52 



BACTERIA IN RELATION TO PI^ANT DISEASES. 



into a carrot-root in small numbers, under favorable conditions usually will rot the whole of 

 it within a week. There are various other bacteria of this type, e. g., Spieckermann's soft 

 rot organism and Bacillus phytophthorus Appel. In general, they are omnivorous in their 

 tendencies. B. aroideae is able to rot the fleshy parts of at least 13 plants belonging to 

 half a dozen widely different families, and B. carotovorus has almost or quite as wide a range 

 of activities. The green leafy parts of these same plants are attacked either not at all or 

 less readily by these soft-rot organisms, for the rapid multiplication of which tissues full of 

 water appear to be essential. 



Fig. 6.* 



Bacillus tracheiphilus, the organism of the cucurb:; ::":.: ;r . : 

 somewhat higher type. It usually attacks the plant through its leaf -surf ace (plate i), and 

 it generally sticks pretty closely to special systems of tissues, especially in early stages of the 

 disease. It is par excellence an occluder of the vascular system (fig. 6), in which it often 

 extends for a distance of several feet from the original point of infection. It usually enters 

 the plant through wounds made by leaf-eating insects (fig. 7) . "Whether it ever gains an 

 entrance through natural openings is still a mooted question. The few experiments made by 



*FiG. 6. — Cross-sections of four inner bundles from a cucumber-stem in stage of wilt shown in plate i, fig 2 

 differentially stained to show bacterial masses confined to spiral vessels, and their vicinity, where cavities are beginning 

 to form in non-lignified vessel parenchyma. A few pitted vessels are occupied. The phloem, most of the xylem and 

 all large-celled tissues between bundles and toward the periphery of the stem are free from bacteria. ' 



