70 KENNETH V. THIMANN 



2. This acceleration causes increased intake of solutes. 



3. The energy for the accumulation of solutes and for the stream- 

 ing is furnished from a sugar breakdown which involves 

 phosphorylation and in which the Krebs cycle participates ; one 

 of the key enzymes here is of the sulfhydryl type. 



4. The intake of solutes is at once accompanied by intake of water ; 

 growing cells do not have any higher osmotic pressure than 

 non-growing ones. 



5. The acceleration of streaming causes also a difference in the 

 rate of wall deposition, i.e. a change in plasticity. 



6. The increased water intake coupled with the change in wall 

 properties means increased cell size — in other words, growth. 



In this way one can begin to see how auxin can have the multiple 

 effects mentioned at the outset. For a cell may not be in a condition to 

 enlarge directly, due either to secondary wall formation or to its un- 

 favorable situation in other tissues, yet the increased streaming rate and 

 increased uptake of solutes may in some way activate numerous proc- 

 esses, causing cell division and leading to root initials and the like. 

 Such a tissue would accumulate organic solutes too, thus providing some 

 basis for the mobilization phenomena which, as was stated above, are 

 probably secondary and not causal. 



This scheme is still largely hypothetical but there is enough circum- 

 stantial evidence to make one believe that it is at least somewhere near 



the truth. 



Note Added in Proof 



The critical role of a sulfhydryl enzyme in growth has been confirmed (Thimann, 

 K. v., and W. D. Bonner, Jr., Amer. J. Bot., 35, 271-281, 1948; and in press). The con- 

 version of tryptophane to auxin has been clarified (Wildman, S. G., M. Ferri, and J. 

 Bonner, Arch. Biochem., 15, 131-144, 1947) and shown to occur in the living leaf. In 

 spite of these and many other recent results, the exact biochemical role of auxin in 

 growth remains elusive. 



REFERENCES 



Albaum, H. G., and B. Commoner. Biol. Bull., 80, 314-323. I94i- 

 Albaum, H. G., and B. Eichel. Amer. J. Bot., 30, 18-23, 1943. 

 Algeus, S. Bot. Notiser (Lund), 129-278, 1946. 



Avery, G. S., J. Berger, and B. Shalucha. Amer. J. Bot., 28, 596-607, 1941. 

 Averyi G. S., J. Berger, and R. O. White. Amer. J. Bot., 32, 188-191, 1945- 

 Berger, J., and G. S. Avery, Jr. Amer. J. Bot., 31, 11-19, 199-203, I944- 

 Bonner, J., and S. Wildman. Grozvth, Supplement, Vol. 10, 51-68, 1946. 

 Colowick, S. P., M. S. Welch and C. Cori. /. Biol. Chem., 133, 359-373 J 



