92 



BACTERIA IN RELATION TO PLANT DISEASES. 



ATROPHIES. 



The olive-tubercle, due to Bad. savastanoi, affords an excellent example of atrophy. 

 When the tubercle occurs on young stems that portion of the stem beyond the abnormal 

 growth is robbed of a large portion of its nourishment and often dies outright. In less 

 severe cases it is very common to find the length of a stem greatly reduced as compared 

 with normal shoots of the same age and origin. The diameter of the stem immediately 

 above the tubercle is also perceptibly less. Often it is not one-half or one-third as big as 

 the stem immediately below the excrescence (see plate 9). Often also the leaves are dwarfed 

 on that part of the shoot beyond the tubercle. Not infrequently when the terminal shoot 

 becomes diseased in this way an inconspicuous side shoot takes the lead and becomes the 

 strong shoot. 



Fig. 28.* 



The same thing occurs in the daisy knot, due to Bad. tumefaciens. As growth of the 

 knot continues, branches are often starved out and die (plate 10). 



There is also a dwarfing of the whole plant in certain cases. This occurs very often 

 in sweet corn attacked by Bad. stewarti, in cane attacked by Bad. vast ularum, in seedling 

 cabbages attacked by Bad. campestre, and in many plants attacked by crown-galls and 

 root-galls. 



ENLARGEMENT OF THE NUCLEUS. 



In many diseases which I have studied I have not been able to make out any change 

 in the form or size of the nucleus, those nuclei near diseased areas being not different from 

 more remote ones. The change in such cases, if any, is a simple disintegration under the 

 action of the bacteria. In olive-tubercle, however, and in the crown-gall there seems to be 



*Fio. 28. Tumors on sugar-beets due to inoculation by needle-pricks with Bad. tumefaciens, plated from Paris 

 daisy. Inoculated about six weeks. Photographed April 1007. Nearly natural size. 



