614 PLANT GROWTH AND PLANT COMMUNITIES 



ize the tumorous state in crown gall. This was accomplished by vary- 

 ing the concentration of cell-enlargement and cell-division factors in 

 an otherwise suitable culture medium on which the normal cells were 

 planted. These artificially stimulated normal cells, in contrast to the 

 tumor cells, are self-limiting growths, and when the externally supplied 

 stimuli are removed, tlieir growth promptly stops. The fact that such 

 artificially stimulated normal cells commonly show histological and 

 cytological characteristics of tumor cells but are themselves self-limit- 

 ing growths indicates that the observed cellular abnormalities are the 

 result, rather than the -cause, of the tumorous state. 



The results of all of these studies demonstrate, then, that it is 

 possible for a cell to acquire a capacity for autonomous growth as a 

 result of the permanent activation of a series of growth-substance-syn- 

 thesizing systems, the products of which are concerned specifically 

 with growth accompanied by cell division. These systems are precisely 

 regulated in all normal plant cells. 



If we now return again very briefly to the other two non-self- 

 limiting tumorous diseases of plants, we find that in the case of Black's 

 wound-tumor disease, as in crown gall, the tumor tissue grows pro- 

 fusely on White's basic medium supplemented with phosphate and 

 thiamin, whereas normal tissue of the type from which the tumor 

 tissue was derived does not grow on that medium. These findings in- 

 dicate that the virus confers upon the cell the capacity to synthesize 

 greater than regulatory amounts of growth substances essential for 

 growth accompanied by cell division. Similarly, tumors that develop 

 on certain Fi hybrids can and do grow indefinitely on the basic culture 

 medium, while normal tissue isolated from either parent cannot grow 

 on that medium. The implication of this finding is clear, in view of the 

 preceding discussion. Thus we see that, although three diflFerent and 

 quite distinct agencies can initiate the tumorous state in plants, the 

 physiological basis for the autonomous growth of the tumor cell ap- 

 pears to be similar in all three instances. 



The finding that the alteration of a normal plant cell to a tumor 

 cell represents a change from a fastidious, nutritionally exacting cell 

 to a variant type which is essentially non-exacting in its requirements 

 indicates that the transformation process redirects cellular metabolism 

 from the normal, precisely regulated course to primitive pathways 

 which permit the synthesis from mineral salts and sucrose of all me- 

 tabolites required for growth and division— a characteristic metabolism 

 of many unicellular forms. It might be postulated, therefore, that as a 

 result of the transformation process the trend of evolution has been 

 reversed. A primitive area of metabolism, which is characteristic of 

 free-living unicellular organisms and on which has been superimposed 

 during the course of evolution the specialized and precisely regulated 



