274 PLANT GROWTH SUBSTANCES 



root formation. Adenine had relatively little effect on growth when 

 supplied alone, but it completely counteracted the inhibiting effect, 

 when it was obtained, from low concentrations of lAA. No bud forma- 

 tion resulted from treatments of the segments. However, in subcultures 

 of the callus produced on the segments, roots were formed in response to 

 lAA treatments, and a few transfers formed buds, but not exclusively in 

 cultures with adenine treatment. In tissues from all three species, there- 

 fore, a very definite interaction between adenine and lAA was demon- 

 strated, but the quantitative requirements for a particular type of 

 growth or organ formation were very different. 



Specificity of adenine for bud formation.— In the case of auxins we know 

 that a large group of related compounds with certain structural features 

 in common have similar effects on cell elongation and also on root 

 formation. It was of interest, therefore, to determine the specificity of 

 adenine in its effect on bud formation. As shown by the curves in Figure 

 7 adenine derivatives such as adenylic acid and adenosine are active. 

 Also guanine, though it acts more slowly, may be nearly as effective as 

 adenine. However, xanthine and the pyrimidine, uracil, are either com- 

 pletely inactive or have a very low activity. Preliminary tests with 

 cytosine indicate that this compound may possess some activity. On the 

 other hand, the buds which were produced in tissues treated with 

 cytosine appeared so much later than from treatments with adenine, that 

 the effect of this substance may be less direct. The results are as yet 

 fragmentary but suggest that the entire purine nucleus together with 

 either substituted NH2 groups in the 2 or 6 position, or else the absence 

 of keto groups in both these positions is required for high activity. 



Arginine, which has been reported to substitute for adenine in the 

 phosphate energy transfer in invertebrate tissues, has also been shown 

 to promote the growth of Avena coleoptiles and pea stem segments 

 (1,4). It also promotes growth of tobacco tissues under the conditions 

 of our experiments, but, in our limited experience with the tobacco 

 tissues, arginine has a general growth-promoting effect which is quite 

 distinct from the effect of the active purines. The specificity of adenine 

 and structurally related compounds for bud formation may well be 

 compared with the specificity of lAA, and other compounds with auxin 

 activity, for root formation. Still, neither group of compounds can be 

 designated as specific organ-forming substances, since both are un- 

 questionably essential for growth of all cells and tissues. Furthermore, 



