610 PLANT GROWTH AND PLANT COMMUNITIES 



strain had their requirements in terms of rapid growth only partly 

 satisfied for substances of that type. 



It was also concluded from these studies that tumor formation 

 takes place in essentially two distinct phases (Braun and Laskaris, 

 1942 ) . In the first phase, normal cells are changed to tumor cells which 

 do not as yet develop into a neoplastic growth. The second phase, ac- 

 cording to this concept, is concerned with the continued abnormal pro- 

 liferation of the tumor cells after the cellular alteration has been ac- 

 complished. The virulent bacteria obviously can accomplish both 

 phases; the attenuated culture, essentially only the first. This study 

 emphasized, then, the need for recognizing a distinction between those 

 factors that render the cells neoplastic and those that affect their sub- 

 sequent behavior. 



Two known requirements must be satisfied to complete the first, 

 or inception, phase. These have been termed "conditioning" and "in- 

 duction." By conditioning is meant that only those plant cells that have 

 been rendered susceptible to transformation as a result of irritation 

 accompanying a wound can be altered to tumor cells (Braun, 1952). 

 An analysis of the conditioning process has revealed that an interesting 

 relationship exists between the period in the normal wound-healing 

 cycle in which the cellular alteration is accomplished and the rate of 

 growth of the resulting tumors (Braun, 1952; Braun and Mandle, 

 1948). It has been found that the host cells gradually become vulner- 

 able to the action of the tumor-inducing principle, reaching a maximum 

 susceptibility about 60 hours after a wound is made. Thereafter the 

 cells gradually become more resistant to transformation as vvound 

 healing progresses toward completion. If the cells are not adequately 

 conditioned, as appears to be the case in the very early and late stages 

 of the wound-healing cycle, the transformation of normal cells to tumor 

 cells is not accomplished, despite the presence of many virulent bac- 

 teria in intimate contact with the host cells. When these findings are 

 compared with histological events in the region of the wound, we find 

 that it is just before or during the earliest stages of active wound heal- 

 ing that normal cells are transformed to tumor cells of the most rapidly 

 growing type (Braun and Mandle, 1948). It is at this period in the 

 wound-healing cycle that the plant cells show the highest rate of meta- 

 bolic activity. 



Induction refers to the actual conversion of conditioned host cells 

 into tumor cells by a tumor-inducing principle elaborated by the bac- 

 teria. Induction, as far as we now know, completes the first phase of 

 tumor formation. 



The second phase of tumor formation, according to this concept, is 

 concerned with the continued unregulated and autonomous growth of 

 the tumor cell once the cellular alteration has been accomplished. This 



