360 PLANT GROWTH SUBSTANCES 



fertilization and are measurable whereas similar changes in a fruit such 

 as the caryopsis with a single ovule might not be measurable with the 

 techniques available. 



Determinations of auxin content by extraction procedures have identi- 

 fied bound forms and precursor forms of auxin in addition to the free 

 auxin measured by the diffusion technique. The results of experiments 

 employing extraction procedures are difficult to interpret because of the 

 uncertainty as to the auxin forms involved. Some of the investigations, 

 however, substantiate the results of the diffusion experiments and show 

 the production of free auxin beginning in the fruit after fertiUzation. 

 Laibach and Meyer (10) extracted auxin with alcohol finding small 

 amounts in pistils of Helianthus anniius before fertilization and none in 

 those of corn. After fertilization they found a sharp increase in the 

 amount of auxin in the pistils of both species. These results have been 

 confirmed for corn by Avery et al. (i) and Wittwer (21) who found 

 that the auxin content increases for 2 or 3 weeks following fertilization 

 and then decreases until maturity. Extracting the auxin from the rye 

 grain with water, alkaline phosphate buffer, and N/50 NaOH, Hatcher 

 (7) found little or no auxin until three weeks after anthesis which agreed 

 with the determinations by the diffusion technique. 



Of particular concern here are the results of the determinations of 

 auxin in the ovary prior to fertilization. In the investigations cited above 

 Httle or no auxin in the free, combined, or precursor forms was found 

 in rye or in corn grains with the exception of some assays of kernels of 

 the Country Gentleman variety of corn (i). In some experiments per- 

 formed by the writer the grains of a hybrid corn at the silk stage yielded 

 only 2 or 3 degrees of curvature by the diffusion technique, but extraction 

 of the lyophilized grains with ether containing 5 per cent water for 10 

 hours at 23° C. gave curvatures of 30 to 40 degrees per grain. With the 

 same extraction procedure high yields of auxin were obtained from both 

 fertilized and unfertilized ovaries of Nicotiana tabaciim 75 hours after 

 anthesis, the yield from the fertihzed ovaries being greater than that 

 from the unfertilized ovaries (Table 2). It has been shown (20) that the 

 yields of auxin by such an extraction are probably due to the conversion 

 of tryptophan or a similar precursor to the auxin, indoleacetic acid. 

 Prehminary experiments indicate that both fertilized and unfertihzed 

 Nicotiana pistils contain an enzyme system which can convert tryptophan 

 to auxin with remarkable facility. Thus the stimulus furnished by the 



