232 B. A berg 



sections as compared with the water controls. ^Ve have observed the 

 same phenomenon with oat first internodes. At very low concentra- 

 tions, lAA depresses the elongation of the sections below the value 

 obtained in bufler plus sucrose alone. The effect is small, and some- 

 times hard to justify statistically. However it does occur time and 

 again. In fact it is the factor which prevented us from increasing 

 the sensitivity of the first internode test beyond 0.3 to 1 /xg. of lAA 

 per liter. 



Dr. Aberg: As yet I have not observed the inhibition described 

 by Dr. Nitsch for Avena coleoptile sections grown in very low con- 

 centrations of lAA, but further experiments will be made. An expla- 

 nation is not easily given, but I want to point to the negative after- 

 effects of lAA applications which have repeatedly been found (Aberg, 

 Ann. Rev. Plant Physiol. 8: 168, 1957; Osborne, Plant Physiol. 33: 

 46-57, 1958). If the lAA amount is very low, it may rapidly disap- 

 pear from the medium, and the negative after-effect may come to 

 the fore. Possibly the final growth result might then indicate an in- 

 hibition. This tentative explanation seems to gain considerable sup- 

 port from the data of Barlow, Hancock, and Lacey (Ann. Bot. 21: 

 257-271, 1957) which show that a low lAA concentration (0.01 p.p.m.) 

 may stimulate the growth of wheat coleoptile sections during the first 

 hours, even if the final result after 20 hrs. is a slight growth inhibi- 

 tion. 



