THE ORIGIN OF THE PLANT TUMOR CELL 609 



initiated in 24 hours, but small, slowly growing, benign tumors are 

 produced in 34 hours. A 50-hour exposure of plant cells to the action 

 of the bacteria results in tumors which grow at a moderately fast rate, 

 while tumors initiated in 72 to 96 hours grow very rapidly and are of 

 a potentially malignant type. 



Sterile tissues isolated from the three types of tumors and planted 

 on a simple inorganic salts-sucrose-containing culture medium retain 

 indefinitely their characteristic growth patterns. Normal cells of the 

 type from which the tumor cells were derived do not grow on the basic 

 culture medium. These results indicate that the transformation of nor- 

 mal cells to tumor cells takes place gradually and progressively, lead- 

 ing in a period of three to four days to a completely autonomous, rap- 

 idly growing tumor-cell type. Cells altered in shorter periods represent 

 a lower grade of tumor-cell change. This, then, appears to be an ex- 

 cellent example of tumor progression in which various degrees of neo- 

 plastic change can be obtained and such cells cultured at will. In all 

 instances the bacteria were killed before there was the slightest evi- 

 dence of tumefaction at the points of inoculation, and yet large, rapidly 

 growing tumors were produced if the bacteria were allowed to act for 

 72 or more hours. The above study, in addition to defining the incep- 

 tion period, also shows us that a factor of considerable biological in- 

 terest passes from the bacteria to the host cells and brings about a 

 profound and heritable change in the subsequent behavior of the 

 affected cells. We have called this factor the tumor-inducing principle. 



In any analysis of a complex series of events such as occurs during 

 tumor formation, it is often convenient to subdivide the total event, 

 insofar as is possible, into a series of contributing events, each of which 

 is essential for the consummation of the completed process. In study- 

 ing part events in the crown-gall disease, use was made of the fact that 

 isolates within different strains of the inciting bacteria may show dif- 

 ferent degrees of virulence. Some isolates are regularly capable of 

 initiating large, rapidly growing tumors, whereas certain sister-cell cul- 

 tures may produce only small, benign growths. It was found that when 

 the small growths initiated by the attenuated culture were supple- 

 mented at a distance with growth substances of the auxin type, they 

 expanded rapidly and were comparable in size and rate of development 

 to tumors initiated by the highly virulent strain ( Braun and Laskaris, 

 1942; Thomas and Riker, 1948). These artificially stimulated tumors 

 expanded rapidly only as long as the source of auxin was present; when 

 the hormone was removed, growth promptly slowed down. They were, 

 therefore, hormone-dependent tumors. These studies suggested that 

 plant cells transformed to tumor cells by the virulent strain of bacteria 

 were themselves capable of synthesizing optimal amounts of growth 

 substances of the auxin type, while those altered by the attenuated 



