THE BACTERIAL DISEASES OF PLANTS. 
2 99 
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 
FIG. 53. The'black rot of cabbage, 
a, a bit of the stem showing^the 
blackened fibro vascular bundles; &, 
cells, highly magnified, showing some 
filled with bacteria; c, the bacteria. 
