278 



PLANT GROWTH SUBSTANCES 



organ formation. So far our results on ribose are restricted to tests on 

 growth of callus supplied with adenine. lAA, and different levels of 

 (AosfAate singly and in different combinations. Interestingly enough, 

 in the lower ccmcentration range ribose may have a growth-promoting 

 ^ect, but even in concentrations as low as lo mg./l. it may be strongly 

 inhibitor\-. as is I.\-\. The tv^o substances together exert an even stronger 

 inhibitor)' effect on growth. However, in the presence of adenine this 

 iahibidoa is counteracted in all cases, and. in fact, v^ith low ribose 

 concentrations more rapid growth, as measured by average increases in 

 fresh and dr\' weights of the tissues, have been obtained than from the 

 addition of adenine or adenine and lAA in the absence of ribose. 

 However, the additional increase in growth obtained from ribose in the 

 presence of adenine and L\.\, has so far been too small to be definitely 

 significant. In spite of this, the results obtained with combinations of 

 any two or all three components of the nucleotides strongly suggest a 

 close relationship between them and auxin in their effect on growth. 



Analyses of tissue composition. — .\nalyses of tissue composition includ- 

 ing "free" and bound auxin, adenine, phosphate fractions, and changes 

 in these components with treatments leading to different t\'pes of growth 

 and organ formation are in progress. In general it may be said that the 

 readily extractable auxin content in the tobacco stem segments decreases 

 rapidly from the original level, and a gradient with relatively higher 

 concentrations present in the basal half than in the apical half is estab- 

 lished. A return to higher auxin levels occurs after buds have formed 

 and, therefore, becomes most marked at first in segments treated with 

 adenine. However, rapid growth of the segments themselves is also 

 associated with increased auxin content. Representative data are shown 

 in Table 5. The effects are particularly marked in horse radish segments 

 (Table 6} whose auxin content is lower than in tobacco at the start and 

 higher after buds have been formed. These changes in auxin content, 

 therefore, run parallel with the growth and n,'pe of organ formation 

 which occur. Low auxin levels are associated with bud formation and 

 high auxin levels with root formation. It is probable, therefore, that the 

 number of buds forming on a segment is limited by the auxin production 

 in the buds which first grow out. Due to difficulties in obtaining quanti- 

 tative estimates of auxin concentrations in carrot tissues no reliable 

 data have yet been obtained on this species. 



From the results presented above, it might be assumed that adenine 



