1919] Coit-Hodgson : Abnormal Shedding of Washington Navel Orange 293 



ever, that while a siuKlcu rise in tt'iiipcraturc may be and often is 

 aecompaDied l)y increased shedding rates, it has been observed by the 

 writers that pnifnse sliedding of the young Navel oranges takes place 

 during periods wiieu 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 discu.ssed in another place. 



Many investigators have noted the marked effect of increase in air 

 teini)eratures on the time involved in the separation process, and we 

 have noted the same phenomenon. The eifeet 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 abscissiou 

 following sudden increa.ses in temperature, as noted by several investi- 

 gators, may be easily explainetl on the gi'ound 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 teiii]ieratiire is the causative stimulus. 



L.VCK OF POI-LIN.VTION .\ND FeRTIL1Z.\TI0N 



While there is a general rule that pollination and I'ertilization is 

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

 spicuous for its exceptions. A uuinbci- of our commercially important 

 fruits, such as biiuauas, Sultanina gi'apes, Jaj)aiiese persimmons, and 

 Navel oranges, arc distinctly parthcnocarpic and do not require the 

 stimulus of pollination to insure the setting of fruits which are usuall.v 

 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 in.stead of 

 dcvcliipiiig inio normal ovules cajialilc nl' l)ciiig I'crtilized.-" 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 oceiu'riiig uudi'r 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 



2" Ikeda, T., On the Parthenocarpy of ritriis Fruits. .lour. Sci. Afir. Soc. Tokyo, 

 vol. 63 (1904). 



