THE BACTERIAL DISEASES OF PLANTS 323 
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 bundles 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 
times by different experiments, leave no room to doubt that the 
bacteria are the cause of the disease in question. 
The method by which the bacteria make their way into the 
plant is interesting. We have learned in an earlier chapter that 
bacteria frequently secrete enzymes. This Pseud. cambpesiris 
secretes such an enzyme and one that has the power of softening 
and dissolving cellulose. As the bacteria multiply at the in- 
oculated point they secrete this enzyme, called cytase, which at 
once softens and disintegrates the walls of the adjacent plant cells. 
The contents of the cells thus exposed are quickly killed by the 
action of the bacteria, a toxin being probably secreted by them 
for the purpose, and the bacterium, feeding upon the food thus 
furnished, multiplies further. More cytase is produced, dissolv- 
ing more cell walls, and the disease progresses as the bacteria thus 
enter the plants. In this way they travel through the plant, 
chiefly in the vascular bundles, and finally may affect the plant 
throughout. The cellulose-dissolving enzyme has been found to 
be secreted not only when the bacterium is growing in the host, 
but also in the laboratory in the bacteriologist’s test-tubes. 
The bacteria have apparently three methods of entering the 
plant. Through the uninjured cuticle they are unable to enter, 
nor can they enter through the stomata of the plant. But if the 
cuticle be broken by a wound or scratch, no matter how tiny, 
the broken cuticle will offer an entrance to the germs. Further, 
