I06 PLANT GROWTH SUBSTANCES 



become a matter of different rates of metabolism being responsible for 

 different growth responses, such rate differences being due to dissimilar 

 hormone concentrations in various tissues. It has never been demon- 

 strated, however, that hormones occur in tissues in concentrations that 

 would inhibit metabolism. But let us look at the literature on the subject, 

 to see what evidence there may be for linking hormone-accelerated 

 respiration with growth. 



The first study of this sort is that of Bonner (6). He suspended small 

 segments oiAvena coleoptiles in solutions of an impure hormone prepara- 

 tion. At suitable concentrations the segments elongated considerably 

 (over the controls), and this speeded growth was accompanied by an 

 increase in the rate of respiration as measured by oxygen uptake in a 

 Warburg respirometer. The author concluded, "it seems possible that 

 the increase in respiration caused by growth substance may be an essential 

 part of its action in growth." Three years later Bonner (7) again reported 

 on the same general experiments, but this time carried out with improved 

 Warburg vessels and crystalline auxenolonic acid (auxin b) from the 

 Utrecht laboratories. From this work he concluded, in agreement with 

 similar results obtained at Utrecht, that auxenolonic acid failed to 

 stimulate respiration; indeed, a threefold purification of the impure 

 hormone he had used in 1933 removed most of its stimulatory power. So, 

 although growth was stimulated in the presence of the purified hormone 

 preparations, Bonner reported no increase in respiration. 



In 1940 duBuy and Olson (11) studied the respiration of single 

 infiltrated Avena coleoptiles and found that indoleacetic acid alone or 

 in the presence of fructose had no accelerating effect on respiration. 

 Sweeney (18) pointed out later that the failure to obtain accelerated 

 respiration was caused by the use of water-infiltrated coleoptile segments, 

 which were low in oxygen tension. 



In 1941 Commoner and Thimann (10), in an experiment of somewhat 

 similar design, reported a small but significant stimulation of respiration 

 in Avena coleoptile segments in the presence of certain four-carbon 

 acids, mahc and fumaric in particular. They found that the acids ac- 

 celerated growth as well as respiration if the hormone indoleacetic acid 

 was present in a concentration of i to 10 mg. per liter. When the data 

 were plotted the curves for growth and respiration were found to 

 parallel one another roughly. The authors concluded that "the four- 

 carbon acids provide a respiratory system which is part of the chain of 



