300 THE PARASITIC DISEASES OF PLANTS. 



times by different experimenters, leave no room to douftt 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, campestris secretes such an 

 enzyme and one that has the power of softening and dissolving cellu- 

 lose. As the bacteria multiply at the inoculated 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, feed- 

 ing upon the food thus furnished, multiplies further. More cytase 

 is produced, dissolving 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, there are on the edges of 

 the leaves minute openings called water pores. Through these pores 

 the bacteria also can enter. Seemingly they can also enter through 

 the roots, especially through the tips of the rootlets which are likely 

 * to be exposed and broken during transplanting. 



No effectual remedy against this disease has yet been found. 

 Its method of distribution from field to field is not well known, and 

 hitherto no means of checking it after it has made its appearance in 

 a field has been discovered. Wet weather, which is best for the 

 growth of the cabbages is, unfortunately, also best for the growth of 

 this parasite. That it lives in the soil from year to year seems 

 proved, and hence after it makes its appearance in a field it is likely 

 to recur year after year. Since it is confined to members of the 



