612 PLANT GROWTH AND PLANT COMMUNITIES 



the normal tobacco-pith cells could not actively synthesize either a cell- 

 enlargement factor or a factor limiting for cell division, following their 

 transformation to tumor cells, both of these substances were produced 

 in greater than regulatory amounts. If this were not true, continued 

 growth accompanied by cell division and hence tumor formation would 

 not have resulted in the test system used in this work. 



That these two growth substances are actively synthesized by 

 growing tumor tissue can be further demonstrated by grafting a frag- 

 ment of sterile tobacco-tumor tissue on a fragment of normal tobacco- 

 pith tissue. The tumor tissue stimulates the pith to divide to such an 

 extent that stimulated pith cells may raise the tumor tissue a consider- 

 able distance above the graft surface ( Braun, 1956 ) . This is reminiscent 

 of the desmoplasias of animal pathology, where the tumor stimulates 

 the normal cells of its stroma to very active division. 



Finally, it is possible to demonstrate the presence of a cell- 

 enlargement factor and a factor limiting for cell division in the tumor 

 tissue by isolation and diffusion techniques. 



All of these studies indicate that the tumor tissue synthesizes 

 greater than regulatory amounts of a cell-enlargement factor and a 

 factor normally limiting for cell division. The permanent activation of 

 these two growth-substance-synthesizing systems, with the resulting 

 production of greater than regulatory amounts of the cell-enlargement 

 and cell-division factors, would appear in itself sufficient to account 

 for the continued abnormal proliferation of the crown-gall tumor cell. 

 More recent work has shown that this is not the entire explanation 

 (Braun, 1958). 



It was indicated earlier in this discussion that the alteration of 

 normal cells to tumor cells is a gradual and progressive process, lead- 

 ing in a 3- to 4-day period to a completely autonomous, rapidly grow- 

 ing tumor cell. Cells altered in a 34-hour period grow very slowly in a 

 host as well as in culture, while those transformed in a 50-hour period 

 proliferate at a moderately fast rate. Normal cells of the type from 

 which the tumor cells were derived do not grow on the basic medium. 

 Thus, although the difference between the three types of tumor cells is 

 quantitative, since all can grow indefinitely although at different rates 

 on the basic medium, the difference between the tumor cells and nor- 

 mal cells is qualitative. Since the three types of tumor cells were de- 

 rived from the same plant species, they were admirably suited and 

 were used for a study of the factors required for rapid autonomous 

 growth. In these studies the fully altered, rapidly growing type of 

 tumor cell was used as the standard. This cell type can synthesize, in 

 optimal or near-optimal amounts, all of the growth factors needed for 

 its continued rapid proliferation from the mineral salts and sucrose 

 present in the basic medium. The moderately fast-growing tumor cells 



