VI CELL ENLARGEMENT 777 



removal or supplementation of the auxin source has an immediate effect on growth. 

 The tropisms, in which light or gravity acting from one side brings about curva- 

 ture of the seedling, constitute the best example, since direct auxin determinations 

 in the organs exposed show that the convex side receives, instead of 50% of the 

 auxin, perhaps 65-70%, — a relatively small change to be responsible for so major 

 a change in the growth pattern. 



The optimum levels of auxin for cell enlargement are of the order of 100 times 

 greater than the above. In tests with isolated coleoptile or stem sections in indole- 

 acetic acid solution the optimum concentration lies between 2 and 10 mg/1. Up to 

 the optimum, the elongation is very roughly proportional to the logarithm of the 

 concentration (see Fig. 7). The same relationship holds for the inward curvatures 

 of slit stems or coleoptiles (Thimann and Schneider, 1938). 



Growth of coleoptile tissues in auxin, in presence of certain buffers, is approxi- 

 mately linear for the first 12 h., though pea stems and especially tuber tissues show 

 a marked lag which may last from an hour to two days (see Hackett and Thimann, 

 1 952a ; Hanson and Bonner, 1 955) . Now the reciprocal of the growth rate of coleop- 

 tile sections during the first hours is roughly proportional to the reciprocal of the 

 auxin concentration, and a parallel has been drawn between this relation and the 

 Michaelis-Menten relation between enzyme concentration and reaction rate 

 (McRae and Bonner, 1953). Thus 



lal + A- 

 V S 



where V = reaction rate and S = enzyme concentration. Here the auxin corresponds 

 to the enzyme and the initial growth rate to the enzymatic reaction rate (see Bonner 

 and Foster, 1956; Bennet-Clark, ig56b). There has been much inconclusive argu- 

 ment over this type of analysis, which logically would imply that growth is a single 

 reaction. It may be worth while to point out that unless the range of concentrations 

 used is very wide it is not easy to distinguish between a double reciprocal and a 

 logarithmic relation. Thus over the range from 1.2-25, i/log n varies closely 

 parallel to K-ijn, where K = about 10. Then, too, the theory should apply only to 

 the initial growth rate, but in most tissues there is a lag after application of auxin, 

 before the growth rate reaches its full value, as noted above. Further, since initial 

 growth rate dind final length do not stand in any simple relationship to one another, 

 and the initial rate especially is greatly altered by the sucrose concentration as 

 well as that of other solutes (Bennet-Clark, 1956b) the matter is indecisive. 



Above the optimum the growth falls off drastically with relatively small incre- 

 ments of concentration. The pH of the solution determines the extent of depression; 

 in unbuffered solutions, in which the auxin is in the free acid form, concentrations 

 above 30 mg/1 are strongly inhibitory and even toxic; the sections become flaccid 

 within 10-20 h. and the contents of the cells flow out into the solution. In pea 

 stem sections it has been shown that the exudate contains reducing sugars, several 

 aminoacids and other organic materials, which makes it probable that in general 

 it is a sample of the cell contents (Christiansen, 1950). In other words the excess 

 auxin has destroyed the normal retentiveness, or semipermeability, of the cell 

 membrane. In less acid solutions, higher auxin concentrations are needed to exert 

 the same effect. The combination of auxin with a growth inhibitor such as iodoace- 



Lileralure p. 8l6 



