THE ORIGIN OF THE PLANT TUMOR CELL 611 



aspect of the tumor problem is concerned with growth, and in order 

 to gain insight into the nature of autonomous growth it is necessary to 

 understand something of the processes involved in normal cell growth 

 and division. 



Growth in all higher organisms results either from an enlargement 

 of the cells or from the combined processes of cell enlargement and 

 cell division. In plant cells the development of these fundamental 

 growth processes appears to be dependent upon specific substances 

 that may be synthesized by the cells. It is now possible, moreover, to 

 delimit, under fully controlled experimental conditions and with the 

 use as a test object of certain specialized plant cell types, these two 

 fundamental growth processes (Jablonski and Skoog, 1954; Steward 

 and Caplin, 1951 ) . W^hen, for example, tobacco-pith parenchyma cells 

 are isolated from a plant and treated with growth substances of the 

 auxin type, they enlarge greatly in size but do not divide (Jablonski 

 and Skoog, 1954.) It is only when a second growth factor, such as 6- 

 furfurylaminopurine or a naturally occurring equivalent of that sub- 

 stance, is supplied to the pith cells in addition to an auxin that a pro- 

 fuse growth accompanied by cell divisfon results. Application of 6- 

 furfurylaminopurine alone is ineffectiv e in encouraging either enlarge- 

 ment or division of the pith parenchyma cells. These findings demon- 

 strate that two growth substances, one concerned with cell enlargement 

 and the other with cell division, act synergistically to promote growth 

 and cell division in tobacco-pith parenchyma cells. Normal tobacco- 

 pith cells do not and cannot synthesize these substances, for if they did, 

 the cells would respond in tlie characteristic manner described above. 



Since the cellular systems responsible for the synthesis of these 

 two growth substances appear to be solidly blocked in normal tobacco- 

 pith cells, it was of interest to learn how such cells would respond 

 when transformed to crown-gall tumor cells. If, for example, only the 

 system synthesizing the cell-enlargement factor is activated as a result 

 of the transformation of normal cells to tumor cells, then the altered 

 pith cells should enlarge greatly, without, however, dividing. If, on the 

 other hand, the system producing the cell-division factor is activated 

 without a corresponding activation of the auxin system, then neoplastic 

 growth should not result, because, as we have seen, the cell-division 

 factor without auxin is ineffective in initiating growth accompanied by 

 cell division in tobacco-pith tissue. Only if both of the growth-sub- 

 stance-synthesizing systems are activated simultaneously, following the 

 transformation of normal cells to tumor cells, will a tumor develop in 

 this test system. The result of studies bearing on this question indicates 

 that when healing pith parenchyma cells are inoculated with crown- 

 gall bacteria, a typical crown-gall tumor develops (Braun, 1956). What 

 does this simple experiment show us? It demonstrates that, although 



