46 



PHYTOHORMONES 



is auxin, on the right-hand arm auxin is in excess, and a 

 second factor, the ''food factor," becomes Umiting. 



The maximum angle is a measure of the maximum rate 

 at which curvature takes place, and is not determined by 

 the maximum amount of curvature which the plant can 

 undergo. After the 90-110 minute period in which the 



curvature is meas- 

 ured, the maximum 

 angle continues to 

 increase, although 

 at a different rate 

 {u). The reduction 

 in the rate of in- 

 crease, which is 

 even more pro- 

 nounced for curva- 

 tures below the 

 maximum (see Fig- 

 ure 21) is due to 

 three factors : 1 , the 

 influence of gravity, 

 which causes a geo- 

 tropic curvature in 

 the opposite direc- 

 tion (this may be 

 seen by comparing 

 plant 1 with plants 

 2 and 3 in Figure 2 of du Buy and Nuernbergk [1930]) ; 2, the 

 lateral transport of auxin from the block across the plant, de- 

 tectable by a growth acceleration on the far side of the plant 

 (du Buy and Nuernbergk, 1930; Laibach and Kornmann, 

 1933a) ; [it seems that this acceleration does not occur immedi- 

 ately on applying the auxin, but, as Nuernbergk (1933) has 

 shown, there is a period after decapitation in which, pre- 

 sumably on account of the wounding, lateral transport 

 is inhibited : this period appears to be of the order of 90 min- 

 utes (u)]; 3, the effect of regeneration. This phenomenon, 



180 min. 



Fig. 21. Change of auxin curvatures with time. 

 In these plants the blocks were applied immedi- 

 ately after decapitation, at time 0. Concentra- 

 tions of indole-acetic acid: 1, 2, 4, and 8 X 0.07 mg. 

 per liter respectively. Ordinate, curvature in 

 degrees. Each curve the mean of 6 plants. 



