2I 4 



BACTERIA IN RELATION TO PLANT DISEASES. 



proved equally virulent to cucumbers, the disease occurring promptly and the signs being 

 typical in all respects, including the presence of the sticky bacillus in the vascular system 

 (plate 15, fig. i). The inoculation-experiments were repeated a few weeks later with the 

 same positive results; squashes and cucumbers being infected with uniform success. Addi- 

 tional studies should be made. 



In the inoculated plant, the primary foliar signs (a dulled green with absence of 

 turgor) always appear first in the punctured area and immediately around it, but never until 

 after a definite period of incubation covering at least several days. The signs of disease 

 gradually extend until the entire blade of the leaf is involved. The loss of turgor and change 

 to dull green is soon followed by shriveling, after which the leaf -blade becomes brown. 

 Subsequently, and usually considerably prior to the collapse of the petiole of this leaf, the 

 blades of other leaves up and down the stem suddenly wilt (plate 16 and text fig. 58). The 

 first leaves to show this secondary wilting are ordinarily those which arise from parts of 

 the stem nearest to the insertion of the inoculated leaf; exceptionally the first leaf to show 

 secondary wilt is one standing over the inoculated leaf rather than one actually nearer but 

 inserted on the opposite side of the stem. Gradually more and more remote leaves are 



destroyed until the whole plant is involved. 

 Whenever this secondary stage of the dis- 

 ease supervenes, the vessels in the stem 

 (which still outwardly presents a green and 

 normal appearance) will be found to be 

 occupied more or less fully by the bacillus. 

 Usually the organism is to be found in the 

 vessels of such plants in extraordinarily 

 large numbers. In the stem of the squash 

 the writer traced the bacterial occupation 

 microscopically in one plant to a distance 

 of 210 cm. from the point of infection, and 

 in another plant to a distance of 240 cm. 



Almost all of the writer's inocula- 

 tions have been made by means of needle- 

 punctures into the blade of the leaf, at first 

 often directly from plant to plant, but in 

 recent years generally from pure cultures 

 (descendants of poured-plate colonies) on 

 agar, carrot, potato or in beef -bouillon, and 

 other fluids. This method closely resembles 

 the natural manner of infection and has given very satisfactory results. These inocula- 

 tions now number over 700. A great many check-plants were held for comparison and 

 the number of accidental infections (when insect-carriers have been excluded) has been 

 practically nil, whereas the number of successful inoculations in susceptible plants has 

 frequently amounted to from 75 to 85 per cent of the total number punctured. In certain 

 experiments (pages 246, 276) every inoculated plant has contracted this disease, which is one 

 of the most infectious known to the writer. So many experiments have been made, under 

 such a variety of conditions, and with such good success (except in case of the squashes 

 already mentioned) that not the least doubt remains, either as to the bacterial nature of 

 this disease or as to the particular organism which causes it. 



Fig. 54.* 



*FiG. 54. Cross-section of small portion of a cucumber-stem attacked by B. tracheiphilus showing condition 

 of one of the outer bundles. The pitted vessels lie in the more heavily shaded lignified part of bundle and only a very 

 few of them are occupied by bacteria. All the spiral vessels are filled and the bacteria have formed conspicuous 

 cavities in the primary vessel-parenchyma which is a living non-lignified tissue. Other tissues are uninjured. Drawn 

 from a photomicrograph. 



