300 THE PARASITIC DISEASES OF PLANTS. 
times by different experimenters, leave no room to doubt that tl 
bacteria are the cause of the disease in question. 
The method by which the bacteria make their way into the plai 
is interesting. We have learned in an earlier chapter that bacter 
frequently secrete enzymes. This Pseud, campestris secretes such 
enzyme and one that has the power of softening and dissolving cell 
lose. As the bacteria multiply at the inoculated point they secre 
this enzyme, called cytase, which at once softens and disintegrates tl 
walls of the adjacent plant cells. The contents of the cells thi 
exposed are quickly killed by the action of the bacteria, a toxin beii 
probably secreted by them for the purpose, and the bacterium, fee< 
ing upon the food thus furnished, multiplies further. More cyta 
is produced, dissolving more cell walls, and the disease progress 
as the bacteria thus enter the plants. In this way they trav 
through the plant, chiefly in the vascular bundles, and finally m; 
affect the plant throughout. The cellulose-dissolving enzyme h; 
been found to be secreted not only when the bacterium is growir 
in the host, but also in the laboratory in the bacteriologist 
test-tubes. 
The bacteria have apparently three methods of entering the plan 
Through the uninjured cuticle they are unable to enter, nor can thi 
enter through the stomata of the plant. But if the cuticle be brok< 
by a wound or scratch, no matter how tiny, the broken cuticle w 
offer an entrance to the germs. Further, there are on the edges 
the leaves minute openings called water pores. Through these por 
the bacteria also can enter. Seemingly they can also enter throu^ 
the roots, especially through the tips of the rootlets which are like 
to be exposed and broken during transplanting. 
No effectual remedy against this disease has yet been foun 
Its method of distribution from field to field is not well known, ar 
hitherto no means of checking it after it has made its appearance 
a field has been discovered. Wet weather, which is best for tl 
growth of the cabbages is, unfortunately, also best for the growth i 
this parasite. That it lives in the soil from year to year seen 
proved, and hence after it makes its appearance in a field it is like 
to recur year after year. Since it is confined to members of tl 
