Abnormal Growth 295 



animals so vulnerable. The animal cancer cell has lost its specificity and 

 become, so to speak, an independent parasitic entity of unlimited 

 growth. What this change involves and what causes it are still not 

 understood. Crown-gall tumor cells are certainly in this same category 

 for they grow indefinitely and do not depend on the continual presence 

 of the factors that induced them. The true cell invasions and metastases, 

 in which bits of cancer tissue are carried away to other parts of the body 

 and there develop new centers of malignancy, are absent in plants, but 

 transfer of gall tissue from place to place by grafting is readily ac- 

 complished. Many students of the problem are inclined to regard crown- 

 gall tumors as basically no different from animal cancers. It is obvious 

 that these examples of abnormal growth provide some of the best ma- 

 terial known for a study of the way in which the higher levels of or- 

 ganization in the plant are broken down. For students of morpho- 

 genesis they long have had a particular interest. 



Root Nodules. Another type of cataplasmatic galls rather different 

 in character from the others here described and of much practical im- 

 portance to man are the nodules formed on the roots of leguminous 

 plants from the invasion of their cortical tissues by species of Rhizobium. 

 They are an example of what has been called "controlled parasitism," 

 for the relation between this bacterium and the plant may better be 

 regarded as symbiosis rather than parasitism since the host plant ob- 

 tains an advantage because of the atmospheric nitrogen fixed by the 

 bacteria. These nodules have a higher degree of organization and pro- 

 duce more specialized structures than do most cataplasmatic galls 

 and perhaps should be included under prosoplasmatic ones (Allen and 

 Allen, 1953). The particular character of the nodule depends upon the 

 host plant and the species of bacterium that invades it. As in crown 

 gall, auxin action mav here be involved. 



Abnormal Growth Due to Other Causes. Manv cases have been re- 

 ported of abnormalities due to other factors than parasitism or chemical 

 stimulation. X rays may produce them ( Sankewitsch, 1953), as may 

 ionizing radiations (Gunckel and Sparrow, 1954). Some resulted from 

 the A-bomb tests in the Pacific (Biddulph and Biddulph, 1953). 



In some plants tumors arise from no recognizable cause and are 

 presumably due to somatic mutations or to a modification of organized 

 development by other genetic factors. The best known case is that of 

 the tumors occurring spontaneously in hybrids between Nicotiarui 

 glauca and N. Langsdorfii (Kostoff, 1930a; Kehr and Smith, 1954). These 

 are small amorphous structures appearing on stems and branches, and his- 

 tologically resembling wound callus and crown gall. Kostoff believes that 

 they are due to a disturbed growth balance, either in nucleus or cyto- 

 plasm, between these two particular species. These tumors, removed 



