TECHNIQUE OF AUXIN DETERMINATIONS 25 



It will be seen that after the first 5-hour period the growth 

 rate of the decapitated plants has increased again. If now 

 the uppermost zone of these plants was cut ofT and placed 

 upon freshly decapitated stumps, the growth of such stumps 

 was accelerated, just as by tips. Other cylindrical zones of 

 the coleoptile, similarly placed upon freshly decapitated 

 stumps, had no such effect. Thus the uppermost zone of 

 plants which have been decapitated for some time behaves 

 Hke a tip in that from it diffuses a substance which accel- 

 erates the growth of the stump. This is the "regeneration 

 of the physiological tip," and, as we shall see later, it ex- 

 plains the earlier observation of Rothert (1894) that tropis- 

 tic sensitivity returns some hours after decapitation. 



The regeneration was studied in greater detail by Dolk 

 (1926), from whose paper the curve of Figure 7 is taken. It 



2 4 6 d 10 12 14 16 18 20 hrS. 



Fig. 7. Growth rate of Avena coleoptiles after decapitation. Ordinate, mm. 

 elongation per hour; abscissa, time from decapitation in hours. (From Dolk, 

 Proc. Kon. Akad. Wetensch., Amsterdam, 29: 1113-1117, 1926.) 



will be seen that immediately after decapitation the growth 

 rate begins to fall, but after 2}/^ hours (at 21° C) a sudden 

 break in the curve occurs and the growth rate rises to about 

 50 per cent of that of the intact plant. This sudden rise 

 can be completely ehminated by a second decapitation 

 2 hours later, in which the uppermost 1 mm. of the stump 

 is removed (Dolk, 1930). This proves that the rise is really 

 due to regeneration of the production of growth hormone 

 in the uppermost zone of the stump. It may be pointed 

 out that this regeneration is not accompanied by any mor- 

 phological change in the uppermost cells of the stump 

 (Tetley and Priestley, 1927; Perry, 1932). 



