The Interaction Between Causative Agents in 

 Diseased Growth 



A. J. RIKER AND J. E. THOMAS 



THE basic problems, what starts off diseased plant growth and what 

 keeps it going, are among the most fundamental in biology. Galls 

 may be incited by various agencies including physical and chemical 

 factors, virus, bacteria, fungi, nematodes, and insects. However, much 

 of the basic research has been done with crown gall caused by Agro- 

 bacterium tumefaciens (Smith and Town.) Bergey et al. 



Many of the numerous working hypotheses given to explain gall 

 formations have been centered on one or another chemical substance. 

 Such ideas have developed naturally from work on other diseases where 

 injury and death were caused by some single factor, such as too much 

 heat, a toxic spray, or a single microorganism. Many of the earlier studies 

 on pathological growth have been reviewed elsewhere, for example, by 

 Riker and Berge (i6) and by Riker, Spoerl, and Gutsche (20). The 

 activity of growth substances has an obvious bearing on diseased growth. 

 The general subject has been covered in a previous paper (22) listing 

 earlier reviews, and by various reports elsewhere in this symposium. 



While a single extraneous factor may cause death, the growth of cells 

 obviously is more complex. For normal growth many physical and 

 chemical factors seem to operate in suitable balance with one another. 



Our purpose in this discussion is to indicate how some of these factors 

 may be thrown out of balance. Further we consider evidence bearing 

 on the suggestion (15) that such a lack of balance may influence subse- 

 quent abnormal growth. 



Histological picture. — Perhaps one of the easiest ways to visualize 

 this problem is to consider what happens when crown-gall bacteria appear 



