VI CELL ENLARGEMENT 797 



consisting of cellulose, hemicellulose and pectins. Green algae with similar wall 

 structure show growth promotion by auxin (Algeus, 1946), but Euglena, with an 

 elastic non-cellulosic membrane (unpublished data), and Phycomyces, with a 

 chitinous wall, are clearly unaffected (Gruen, 1956). 



The only difficulty with this view is that it does not allow for the other effects 

 of auxin, such as promotion of cell division or of respiration. The latter might of 

 course be indirect, but the action on cell division is direct and so are certain other 

 effects. There is, for instance, a marked effect on the rate of cyclosis or cytoplas- 

 mic streaming in Avena, which appears within a few minutes after indoleacetic 

 acid is applied (Sweeney and Thimann 1937, 1938; seeThimann^/a/., 1952, for brief 

 review). Physiological concentrations are optimal; the effect is dependent upon 

 oxygen and sugar, and is inhibited by iodoacetate and by dinitrophenol. The 

 inhibition by iodoacetate can be prevented by malate. Furthermore, weaker 

 auxins have a correspondingly weaker effect on streaming. The effect thus has 

 remarkable parallelism with the effects on cell enlargement, except that it precedes 

 them. Recently very similar effects in the staminal hairs of Tradescantia have 

 been reported (Turner et al., 1954) and extensive work from the author's 

 laboratory has indicated the same process in the cambium of white pine [Pinus 

 Strobus) (Thimann and Kaufman, 1958). 



Phenomena such as this suggest that the initial site of action of auxin may be in 

 enzyme systems in the cytoplasm, and that only secondarily are enzymes which 

 metabolize the cell wall affected. The role of auxin as "coenzyme" or "protective 

 agent" in an enzyme system of fundamental importance, a "master reaction," 

 seems more attractive than ever, but now it appears that this "master reaction" 

 may be one which forms enzymes rather than merely acting upon them. 



An unsolved question is whether auxin acts directly or only after conversion to 

 some active form, like, e.g. indoleacetyl phosphate or indoleacetyl coenzyme A. 

 It has been suggested (Thimann, 1956c) that the optimum curves of Fig. 7 depend 

 on such conversion, the excess of unconverted lAA at high concentrations acting 

 to antagonize the active form; however, as yet, no direct evidence exists to support 

 this suggestion. 



VII. GROWTH OF SPECIFIC ORGANS 



{a) Growth of leaves 



The center of the vegetative apical meristem contains many non-dividing, 

 vacuolate cells, and most of the activity is on the surface or tunica, particularly 

 in a ring around the flanks (see Fig. 3, p. 766) Leaf primordia arise as ovitgrowths 

 of this ring on the flanks of the meristem, each forming a group of meristematic 

 cells which curves up around the apex like a finger-nail. Underneath each growing 

 primordium, the meristematic tissue of the apex begins to produce a line of elon- 

 gated cells which form the procambium of a vascular strand, and quickly differenti- 

 ate into xylem and phloem tissue which extend up into the leaf. As the primordium 

 enlarges — more in length than in width at first — the basal part, in dicotyledonous 

 plants, becomes less active and later forms the petiole; the upper part goes on to 

 form the lamina. The most apical part of this in turn presently also becomes less 



Lllerature p. 8i6 



