Polar Transport of Three Auxins 417 



what different conclusions. They interpreted their experiments as 

 indicating that IBA transport was 25 per cent slower than lAA. Our 

 experiments indicate IBA to be about 50 per cent slower. They found 

 NAA to be transported 58 per cent more slowly than lAA, and oin- 

 experiments indicate the rate to be almost the same as for lAA. Ex- 

 amination of the method of determination of velocity used by Went 

 and White suggests that precise calculations would be very difficult to 

 make by their method. It involves the determination by photokymo- 

 graph of the moment of initial curvature of Avena coleoptiles when 

 the transporting coleoptile sections are placed unilaterally upon the 

 tip and a donor agar block on top of that. The determination of the 

 moment of initial curvature is complicated by nutational movements 

 of the coleoptile, by initial negative curvatures of the coleoptile, and 

 by uncertain rounded sections of the time curve which make defini- 

 tion of the unresponding and responding sections rather uncertain. 

 Using the more precise method of van der Weij (9), we feel that the 

 velocities reported here are quite exact within 1 mm. per hour vari- 

 ability at most. The possibility exists, of course, that the velocity of 

 NAA transport in Avena coleoptiles may be slower than in sunflower 

 stems. The strictness of polarity of transport of lAA reported by the 

 earlier workers is confirmed here, and in addition the same is found 

 for IBA and NAA. The report by Went and White (10) of lack of 

 transport for indole-3-propionic acid is also confirmed here. 



In view of the close similarities of the characteristics of transport 

 of lAA, NAA, and IBA, it can be deduced that the transporting tissue 

 cannot distinguish between these three auxins, except for the velocity 

 difference for IBA as discussed above. Molecules with closely similar 

 structures can be effectively differentiated against. Thus there is ap- 

 parently no transport of indene-3-acetic acid (8), of indole-3-propi- 

 onic acid, cis-cinnamic acid (10), the phenoxyacetic acids (5), or of 2- 

 naphthoxyacetic acid. Indirect evidence may permit the inclusion of 

 phenylacetic acid in this list (2). It appears, then, that the polar 

 transport system is not specific for the indole ring, but it may be spe- 

 cific for a two-cyclic ring since none of the phenyl and phenoxy auxins 

 seem to be transported in this system. The inhibition of apparent 

 transport by two benzoic acid auxins (3) indicates that these mono- 

 cyclic auxins may not be transported either. 



It has been suggested by Niedergang-Kamien and Leopold (7) that 

 an adsorptive phase may be involved in the polar transport system. 

 Since the most likely site of adsorptive attachment of an auxin would 

 be on the aromatic ring, it is reasonable to suppose that the speci- 

 ficity of the transport system for certain ring structures may be related 

 to the adsorptive characteristics produced by the ring. 



