110 BACTERIA IN RELATION TO PLANT DISEASES. 



Not all of the plants showed bacterial slime. On sectioning and examining with the 

 hand-lens, about 160 were apparently free from the yellow bacteria, and all of these showed 

 absence of brown stain in the nodes, except in some cases a very slight stain at the extreme 

 base of the stem, which may have been due to incipient or arrested infection. It is not 

 known why a portion of these plants escaped. The following explanations are offered : 



(1) Possibly most of the remainder were slightly infected and some undoubtedly would have devel- 



oped the disease in course of another month if they had not been killed by frost. 



(2) The infectious material was washed off these plants by copious flow of fluid from the water-pores 



during the seedling stage. 



(3) The inoculated leaves died and dried out before the bacteria had opportunity to reach the stem 



by way of the foliar bundles. 



(4) The bases (nodes) of these particular plants were exceptionally hard and offered unusual physical 



resistance to the movement of the bacteria. This is a very important point, as we shall see 

 later. (See also ante, Cobb's disease of sugar-cane, p. 37). 



(5) The juice of these plants was more acid, or in some other chemical way was more injurious to the 



bacteria than that of the plants which became diseased. 



Total number of plants found diseased to such an extent that a microscopic examina- 

 tion was not necessary to detect the bacterial slime, although it was made frequently, 266. 

 Add to the above 30 that were plainly diseased when examined hastily under a compound 

 microscope, and 18 doubtful cases which should probably be classed with the diseased, i. e., 

 in which the bacteria could probably have been demonstrated by a more careful examina- 

 tion (only a few minutes study was given to each one) , and we have the following result : 

 Diseased plants, 296; doubtful plants, 18 (probably diseased); sound plants, i. e., not any 

 visibly infected, unless the occasional slight brown stain at the extreme base be so inter- 

 preted, 159; total, 473; per cent of diseased, excluding doubtful, 62. 



In this connection it must not be forgotten that the experiments were begun late and 

 were cut short by a frost. Probably if they had been gotten under way 3 weeks sooner 

 or if the frost had held off that much longer there would have been a considerably greater 

 number of cases. 



At the end of the experiment all of the refuse was carefully gathered together and 

 burned. Undoubtedly some of the organisms were left in the soil, but no diseased plants 

 were left to be blown about or eaten by animals. Only the smaller roots were left in the soil. 



SERIES III TO VII, 1902. 



Employing the same strain of organism and susceptible varieties of sweet corn and 

 making the inoculations during the seedling stage with young cultures in the same way as 

 in Series I, but growing the plants to maturity in small pots in the hothouse under very 

 different physical conditions, a very different result was obtained, most of the plants failing 

 to take the disease. Of course one might attribute the failure to loss of virulence on the 

 part of the organism, but, in the present state of our knowledge, this would be purely hypo- 

 thetical, especially as no great amount of time intervened between the two experiments. 

 I am, therefore, inclined to ascribe it rather to the very slow growth of the plants, this view 

 being more in accord with other facts observed by the writer, e. g. , see Series XI to XV and 

 XX to XXIV. Certainly the hard, dry tissues of these plants would afford a much greater 

 physical obstacle to the movement of the bacteria than would soft juicy tissues. We do 

 not know what chemical substances are stored in the plant during such phases of growth, 

 but if acids are stored in excess, as might well be the case, then there would be also chemical 

 difficulties in the way of general infection of the plant. All such problems remain to be 

 worked out. The facts are as follows : 



