PLANT HORMONES AND GROWTH 



results in increased cj,ro\vth. Fnrther examination was therefore 

 made of the relationship between streaming and respiration. 



lodoacetate, in the concentration which inhibits growth, com- 

 pletely inhibits the acceleration of streaming by auxin, and the in- 

 hibition, as with respiration and growth, is removed by the addition 

 of malate. Malate alone has no detectable effect on the streaming 

 rate; but in the presence of auxin it has the following very significant 

 effects. After coleoptiles reach a certain age (about five days at 

 25° C. in weak red light), they no longer show any acceleration, either 

 of growth or of streaming, when auxin is added. But when malate 

 is added, auxin is able to produce its normal acceleration of streaming 

 in this old material. Similarly, if young sections are given prolonged 

 soaking in water they behave like old coleoptiles, and here too the addi- 

 tion of malate restores the sensitivity of streaming to the influence of 

 auxin (4). Apparently, therefore, aging or prolonged soaking in 

 water depletes the coleoptile of its store of malate or other four-carbon 

 acids. 



From these observations it can be concluded that there is a 

 special part of the respiratory system whose activation results in growth. 

 This growth is probably brought about in the first instance through 

 acceleration of the rate of protoplasmic streaming. Finally, the system, 

 involving streaming, a part of the respiration, and growth, is mediated 

 by the four-carbon acids, and auxin apparently acts as an essential 

 coenzyme. It should be added that some unpublished experiments 

 with other materials strongly indicate that this growth-controlling 

 system is present in similar form in materials quite other than the 

 coleoptile. 



It will be seen that, while a good deal of information is now 

 available about the mechanism of auxin action, and therefore the 

 mechanism of growth, the interrelationships between the various 

 factors and processes are by no means clear. How, for instance, can 

 a process of respiration be linked to one of growth? 



It is important to consider first what constitutes "growth" in 

 its simplest form. To a zoologist growth is commonly thought of in 

 terms of cell multiplication. In the oat coleoptile, however, the 

 number of cells reaches its final value at an early age and thereafter 

 growth is by cell elongation only. Such a situation, with modifica- 

 tions, is common in plant tissues. Furthermore, a coleoptile section 



