616 PLANT GROWTH AND PLANT COMMUNITIES 



are not a mixture of normal and tumor cells but are composed entirely 

 of tumor cells which retain, despite their alteration, highly developed 

 capacities to organize morphologically abnormal leaves and buds. The 

 cells of these complex tumors, like those of the unorganized type, grow 

 profusely and indefinitely on a basic culture medium which does not 

 support the continued growth of normal cells. 



An attempt was made to distinguish between somatic mutation at 

 the nuclear gene level and the presence in the tumor cell of cytoplas- 

 mic changes which had assumed control of the cells and were respon- 

 sible for the continued abnormal proliferation of the afiFected cells. It 

 is well known in biology that certain self-replicating cytoplasmic en- 

 tities can be eliminated from cells under conditions that favor the in- 

 creased multiplication of those cells in relation to the multiplication of 

 the self-duplicating factor. 



The primary growth of higher plants is the result of the very rapid 

 division and subsequent elongation of meristematic cells found at the 

 extreme apex of a shoot or root. Since normal meristematic cells at the 

 apex of a rapidly growing root or shoot divide at a far faster rate than 

 do most crown-gall tumor cells, it was hypothesized that if the abnor- 

 mal tumor buds found to develop from teratomata could be forced into 

 very rapid growth, recovery of the tumor cells might be accomplished, 

 provided that the factor responsible for the continued abnormal growth 

 of the tumor cell was subject to the effects of dilution in very rapidly 

 dividing cells. The results of this study demonstrated that when tumor 

 shoots derived from tumor buds were forced into very rapid growth as 

 a result of a series of graftings to healthy plants, they gradually re- 

 covered and ultimately became normal in every respect. 



These findings indicate that the crown-gall tumor cell contains, 

 potentially at least, all of the factors, both genetic and non-genetic, 

 that are present in the normal cell. In this instance nothing has been 

 permanently lost as a result of the cellular alteration. These results 

 make somatic mutation at the nuclear gene level appear highly un- 

 likely as a possible explanation of the cellular alteration in crown gall. 

 They suggest, instead, that cytoplasmic changes, which may, however, 

 be more or less under control of the nuclear genes, are responsible for 

 the continuity of the tumorous properties from one cell generation to 

 the next. 



These results may be interpreted in terms of steady-state chem- 

 istry: that is, that alternative areas of metabolism compete with one 

 another, leading to biosynthetic states which commonly show very high 

 degrees of stability but which under certain special conditions may be 

 reversible. If this is true of the generality of tumors, it could have in- 

 teresting implications, for it would mean that the malignant tumor cell 

 is not, as is now commonly believed, an irreversibly altered cell. A 



