Groxi'th Reirulatiug Effects of Phenoxy Compounds 



223 



10 



10 



10'^ 

 MOLES 



lO''^ 10"8 

 PER LITER 



10-6 



10-* 



Fig. 1. Growth effects upon Aveua coleoptile cylinders (A), wheat roots (W), and 

 flax roots (F) of indole-3-acetic acid (lAA) and 2-naphthyhnethylseIenoacetic 

 acid (2-NMSeA). Growth values expressed as per cent control. The dashed F-curve 

 shows effect of 2-MNSeA on flax roots inhibited by 2,4-D (growth in IQ-'M 2,4-D 

 alone is 100). Vertical arrows indicate growth restoration obtained by addition of 

 10-"M a-(l-naphthylmcthylthio)propionic acid (1-NMSP). 



is eliminated by the presence of IQ-^M lAA. At higher concentra- 

 tions the growth rises again and may reach a level considerably ex- 

 ceeding the control growth (Figure 2). To the initial inhibition of 

 oat coleoptile cylinder growth corresponds an initial stimulation of 

 wheat root growth. Flax root growth is inhibited, and the inhibition 

 is usually restored to a less extent by the antiauxin 1-NMSP than is 

 the inhibition caused by a typical auxin. The growth of 2,4-D-inhib- 

 ited flax roots is usually stimulated. 



As mentioned, a typical auxin usually causes a long series of 

 growth responses, and a substance giving one of these responses can 

 normally be expected to give also the other ones. The same is also 

 valid for the typical antiauxins. The occurrence and strength of the 

 auxin, or antiauxin, responses do not show an absolute connection, 

 however, and for the intermediate substances the behavior is some- 

 times irregular. No stimulation of wheat root growth could thus be 

 detected for 2-BrPOA and 2-MePOA in spite of their clearly interme- 

 diate character in the oat cylinder test. 



Apparently such irregularities may to some extent be caused by 



