452 



J. van Overbeek 



CYTO SKEL£ TON O 



i 



Fig. 2. Upper: Comparison Ijclween models of molecules of 2,3,6-trithloio sub- 

 stituted phenoxyacetic (POA), phenylacetic (PA), and benzoic (B) acids as they are 

 envisioned to fit into the cytoskeleton and its accompanying layer of H-bonds. 

 Because of the two ijulkv chlorine atoms on both nrtho positions, the rings have 

 penetrated only relatively shallowly. The chlorine in the 3 position (below the 

 plastic) helps in anchoring the ring. This shallow penetration locates the carboxyl 

 groups of PA and B just right for becoming part of an oscillating H-ijond system 

 at the surface of the cytoskeleton, indicated by the square pieces of foam rubber. 

 PA and B are therefore active. The side-chain of POA is too long for such shallow 

 penetration in the cytoskeleton; it protrudes too far and its carboxyl group is out of 

 reach of the H-bond network. POA is therefore inactive. Lower left: Rear view 

 of the cytoskeleton model showing 2,4-dichlorophenoxyacetic acid and 2,6- 

 dichlorophenoxyacetic acid. 2, ID has a slender ring configuration and passes 

 deeper into the cytoskeleton than the bulky 2,6-D. The carboxyl group of 2,4-D 

 makes contact with the H-bond network, while that of 2,6-D extends ineffectively 

 above it. Lower right: Side view of model showing 2,6-dichlorophenylacetic acid. 

 While the ring is anchored in the cytoskeleton, the side-chain sticks out laterally 

 and its carboxyl group becomes part of the H-bond network. 



1l is ihc limclion ol the auxin ring lo anchor tlic molecule in the 

 cytoskeleton and thereby hold the polar grotip in plate. By means of 

 a model (Figure 2) we will see how this explains many questions of 

 auxin physiology that heretofore have remained imanswered. 



(1) Before the auxin molecule can la II in plate (solubilizc) in the 

 cytoskeleton, it must first arrive there. Since the membrane of the 

 cytoskeleton is probably a lipoprotein, the auxins nuist have a par- 

 tition coefficient favorable for partitioning into fats. It is \\cll known 



