/;/ Vitro Experiments on Tissues of Pathological Origin 



ALBERT C. HILDEBRANDT 



THE role of growth-regulating substances in pathological growth has 

 received wide attention. Much of this work has centered around the 

 bacterial plant-gall disease incited by Agrobacterium tumefaciens (Smith 

 and Townsend) Conn. The early Uterature on crown gall has been re- 

 viewed by Riker and Berge (20) and by Riker, Spoerl, and Gutsche (21). 

 Such gall formation may result from a balance of factors among which 

 growth-regulating substances may be important (19, 10). 



Plant-tissue culture provides certain advantages over whole plants 

 to study the fundamental aspects of normal and abnormal growth. 

 For example, strains of callus cultures may be grown vegetatively in 

 vitro for unlimited periods. The media for such cultures are easily 

 reproducible since they initially contain only nutrients whose chemical 

 formulae are known. This provides a fairly simple medium for closely 

 controlled studies. Such cultures may be maintained in a relatively 

 undifferentiated condition. In certain cases changes in the morphological 

 character of the cultures were induced under controlled conditions. 

 The cultures may be subjected over long or short periods to wide ranges 

 of concentrations of supplements, including cell stimulating or inhibiting 

 materials under otherwise constant conditions. Such cultures once es- 

 tablished are no longer directly influenced by the association of other 

 parts of the plant. These and other conditions seem to offer important 

 opportunities for critical studies under closely controlled environments. 

 Whole plants may respond in a variety of ways to the presence of 

 growth-regulating substances. Such responses may include formation 

 of adventitious roots, epinasty, stimulation of cambial activity, inhibi- 

 tion of bud development, or delayed abscission of old leaves. A number 

 of natural and synthetic growth-regulating compounds may induce 



