Abnormal Growth 



293 



secondary galls develop after growth in length is over but always in close 

 association with the xylem, as though the latter were the pathway of 

 induction. Secondary tumors behave much like primary ones in grafts 

 and in culture, and there seems to be no very fundamental difference 

 between the two. 



Remarkably enough, many crown galls, particularly secondary ones, 

 seem to be free from bacteria. There is evidence, however, that bacteria 

 must always be present at the very beginning of tumor growth but 

 that they soon disappear. Braun and White (1943) made Vinca rosea 



Fig. 11-11. Section of a young crown gall on Pelargonium, showing a nest of vascular 

 cells. ( From Noel. ) 



galls free of bacteria by heat treatment. Such tissues retained their gall- 

 producing properties when grafted into healthy plants (White, 1945). 

 Although bacteria are required for the inception of crown gall, once the 

 change is induced they no longer seem necessary for the growth of gall 

 tissue. 



Crown gall (and presumably other amorphous galls of this general 

 type) does not result from a single cause but involves a series of factors. 

 Klein and Link ( 1955 ) discuss this in their extensive account of the 

 etiology of crown gall (Fig. 11-12). There is first a conditioning phase, 

 perhaps induced by wounding and involving wound hormones. This 

 makes the cells susceptible to conversion into tumor cells. It is possible 



