608 PLANT GROWTH AND PLANT COMMUNITIES 



pendent of the inciting bacteria. This leads to the development of 

 massive tumorous growths, which in extreme instances may weigh as 

 much as 100 pounds. Such tumors are potentially malignant, in that 

 they seriously damage or kill the plants in which they develop. In cer- 

 tain hosts, such as the sunflower ( Braun, 1941 ) and Paris daisy ( Smith 

 et al., 1912), there may be produced, in addition to primary tumors, 

 secondary tumors which arise at points distant from the seat of the 

 primary growth. These secondary tumors are interesting because they 

 are frequently free of the bacteria that initiate the primary tumor 

 (Braun, 1941; Smith et al, 1912). 



The finding that many of the secondary tumors are bacteria-free 

 permitted the unequivocal demonstration of the truly autonomous 

 nature of the crown-gall tumor cell (Braun and White, 1943; White 

 and Braun, 1942). Sterile tissue isolated from secondary tumors grows 

 profusely and indefinitely on a defined culture medium which does not 

 support the continued growth of normal cells of the type from which 

 the tumor cells were derived. This indicates that a profound and herit- 

 able change has occurred in the plant cells as a result of the localized 

 presence of the inciting bacteria in susceptible host tissues. Small frag- 

 ments of such tumor tissue, implanted into a healthy host, develop 

 again into tumors comparable to those initiated by the bacteria in 

 every respect except that the implants are sterile. Since such sterile 

 tumor cells, isolated not only from secondary tumors but subsequently 

 also from primary tumors of many diflFerent plant species (de Ropp, 

 1947; Gautheret, 1947; Hildebrandt and Riker, 1949; Morel, 1948; 

 White, 1945 ) , have not in the more than ten years that they have been 

 kept in culture shown the slightest tendency to become less autono- 

 mous, they have generally been regarded as being permanently altered 

 cells which reproduce true to type and against the growth of which 

 there is no control mechanism in the host. These are the characteristics 

 by which the malignant animal cells are distinguished from healthy or 

 merely inflammatory cells. 



After it had been definitely established that normal cells were 

 changed to tumor cells under the influence of the bacteria, the next 

 problem that engaged our efforts was concerned with the period neces- 

 sary for the bacteria to accomplish the cellular transformation ( Braun, 

 1943,1947,1951a). 



In order to study this question, an experimental method was de- 

 vised which permitted the selective killing by thermal treatment of the 

 inciting bacteria at any desired time following their introduction into 

 the heat-resistant host, Vinca rosea, without affecting the capacity of 

 the host cells to respond with tumor formation to any alteration that 

 had occurred prior to the killing of the inciting organisms. With the 

 use of this method, it was possible to demonstrate that tumors are not 



