35^ PLANT GROWTH SUBSTANCES 



extracts of green, unripe anthers gave much higher auxin yields than 

 similar extracts of yellow, ripe anthers, and extracts of mature pollen 

 grains did not contain active hormones. By grinding the mature pollen 

 grains with glass and acidifving the mLxture before extraction with 

 chloroform, auxin is obtained from the pollen of Xicotiana tabacum, 

 AmirThinum majtis. Cyclamen persicum, and Datura suaveolens (15). 

 L sually more auxin is obtained if the pollen grains are germinated on 

 I per cent agar containing sucrose before extraction, and if hydrolysis 

 with i.o N NaOH precedes extraction of the ground pollen grains, the 

 yields of auxin are uniformly high. The poUen of all plants probably 

 contains auxin, but it may be present in variable amounts as free, bound, 

 or precursor forms, and in some species several auxins may be present. 

 The extracdon procedures used are responsible for the unsuccessful 

 attempts to obtain auxin from pollen which have been reported. 



TTie gro\M:h hormones of the pollen are not the principal hormones 

 involved in the enlargement of the ovar}' as is shown by a comparison 

 of the amount of auxin present in the ovar\' after fertiHzation and 

 the amount present in the pollen of a normal pollination. The as- 

 say of auxin in the ovary can be accompUshed by placing the tissue 

 on an agar block and allowing the auxin to diffuse into the agar for 2 or 

 3 hours. Numerous determinations of diffusible or free auxin in pistils 

 of Nicotiana tabacum have shown that relatively small quantities are 

 present at full anthesis (14). Similar results are obtained with pistils of 

 Aruirrhinum and corn. Following pollination in Nicotiana tabacum an 

 increase in diffusible auxin is detected in the style accompanying the 

 penetration of the pollen tubes and when fertilization occurs there is a 

 marked increase in the amount of diffusible auxin in the ovary. The 

 amount found in the style is 30 times the maximum amount obtained 

 by extraction of the pollen and the amount in the ovary is 100 times as 

 great (15). Thus the stimulus for fruit development provided by the 

 pollen is something other than the growth hormones which it contains. 



TTie high level of diffusible auxin content is maintained in the ovary 

 oi Nicotiana for at least 45 hours following fertilization although a down- 

 ward movement from the ovary to the stem must occur since considerable 

 amounts of auxin can be diffused from the pedicel alone (14). This auxin 

 inhibits the development of the abscission layer (ii,i6j and thus one of 

 the primary requirements for the development of the fruit is fulfilled. 



