294 The Phenomena of Morphogenesis 



that the activation process in ordinary wound healing and in the incep- 

 tion of tumors may be the same. This is followed by an induction phase 

 in which a tumor-inducing substance of some sort enters the host from 

 the bacterium. A heat-labile product of virulent crown-gall bacteria has 

 been found to alter conditioned cells into incipient tumor cells. How this 

 is done is not clear. The substance may itself be the agent of change, 

 possibly a virus or a macromolecule of DNA or even a gene or a 

 hereditary agent in the cytoplasm; or it may induce the change by 

 causing gene mutation or the production of permanent, self-reproduc- 

 ing bodies, sometimes called plasmoids. Finally, in the promotion phase 

 the gall grows to completion. Here auxin is involved, in the promotion of 

 an incipient into a primary tumor cell, in the multiplication and per- 



PRIMARY TRANSFORMATION PERIOD 



CONDITIONING 

 PHASE 



INDUCTION 

 PHASE 



PROMOTION and COMPLETION 

 PHASE 



CONDITIONED 

 CELL | 



MtltH 



Tumor- inducing 



principle 



INCIPIENT 

 TUMOR CELL 



* t.t 

 Auxin 



Auxin 



Tumor-inducing 

 I principle i | 



Mill' 1 



- $■ *" 



AGROBACTERlUM TUMEFACIENS 



PROMOTED 

 CELL 



PRIMARY 

 TUMOR CELL 



WOUNDING 



N0CULATI0N 



Fig. 11-12. Diagram of probable interrelations of various factors in the transformation 

 of a normal cell into a primary tumor cell. ( From Klein and Link. ) 



haps the differentiation of tumor cells, and in causing various host effects 

 which accompany tumor formation. The physiology of crown-gall for- 

 mation has been further discussed by Klein (1958). 



A question often raised is whether crown gall and its derivatives are 

 really plant cancers, as Smith vigorously maintained they were, or if 

 something different from true cancer is here involved. This question 

 has been discussed by Levine (1936), White and Braun (1942), and 

 others. It should be remembered that such a condition as malignancy 

 is difficult to define in the same terms in organisms as different in struc- 

 ture and organization as plants and animals. The unrestrained, invasive 

 type of growth characteristic of animal cancer, with its metastases and 

 lethal quality, could hardly be expected in a plant, which has no true 

 circulatory system and lacks the high degree of organization that makes 



