PLANT GROWTH HORMONES 6/ 



on the growth process itself. Furthermore this inhibition of growth is 

 removed by malate and other four-carbon acids, and also by pyruvate 

 and (as we have found later) isocitrate. Also, the growth of the unin- 

 hibited sections in auxin and sugar is clearly promoted by four-carbon 

 acids after some hours. It is clear then that growth is not a function of 

 respiration as a whole but of some specific part of it, i.e. of a respiratory 

 system or at least a hydrogen-transferring system, involving those 

 organic acids which participate in the Krebs cycle. 



Confirmation of this came from the study of protoplasmic streaming 

 in the coleoptile. Mrs. Sweeney and I (1938, 1942) studied this in the 

 epidermal cells, at first by direct observation with a stop watch and later 

 with a special device which provides an artificial comparison stream 

 whose rate can be adjusted to synchronize with that of the protoplasm 

 particles. The rate of streaming is accelerated by auxin. The reaction is 

 closely related to growth because : 



(a) it takes place before visible growth occurs, being clear within 

 a few minutes, 



(b) oxygen is required, as in growth, 



(c) sugar is needed for the eflfect to last, just as in growth, 



(d) the dependence on auxin concentration is parallel to that of 

 growth, 



(e) the increased rate is prevented by iodoacetate, 



(f) the auxin effect is promoted by malate, especially in old or 

 starved coleoptiles, whose sensitivity to auxin has been greatly 

 reduced. 



The parallel is thus very complete ; the effect of auxin on streaming is 

 almost undoubtedly a part of its effect on growth. 



If acceleration of growth and acceleration of streaming are due to a 

 dehydrogenation process, the effect of auxin on respiration should be 

 interesting. 



All investigators are agreed that auxin does not raise the oxygen 

 consumption of coleoptile tissue in buffer solution ; but we found that in 

 sugar (sucrose and fructose) after a few hours' soaking, auxin does 

 produce a rise of some 10% in oxygen consumption. Furthermore, if 

 the tissue is soaked in malate or fumarate, the rise is larger (about 

 22%). This effect of auxin on respiration in the presence of malate 

 closely parallels the effect on growth. 



The effect of auxin on respiration in presence of sugar has recently 

 been confirmed by Avery and Berger, though not however the influence 



