ALBERT C. HILDEBRANDT 403 



was critical in determining whether the cultures continued to produce 

 a callus mass or whether they differentiated roots, stems, and leaves. 

 Weak concentrations of growth-regulating substance favored cambium 

 development and cell division; a slightly stronger concentration was 

 beneficial for cambial growth, but stimulated root formation in addition; 

 while even stronger concentrations stopped cell division and favored 

 cell enlargement or stopped growth completely. 



Cultures of tissue from normal plants of certain species only required 

 added growth-regulating substance in the medium to support growth of 

 the original isolate or of the first few transfers in vitro. These cultures 

 following the original incubation period on a medium containing supple- 

 mentary growth-regulating substance grew indefinitely when transferred 

 to a basal medium lacking added growth-regulating substance. Such 

 cultures were described as habituated and were isolated from several 

 plant species. 



Tissue cultures of primary or secondary crown-gall origin from several 

 species and tissues from the genetically unstable tobacco hybrid were 

 capable of unhmited growth in vitro on media lacking any added growth- 

 regulating substance. Different concentrations of supplementary growth- 

 regulating substance induced macroscopic changes evident only as in- 

 creased weight at optimal high dilutions or as inhibition or death at 

 low dilutions. No leaves, stems, or roots were formed on cultures of 

 crown-gall origin, but under certain conditions they were formed on the 

 tobacco tissue. Different concentrations of supplementary growth-regu- 

 lating substance influenced histologically the sunflower tissue of crown- 

 gall origin. 



There appeared therefore certain similarities and differences between 

 tissue cultures from normal plants and those of pathological origin. 

 Some of these similarities and differences in the requirements of the 

 respective types of tissue cultures for growth-regulating substance were 

 indicated. The growth-regulating-substance activity of the extracts of 

 the three types of cultures in the Avena test was also reported to vary 

 with the three types. Thus, the importance of growth-regulating sub- 

 stance for normal and pathological growth was suggested and has re- 

 ceived considerable attention. However, the fundamental role of these 

 materials in normal and pathological growth is still to be worked out. 

 These and further studies with plant tissue in vitro may clarify the 

 nature of the balances resulting in normal or pathological growth. 



