K. V. THIMANN 



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same rans^e of concentrations and to the same degree as it inhibits 

 respiration. Recently, however, certain dehydrogenase inhibitors 

 have been found to inhibit growth at concentrations much lower than 

 those which are effective on respiration. lodoacetate is particularly 

 effective, inhibiting all growth in coleoptile sections at a concentration 

 of 5 X 10~^ M, a concentration which produces only a very slight 

 decrease in respiration rate (2). This growth inhibition is completely 

 removed by adding malate, fumarate, or to a lesser extent succinate 

 or pyruvate. The four-carbon acids, in fact, accelerate growth some- 

 what over that of controls, in presence of auxin and sugar. Thus a 

 typical respiratory system appears to exercise control over growth. 

 Also, in the presence of malate, and particularly if the sections are 

 soaked beforehand in solutions of malate, the addition of auxin causes 

 a definite small increase in respiration rate. It is of importance also 

 that the auxin concentrations which are effective in stimulating res- 

 piration closely parallel those concentrations at which growth is 

 affected. From the fact that growth can be completely inhibited with 

 only a small decrease in respiration it follows that growth is not con- 

 trolled by the respiration process as a whole, while the influence of 

 iodoacetate and the four-carbon acids suggests that the dehydrogena- 

 tion of the four-carbon acids is a controlling process in growth. Evi- 

 dently this system accounts for only a small part of the total respiration 

 of the coleoptile tissue. 



Striking support for these conclusions comes from a study of the 

 effect of auxin on the rate of protoplasmic streaming. The long 

 epidermal cells of the coleoptile are particularly favorable for measure- 

 ments of streaming rate, which, under controlled external conditions, 

 is nearly constant and reproducible. When auxin solution is applied 

 to the coleoptile, there is an immediate rise in the streaming rate, 

 which, however, if no other materials are present, returns to normal 

 after ten to twenty minutes. The presence of sugar allows the in- 

 creased rate to be maintained for several hours. Oxygen is necessary 

 for the reaction; and if both oxygen and sugar are available there is 

 a good parallel between the concentrations of auxin which accelerate 

 streaming and those which accelerate growth. Since, also, the 

 increase in rate of streaming occurs before any increase in growth rate 

 can be detected, it is reasonable to conclude that the effect on stream- 

 ing is one of the first stages in the process, caused by auxin, which 



