Developmental Effects of Auxins 1 07 



Root formation has been induced in stem cuttings for many years 

 by the application of auxins. The factors controlling the differentia- 

 tion of root primordia will be discussed in chapter X, but it may be 

 of interest to mention here the hypothesis of Bouillene and Went 

 (1933) that a special hormone, rhizocaline, was involved in this type 

 of morphological differentiation. They found that some substance 

 which brings about differentiation of roots is formed in leaves and 

 in buds, and moves in a strictly polar manner toward the physiological 

 base. Although the polarity of this compound was similar to auxin and 

 the loci of its formation again similar to those of auxin, the investiga- 

 tors were unable entirely to replace the effects of leaves and buds by 

 simply adding auxin. Consequently they concluded that such a special 

 hormone existed. Since that time, however, Thimann and Poutasse 

 (1941) and more particularly van Overbeek et al (1946) have shown 

 that the effects of leaves and buds in bringing aljout root formation 

 can be entirely supplanted by organic nitrogenous compounds, sugars, 

 and even in part by inorganic nitrogen sources if applied with auxin. 



Flower formation as a type of organ differentiation has not been 

 clarified with the same precision as bud and root formation. Clark 

 and Kerns (1942) discovered that the pineapple plant could be forced 

 to differentiate flowers by the application to the plant of small quan- 

 tities of auxin. It is very suggestive to note that applications of higher 

 auxin concentrations have the opposite eflect, that is, they can abso- 

 lutely prevent flower initiation in the pineapple. While we are luiable 

 to fit this phenomenon into agreement with the concept of Skoog and 

 Tsui, it is tempting to believe that the auxin effects on flowering are 

 being obtained through a change in balance or interaction between 

 auxin and some other plant constituents. There are several reasons 

 for believing this. Pineapple plants grown to different ages will re- 

 quire diflerent dosages for initiation of flowers. Plants moderately high 

 in carbohydrates require less auxin for flower initiation than do plants 

 low in carbohydrate. Furthermore a given amount of auxin applied at 

 different times of day, under different conditions of light and tem- 

 perature, can produce entirely different results in forcing or inhibit- 

 ing flowering. Most revealing of all is the finding that flower forcing 

 techniques fail to bring about flowering unless at least one leaf re- 

 mains on the pineapple plant (Traub et al, 1939). It appears that the 

 action of auxin is not a simple one in this case either, but that an 

 interaction or balance with other plant constituents is crucial to its 

 function. 



More direct evidence of an interaction between auxin and other 

 plant constituents in flowering has been produced recently by Leo- 



