THE BROWN ROT OF SOLANACEAE ! HISTOLOGY 197 



How are the cavities produced? On the root-infected plants 

 (tomatoes, Daturas, Tropaeolum, castor oil-plants), trace the 

 bacteria from vessels in the broken roots into the stem and 

 up the latter into the leaves. Are the unbroken roots sound? 

 Also in stem-punctured plants follow the upward and downward 

 movement of the bacteria. How far in advance of the brown 

 stain can you trace the bacteria? Can you judge from the 

 relative abundance of the bacteria in the vessels of the stem 

 (above and below the point of inoculation) whether the infec- 





4 



f; 



vji 



Fig. 139. — Empty and full (bacterially occluded) vessel in a potato plant. From 

 same series of inoculations as Fig. 138. 



tion is moving up or down the stem? Can you trace the bac- 

 teria along the vessels of the rhizome into the developing potato 

 tuber? Is the starch in the tuber destroyed by the bacteria? 

 Is it removed by the plant? Can you find any evidence of an 

 attempt to ''cork out" the parasite? Study early stages of 

 cavity formation, appearance of tyloses, and other effects of 

 the organism on the tissues. 



Have you observed any bacterial motility in the plant? 

 To see this you must ordinarily take parts only recently occupied. 



