris (Fig. 53). It is easy to 

 keep this growing in the laboratory 

 for months under strict observation. 

 Having thus obtained a pure culture 

 it can be demonstrated at any time 

 that it will produce the disease. It 

 is only necessary to dip the tip of 

 a needle into the pure culture and 

 then prick the leaf of a healthy plant 

 with it. This inoculation is followed 



in a few days by the appearance of the characteristic symptoms of 

 the disease, starting at the point of the needle and travelling down 

 the plant in the usual way. By proper study it is possible to show 

 that the bacteria multiply in the plant following, the vascular bun- 

 dles which they first turn black and then destroy. Since these 

 bundles convey the water to the plant their destruction shuts off the 

 usual water-supply, and the plant wilts. It is possible at any time to 

 isolate the bacterium from these diseased plants and obtain it again 

 in pure culture. Cabbage plants pricked with sterilized needles 

 show no evil result, proving that it is the inoculated bacteria that 

 produce the disease. Such experiments as these, repeated many 



a 



FIG. 53. The black rot of cabbage, 

 a, a bit of the stem showing^the 

 blackened fibrovascular bundles; &, 

 cells, highly magnified, showing some 

 filled with bacteria; c, the bacteria. 



