200 



PHYTOHORMONES 



tings; the bulk of the primordia are on those sections some 

 distance from the apex, which therefore contain the most 

 of this factor. However, when auxin was appUed to the intact 

 cutting 30 primordia were formed at the top, but when 

 apphed to the uppermost one eighth of a cutting, only 

 about 7 were formed at the top: the auxin may, therefore, 

 mobilize some of the other factor from the lower parts of 



-3 -4 ' -5 -6 -1 -« 



Fig. 56. Root formation on etiolated pea cuttings. Ordinate, average 

 number of roots per plant formed near basal (crosses) and apical (circles) cut 

 surface; abscissa, log. of the indole-acetic acid concentration in moles per liter. 

 (From Went, "Allgemeine Betrachtungen fiber das Auxin-Problem," Biol. 

 Zentralbl. 56: Fig. 1, P. 479, 1936; Verlag Georg Thieme, Leipzig.) 



the cutting. This suggests that an important function of 

 the auxin is to control the movement of this other factor. 

 This is supported by another fact, namely that with in- 

 creasingly high auxin concentrations, applied at the tip, the 

 number of roots at the base reaches a maximum and then 

 decreases; at that concentration at which the decrease be- 

 gins, roots begin to appear at the top of the cutting, i.e. at 

 the point of apphcation of the auxin (see Figure 56). Ac- 

 cording to Went (1936) this means that the other factor is 

 becoming mobilized at the tip by the very high auxin con- 

 centration and thus not enough is available for the base. 



