130 Barlow, Hancock, and Lacey 



(either expressed as an actual or a relative difference in length be- 

 tween treatments with and without inhibitor); at concentrations 

 above the optimum, NalAA reduces the effect of the inhibitor, e.g., 

 the average effect of the highest inhibitor dosage (27 ints/ml) over all 

 concentrations of NalAA from to 5 p. p.m. was a 32.6 per cent re- 

 duction below similar solutions without inhibitor, but in 500 p. p.m. 

 NalAA the reduction was only 21.1 per cent. Conversely, the effect 

 of NalAA (as a percentage of the response without auxin) A\as in 

 general the same at all inhibitor levels, except that at 500 p. p.m. 

 NalAA the depressing effect of the high auxin concentration was re- 

 lieved by adding inhibitor, e.g. with increasing inhibitor from 0. 1/27 

 ... 27 ints/ml the percentage differences between and 500 p. p.m. 

 NalAA were respectively— 4.4, —2.6, +1.2, 1.7, 12.9, 12.5. 13.5, and 

 8.4. In another experiment the response in 500 p. p.m. NalAA ^\•as 1.3 

 per cent above that in water, while in successive inhibitor concentra- 

 tions of 2, 10, and 50 ints/ml it was respectively 1.8, 11.3. and 11.5 

 per cent above. 



Time-course studies have been made in some detail and ^\■ill be 

 presented elsewhere; here it is sufficient to say that during the first 3 

 hrs. of growth under our conditions, there seems to be an increase 

 in length of up to 10 per cent, which is not reduced by iliis inhib- 

 itor, low temperature (1), or low oxygen tensions; after this phase 

 is over, and the sections are growing quickly, the inhibitor has a 

 rapid effect upon extension, a reduction in growth rate being detect- 

 able within 1 hr.; conversely, inhibited sections transferred to water 

 or NalAA show rapid recovery by an increased growth rate. The 

 final lengths achieved by such sections depend upon the degree 

 and period of inhibition, as shown in Figure 2. Sections were groAvn 

 in water, and inhibitor at 0.5, 1, and 10 ints/ml for 0, 3, 8, 13, and 19 

 hrs., and then washed in running water (representing about 100 times 

 the original volume of solution) and transferred to 5 p. p.m. NalAA; 

 after a further 24 hrs. final lengths had been attained. Open tubes 

 were used to avoid the complications of uncorking the tubes after 

 different times. Growth uj) to the time of transfer is shown bv the 

 basal portions of the histograms and the subsequent growth in NalAA 

 by the upper plain part; it will be seen that although the final length 

 attained falls with increasing time in any of the solutions, including 

 water, the growth made in NalAA is about the same after transfer 

 from inhibitor as from water at 3 and 8 hrs., slightly more at 13 hrs., 

 and considerably more at 19 hrs. 



A similar situation was found on transfer to 0.5 p.p.m. NalA.A, ex- 

 cept that the amount of growth made after transfer was not so great, 

 particularly from the highest iuhibitor concentration; this trend was 

 even more marked on transfer to water. 



