204 CONTROL MECHANISMS IN CELLULAR PROCESSES 



rate; as seen in Fig. 7-2, this cycle could be repeated. The lag may 

 represent the time required to deplete the tissue, by diffusion, to 

 below a practically "saturating" concentration of KCN for the af- 

 fected system, although we have not studied this extensively. The 

 rapid action of KCN when added indicates that the growth rate can 

 be affected by changes in rates of presumably enzymatic processes 

 within a very few minutes. At 12° C similar results were obtained, 

 except that the half-time for approach to the inhibited rate was 

 slightly longer, about 7 minutes. It would seem that temperature 

 effects on metabolic processes should have showed themselves within 

 the period of 12° treatment in an experiment such as that of Fig. 7-1. 

 Therefore, the change in rate of metabolism or some part of it, 

 caused by changing the temperature, must have already exerted its 

 effect on the growth rate within the first minute. It seems difficult 

 to escape the conclusion that the growth rate follows changes in 

 metabolic processes essentially immediately. If the growth rate 

 were truly controlled by a physical "plasticity," this would seem to 

 have such a short metabolic time constant as to be tantamount to a 

 m.olecular mosaic mechanism. 



Fig. 7-3 shows an experiment testing the response to depriving 

 the coleoptile section of O2. We mounted the section on the end of 

 a syringe needle of appropriate size, so as to be able to gas it in- 

 ternally with N2 at the time of changing from an aerated auxin 

 solution to one gassed with prepurified ( > 99.996 per cent) Ni-; the 

 growth chamber was closed and was flushed all the while with N2, 

 so that the section had to obtain all its Oi> from the auxin solution. 

 After transfer to 02-f ree solution ( and brief gassing of the section ) , 

 there always followed a period of about 10 minutes before any pro- 

 nounced effect on the growth rate occurred; then it rapidly fell to 

 an inhibited value (complete inhibition did not occur). Upon re- 

 turn to 02-containing medium, response was very rapid, the original 

 rate being recovered in about 3 minutes. This behavior is the 

 opposite of that with KCN, and it may reflect the fact that depletion 

 and entry kinetics are just the opposite in the two cases. In any 

 event the rapid response on return to O2 shows clearly how rapidly 

 the growth rate responds to change in the rate of some metabolic 

 process and seems to reinforce the conclusion reached above. 



