104 Fundamentals of Auxin Action 



is a clear demonstration that the effect of a given auxin application 

 will be determined not by the auxin alone, but by the other materials 

 present as well. It is also suggestive that the differences in auxin sensi- 

 tivity of various tissues shown in figure 46 may be essentially attribut- 

 able to diflEerences in chemical composition of the tissues. 



Carrying this concept of interaction of materials with auxin 

 directly to the study of organ differentiation, Skoog and Tsui demon- 

 strated that auxin applications to tobacco stem sections could bring 



a 



CD 



Fig. 48. The separate effects of adenine (40 rag./l.), auxin (0.02 rng/l- naphtha- 

 leneacetic acid) and both upon bud and root formation in tobacco stem segments 

 (from Skoog and Tsui, 1948). 



about the development of roots, callus, or buds, depending upon the 

 materials which were added with the auxin. From their work they 

 concluded that the type of growth obtained is controlled at least in 

 part by the ratio of auxin to organic nitrogenous compounds, par- 

 ticularly purines. Figure 48 exemplifies such an interaction in morpho- 

 logical differentiation, where bud and root formation on tobacco stem 

 sections are compared. The addition of adenine to the culture medium 

 results in a large increase in number of buds differentiated. The addi- 

 tion of auxin (naphthaleneacetic acid) favors root formation. When 



