ROOT FORMATION 203 



(1935) described the formation of laterals on the aerial roots 

 of Cissm, after apphcation of various auxins and auxin-like 

 substances either as paste or in water solution to the growing 

 zone. Application on the basal side of the elongating region 

 was inefifective. The apphed auxin, however, inhibits the 

 gro\\i;h of the main root, and to this they ascribed the 

 branching; as soon as the main root recovered its original 

 growth rate, the effect of the auxin paste on branching dis- 

 appeared. The same phenomenon on a number of seedling 

 roots was described by Faber (1936; see also Laibach, 1935), 

 and by Thimann (1936). Faber obtained profuse branching 

 of the roots at the place of application of the auxin paste. 

 However, Thimann (1936) showed that in Pisum roots 

 branching is independent of the growi^h of the main root. 

 He applied aiLxin paste to the stump of the epicotyl, which 

 produced no inhibition, but even a sHght acceleration of 

 the main root; nevertheless it increased branching from 

 in the controls to an average of 2.7 in the treated plants. 

 On de-tipped roots the effect was still greater. This fact 

 gives good e\'idence for acropetal auxin transport in the root 

 (see IX C). 



Thimann also found that branching in both Avena and 

 Pisum roots is inhibited by the presence of the root tip; 

 since the root tip is a source of auxin this is probably due to 

 factors other than auxin. Correspondingly, Katunskij (1935) 

 reported that in Zea roots branching is inhibited by the 

 apphcation of coleoptile tips. In Avena branching occurs 

 very readily and is not increased by auxin application, so 

 that auxin appears not to be the limiting factor for branching 

 in these roots. 



If an incision be made on one side of Vicia Fdba roots 

 the formation of laterals is prevented on the apical side of 

 the incision for a considerable distance (de Haan and Petrick, 

 1935). This indicates that formation of laterals is controlled 

 by something coming from the stem or cotyledons. All the 

 above facts indicate that root branching is controlled both 

 by auxin and by other factors. 



