28o 



PLANT GROWTH SUBSTANCES 



increases in the phosphate content of the segments and particularly 

 counteracts the lowering in phosphate content which is apt to result 

 from addition of lAA alone. However, the main increases in phosphate 

 content obtained by the treatments have been in the inorganic fraction. 

 Significant increases in organic phosphate fractions, therefore, either 

 do not occur or the compounds formed are too labile to be detected in 

 spite of all attempts with the several methods which were employed. 

 In any case the results point to a sensitive equilibrium between auxin, 

 adenine, and phosphate contents in the tissue, according to which 

 phosphate uptake may be increased or decreased by addition of either 

 one of the two organic compounds, depending on the relative supply 

 of the other. Typical results of the phosphate content and distribution 

 for different treatments and at successive stages of growth are shown in 

 Figure 9, and the proportions of the total phosphate present in different 

 fractions at the start and after 14 days of growth are shown in Table 7. 

 Further evidence on the localization of the phosphate in the tissues is 

 being obtained by isotope analysis. Data on adenine content and carbo- 

 hydrate fractions are still too fragmentary to be interpreted. 



Discussion and conclusions. — The evidence obtained on growth and 

 organ formation in tissues cultured in vitro demonstrate that under 

 the conditions of these experiments the relative growth of different 

 tissues and the type of organ formation which will occur depend on 

 the composition of the nutrient medium. Generally the application 

 of lAA or NAA leads to rapid growth of parenchymatous tissues, to 

 root formation, and to suppression of bud formation, whereas the appli- 

 cation of adenine leads to bud formation. Clearly, however, these 

 substances are not specific either for the formation or growth of par- 

 ticular organs. Both are required for all types of growth. The responses 

 elicited by their application to the tissues depend on the proportions 

 and concentrations in which they are supplied as well as on the propor- 

 tions and concentrations of these and other essential nutrients present at 

 the start or supplied during the growth period, through synthesis or 

 from external sources. 



In regard to the specific actions of auxin and adenine in growth 

 the only new information provided by the present experiments is the 

 definite interaction of these substances and other components of the 

 nucleotides. This strongly supports the original assumption that auxin 

 acts as a coenzyme. If we accept the known function of adenine phosphate 



