1919] Coit-Hodgson : Abnormal Shedding of Washing to it Xarrl Orange 293 



ever, that while a sudden rise in temperature may be and often is 

 accompanied by increased shedding rates, it has been observed by the 

 writers that profuse shedding of the young Navel oranges takes place 

 during periods when no sudden changes or abnormally high tempera- 

 tures occur. It has also been noted that abscission of the interior 

 and well shaded fruits takes place simultaneously with that of fully 

 exposed fruits. It is altogether unlikely, therefore, that the June drop 

 can be explained on these grounds alone. The relation between 

 abscission and tissue temperatures as affected by water deficits will 

 be discussed in another place. 



Many investigators have noted the marked effect of increase in air 

 temperatures on the time involved in the separation process, and we 

 have noted the same phenomenon. The effect of course, as would be 

 expected, is an acceleration conditioned by the magnitude of the tem- 

 perature change. It appears therefore to the writers that abscission 

 following sudden increases in temperature, as noted by several investi- 

 gators, may be easily explained on the ground that the stimulus to 

 abscission had been activated at some time prior to the sudden change 

 in temperature, and the acceleration of the abscission process, produc- 

 ing marked results in a comparatively short period, has led them to 

 believe that the change in temperature is the causative stimulus. 



Lack of Pollination and Fertilization 

 While there is a general rule that pollination and fertilization is 

 essential to the setting and development of fruits, the rule is con- 

 spicuous for its exceptions. A number of our commercially important 

 fruits, snch as bananas, Sultanina grapes, Japanese persimmons, and 

 Navel oranges, are distinctly parthenocarpic and do not require the 

 stimulus of pollination to insure the setting of fruits which are usually 

 seedless. The Navel orange does not produce viable pollen, and pollen 

 from other varieties will only occasionally accomplish fertilization for 

 the reason that nearly all of the embryo sacs disintegrate instead of 

 developing into normal ovules capable of being fertilized. 20 Occasion- 

 ally a few normal embryo sacs may be produced and seeds result pro- 

 vided the particular fruits having the normal embryo sacs happen to 

 be pollinated with viable pollen from congenial varieties. It is the 

 remoteness of the chance of this occurring under ordinary field con- 

 ditions that accounts for the comparative seedlessness of these fruits. 

 Apparently there is nothing in the structure of the blossom of the 



20 Ikeda, T., On the Parthenocarpy of Citrus Fruits, Jour. Sci. Agr. Soc. Tokyo, 

 vol. 63 (1904). 



