VI CELL ENLARGEMENT 795 



also would increase the permeability to solutes. Parts of the chemical argument 

 have been criticized (Thimann, 1951a) and the views originally expressed have 

 been somewhat modified, but Veldstra (1956) still interprets auxin action in 

 essentially physical terms as acting "at a functioning macromolecular surface and/ 

 or interfaces in the cell". 



With the discovery that auxin had actions other than that on cell enlargement, 



notably to promote cell division in cambial and 

 other tissues, and to inhibit enlargement in 

 lateral buds (section Vlld) attention became 

 focussed on some more fundamental action, a 

 "master reaction" from which various ob- 



/ \ \ served effects might follow according to the 



I'liiii' " '^ nature of the cell acted on and the other 



materials available {cf. Audus, 1954). This 

 Fig. 10. Veldstra's conception of the viewpoint became, indeed, essential when it 

 orientation of the ring of the auxin developed that auxin acts on respiration, and 

 molecule in a lipoid membrane, and * 



the projection of the carboxyl dipole "ow we know that many phases of metabolism 

 at an angle to it. may be influenced. 



As far as cell enlargement is concerned, 

 the argument in the preceding section has focused attention on the cell-wall. 

 This might, of course, be considered as being extended passively by the entering 

 water if it could be shown that auxin increases the osmotic concentration 

 in the cell. However, we have seen above that the reverse is the case. The 

 wall might also be extended passively if there were an active water-secreting 

 mechanism which used metabolic energy to "pump" water into the cell. 

 This possibility has been discussed before (Thimann, 1951b, 1954a) and it is 

 enough to say here that it would necessitate the "pump" competing with 

 the natural permeability of the cell wall to water molecules. Recent experi- 

 ments with isotopically labeled water virtually rule out this possibility. For 

 example, using tritium water (Thimann and Samuel, 1955) the water permeabil- 

 ity of the walls of potato tuber tissue was found to be so great that the tritium 

 water reaches 50°o equilibrium with ordinary water throughout a i mm thick 

 section in less than a min. Furthermore, auxin treatment increases this permeability 

 still further. Coleoptile sections, measured with deuterium water, show similar 

 though less extreme behavior (Ketellapper, 1952; Buffel, 1952; Ordin and Bon- 

 ner, 1956); the permeability increases markedly with decreasing hydrogen ion 

 concentration below pn 4.5. Ketellapper (1952) concluded that auxin did in- 

 fluence water permeablility, but this would let water out as much as in, and 

 cannot therefore explain growth promotion. The critical point is that the 

 permeability is already too high to be a limiting factor. It seems certain, there- 

 fore, that no pumping or secretion mechanism could operate against so "leaky" 

 a membrane. 



The possibility that water might be taken up by the same mechanism as that 

 which absorbs ions, i.e. as H"" and OH~ ions, is eliminated by the mere fact that 

 the respiratory energy is insufficient. During 4 days, i g of potato disks in auxin 

 takes up 20 millimoles of HjO while the entire respiration uses only one half a 



Literature p. 816 



