conspectus: reaction of the plant 51 



growths, which are hyperplasias, there is a great multipUcation 

 and simphfication of the parenchyma and a great reduction of 

 the vascular system, but in crown galls produced in the torus 

 of the sunflower (which is a very vascular tissue) there is an 

 excessive number of vessels. There are also various other 

 phenomena, chemical and physical, nearly related to what takes 

 place in certain insect galls, that is, increase of sugars, starches, 

 enzymes, acids ; and structural simplifications and reversions 

 to more primitive forms. What I mean by reversions may be 

 seen by consulting my figures illustrating insect galls. In 

 crown gall, cell-division under compulsion proceeds at such an 

 abnormally rapid rate that the cells are forced to divide while 

 still immature, and in this way masses of small-celled, unripe 

 (anaplastic) tissue arise (Figs. 353, 354) and these develop 

 tumor-strands (Figs. 319 and 323 to 325) in which secondary 

 tumors form — phenomena suggestive of what occurs in malig- 

 nant animal tumors (Consult text of No. XIV and various 

 plates and figures and, especially, read Jensen's recent (1918) 

 Danish paper referred to under Literature of No. XIV). 



prevalence and geographical distribution 



Economically considered, bacterial diseases of plants may be 

 classed as major or minor. Most of the leaf-spots would fall 

 into the latter class. Various soft rots, blights and vascular 

 diseases, being wide-spread and destructive to plants of great 

 economic importance, may be classed as major diseases. 

 Cankers and tumors would fall midway in such a grouping. 

 Occasionally a minor disease, e.g., lettuce rot, celery rot,^ under 

 conditions favorable to the parasite may assume great importance. 

 This is especially true of leaf-diseases which attack the fruit, e.g., 

 the black spot of plum and peach due to Bacterium pruni, the 

 bean-blight due to Bacterium phaseoli, the angular leaf-spot of 

 cotton due to Bacterium malvacearum, the African mango dis- 

 ease due to Bacillus mangiferae, the black chaff disease of wheat 



1 " The loss from this disease in the field where 1 gathered the specimens was 

 ISO crates out of every 700 crates packed." (Dr. J. Rosenbaum, Hastings, 

 Fla. Letter of April 4, 1916.) 



