HORMONAL REGULATION OF PLANT CELL GROWTH 



205 



480 



CO 

 O 



cr 

 o 



360 



5 

 o 

 cr 



240 



120 



Q 







20 



40 60 



TIME MIN. 



80 



100 no 



Fig. 7-3. Effect of transfer from aerated auxin solution to auxin solution 

 gassed with prepurified N:2 {first arrow), and back to aerated auxin solution 

 (second arrow). These transfers repeated at third and fourth arrows. Coieoptile 

 section 9 mm long. 



Regulation of the Growth Rate 



This technique can be used to study other aspects of the growth 

 process and its control by auxin. One fact which was clearly estab- 

 lished was the time required to get a change in growth rate after 

 auxin is added. When transfer is made between water and 3 mg 

 per liter indoleacetic acid, the previously prevailing rate of growth 

 (usually 2-3 divisions per minute) persists for from 10 to 15 min- 

 utes, and then a rapid increase in rate occurs, usually reaching a new 

 steady rate, 2-4 times greater than previously, within 4 to 6 minutes. 



Another point we have examined is osmotic inhibition of growth. 

 We first follow the growth rate in water or auxin, then transfer 

 stepwise to increasing concentrations of mannitol (or mannitol plus 

 auxin), waiting each time until a new steady rate of growth is es- 

 tablished. The response is usually rather rapid (3-6 minutes). 



