322 THE PARASITIC DISEASES OF PLANTS 
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 Crucifere. 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 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. 52). Sometimes the veins 
alone are affected. Sometimes 
the trouble does not appear in 
the growing plant, but only in 
the cabbage after storing, extend- 
» ge qineblackrotcfeabbege, ing through them rapidly and 
blackened fibrovascular bundles; 5, ruining them. 
filed ith bacteriay & the bacteria, 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. 52). 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 
