Growth Substances 



393 



(Fig. 18-17). When slices were repeatedly trimmed off from the lower 

 surface, shoot primordia began to appear there, presumably because of 

 the removal of auxin and the consequent reduction in its concentration. 

 When concentrations of growth substances are much higher than in 

 natural conditions, they tend to have a local effect and to produce a 

 downward bending of the leaves (hyponasty ) and the formation of callus. 

 Upon the latter, root primordia often appear. Growth substances ap- 

 plied to the soil may be absorbed, carried up in the transpiration stream, 

 and affect the structure of the growing plant. 



.INCREASE 

 AUXIN 



UNTREATED 



DECREASE 

 AUXIN 



INTACT 

 ROOT 



ROOT 

 SEGMENTS 



AFTER 

 REGENERATION 



Fig. 18-17. Auxin and regeneration. In a root segment of Taraxacum, placed hori- 

 zontally (2), regeneration is normally polar, shoots developing at the proximal end 

 and roots at the distal one (see Fig. 6-4). When the amount of auxin is increased at 

 the proximal end, roots are produced there ( 1 ) . When it is decreased at the distal 

 end, shoots develop there ( 3 ) . ( From Warmke and Warmke. ) 



It is recognized that growth substances are not the only factors con- 

 cerned in root formation. A supply of sugar is necessary. Indeed, the 

 stimulating effect of leaves on root formation may be due in part to their 

 production of nutrients. In grafting experiments between rooting and 

 nonrooting varieties of Hibiscus, van Overbeek and Gregory (1945) 

 found that something formed in the leaves, in combination with auxin, 

 is required for root growth, and van Overbeek, Gordon, and Gregory 

 (1946) showed that this is not a hormone since it can be replaced by 

 sucrose or nitrogenous substances. The importance of a high carbohy- 

 drate-nitrogen ratio in root determination has already been discussed 



