Developmental Effects of Auxins 103 



As in the auxin control ot growth, a dualism is exerted in the 

 auxin control oi organ differentiation. A relatively high concentration 

 ol auxin, physiologically speaking, can bring about dedifferentiation 

 instead ot ditlerentiation of cells. Auxin applications to plants will 

 sometimes cause large areas of differentiated parenchyma, pericycle, 

 or endodermis tissues to dedifferentiate into callus. Again, under cer- 

 tain circumstances auxin levels can exert a control over the redif- 

 ferentiation of these callus tissues and meristems, giving rise to roots, 

 buds, or flowers. Carrying the feature of dualism still further, we 

 know of comse that the appearance of a given organ may be promoted 

 by one range of auxin concentrations and inhibited by higher con- 

 centrations. 



in 



z> 

 _j 



< 

 o 



10- 



. • 



Auxin Added 



l 



No Auxin 



12 



16 20 MG/L 



CONG. OF ADENOSINE 



Fig. 47. The effect of adenosine on the gifmth of tobacco stem calkis in ihe 

 presence and absence of auxin, 0.25 nig. /I. najilithaleneacetic acid (Skoog and 

 Tsui, 1948). 



Before discussing the interaction of auxins with other plant con- 

 stituents in the control of organ differentiation, it might be well to 

 mention a graphic case to emphasize how sharply auxin action may be 

 altered by other plant constituents present. Skoog and Tsui (1948) 

 have shown that applications of auxin alone to cultures of excised 

 tobacco stems will inhibit growth of the stem sections. When applied 

 in conjunction with adenosine the same auxin applications will stimu- 

 late growth (figure 47). This interaction lays a cornerstone for the 

 understanding of the interactions of other compounds with auxin. It 



