232 Auxins in Agriculture 



fruits may be seen in the case of pears. Bartlett pears set fruit in re- 

 sponse to auxin sprays and consequently an increased set is obtained 

 from such treatment. On the other hand Anjou and other pear varieties 

 do not set fruit parthenocarpically and are thinned by auxin sprays 

 (Griggs et al, 1951). 



THE PHYSIOLOGICAL BASIS FOR FLOWER AND 



FRUIT THINNING 



The thinning of flowers and fruits by auxins appears to be a func- 

 tion of three physiological factors. These are (1) the thinning of flowers 

 by the prevention of natural pollination, (2) the abortion of young 

 embryos and a consequent dropping of the fruits, and (3) the direct 

 forcing of abscission by alteration of the auxin gradient at the abscis- 

 sion zone. 



100 



100 MG/L 



CONG. OF AUXIN 



Fig. 98. The extent of incompatibility of pollen from untreated Kingston Bitter 

 apple flowers when applied to styles of Crawley Beauty flowers which had been 

 sprayed with various concentrations of naphihaleneacetic acid (data of Luckwill, 

 1953). 



