THE ORIGIN OF THE PLANT TUMOR CELL 607 



mammary carcinoma virus and yet show an inherent tendency to de- 

 velop mammary tumors. 



The second non-self-Hmiting tumorous disease of plants is one in 

 which the genetic constitution of the host, and more particularly the 

 genetics of all of the cells comprising the host, plays a primary role 

 (Kehr, 1951). No external agent, such as, for example, a virus, is in- 

 volved in the tumor inception or development. This condition, known 

 as the KostoflF genetic tumors, results regularly in certain interspecific 

 hybrids within the genus Nicotiana. W'hen, for example, two plant 

 species such as iV. glaiica and N. langsdorffii are crossed and the seed 

 of the hybrid is sown, the resulting plants commonly grow normally 

 during the period of their active growth and in the absence of irrita- 

 tion. Once the plants reach maturity and terminal growth ceases, a pro- 

 fusion of tumors invariably breaks out on all parts of the plant. These 

 tumors commonly arise at points of natural wounds. Irradiating such 

 hybrid plants hastens the onset of tumor formation and increases sig- 

 nificanth- the number of tumors that develop ( Sparrow and Gunckel, 

 1956; Sparrow et aJ., 1956). Recently evidence has been provided to 

 indicate that in these hybrids neither sp'ontaneous tumor formation nor 

 radiation-induced tumor formation involves a process of somatic mu- 

 tation at the nuclear gene level ( Smith, 1958 ) . 



It has been found, further, that the parents of the tumorous hy- 

 brids can be divided into t\\'o groups, which have arbitrarily been 

 designated as "plus" and "minus" ( Naf , 1958 ) . If an intragroup cross 

 is made between two "plus" species or between two "minus" species, 

 the resulting offspring will not develop tumors. On the other hand, 

 crosses between a "plus" and a "minus" species produce tumor-bearing 

 offspring. Of a total of more than 50 such crosses studied, very few 

 exceptions to this rule ha\'e been found. From these studies it was con- 

 cluded that the critical contributions of the "plus" parents differ from 

 those of the "minus" parents. These contributions, although primarily 

 genetic in nature, should also be reflected in parental metabolism, and 

 attempts are being m.ade by Naf to characterize these at a physiological 

 level. These tumor-bearing hybrid plants appear to be comparable to 

 the situation described in animals by Gordon (1958) in platyfish- 

 swordtail hybrids. 



The third non-self-limiting tumor disease of plants, and one that 

 I should like to discuss in some detail, is the so-called crown-gall 

 disease. This disease is initiated by a specific bacterium known as 

 Agrohacterium tumcfnciens. The crown-gall bacterium possesses the 

 remarkable ability to tranform normal plant cells into tumor cells ir- 

 reversibly in short periods of time. Once this cellular transformation 

 has been accomplished, the continued abnormal proliferation of the 

 tumor cells becomes an automatic process which is completely inde- 



