SKOOG AND TSUI 265 



on the other hand, bud formation occurs also in the presence of lAA 

 and up to about the same extent as in the control cultures without added 

 lAA. In these experiments the concentration of Fe2(S04)3 was also 

 increased to maintain available iron in the medium. Control series with 

 increased Fe2(S04)3 alone were negative, so that the effect on bud 

 formation is ascribed to the KH2PO4. * 



TABLE 1 



Effect of indoleacetic acid, and changes in concentrations of KH2PO4, 

 Fe2(S04)3, and sucrose on growth and bud formation in tobacco callus in vitro. 

 Initial pH adjusted to 5.0. Cultures started 12/22/41. Final measurements 



3/19/42 



Control 4FE4P 4FE8P 4FE8P2S 



Indoleacetic acid mg./l. 



1. 00 1. 00 1. 00 i.o 



4 Fe = Fe2(S04)3 10 mg./l.; 4 P and 8 P = KH2PO4 50 and 100 mg./l., 

 and 28= sucrose 40 g./l. 



The effectiveness of phosphate in counteracting the inhibitory effect 

 of added auxin suggested that organic phosphates might be limiting 

 components of the auxin reaction system. Since an important influence 

 of phosphate in respiration is through its role in purine metabolism, it 

 seemed desirable to test phosphorylated adenine derivatives. None were 

 immediately available and, therefore, adenosine in combination with 

 inorganic phosphate was first tried. The results of one experiment are 

 shown in Figure i . It may be seen that either adenosine or NAA added 

 singly reduces the growth of the callus, but the two added in combination 

 give as good or better growth than the controls. However, no buds were 

 obtained on either control or treated tissues in this experiment. In 



♦Recently R. W. Howell has compared the effects of NaH^PO^ and KH^O^ 

 on bud formation in tobacco stem segments. He finds both equally active so that 

 the effect of the salts may be ascribed definitely to the {ll^O~) radical rather 

 than to the accompanying cation. 



