1919] Cuit-Hodffson : Abnormal Shedding of Washington Navd Orange 329 



If the stimulus leading to abscission be abnormal water relations, why 

 then do not these two other members of the genus shed their fruits 

 to the same extent as the navel variety? Our observations made in the 

 field in orchards where those varieties are mixed have shown that such 

 is not the case, and experiments performed in our laboratories have 

 shown that abscission is much more easily induced in the navel variety 

 than in the others. Shoots bearing flowers and young fruits of each 

 variety have been ])laeed in moist chambers and kept at room tciiqiera- 

 ture. In the case of the navel variety abscission of all the flowers and 

 fruits has invariably occurred within sixty hours, while in the 

 Valencia variety and with lemons frequently no absci.s.sion occurred 

 within five to eight days. Apparently the navel variety is nuich more 

 susceptible to stimuli which lead to absci.ssiou. In this coiuiection it 

 seems desirable to call attention to the fact that other investigators 

 have found in the case of hybrids ab.scission is much more prevalent 

 and nuich more easily brought about than in the ease of the parent 

 varieties. Thus Goodspeed and Kendall"'' have shown that in the ea.se 

 of certain tobacco crosses in which (mly a small proportion of the 

 ovules are normally matured and capable of fertilization, which con- 

 dition obtains in the navel orange variety, practically all the fiowers 

 and young fruits are abscissed. May not this sensitiveness to stimuli 

 which cause abscission constitute further evidence that the Washing- 

 ton Xavel variety is of hybrid origin? 



Methods op Amelioration 



From the preceding discussion it is obvious that all methods of 

 preventing the June drop of our present strains of Washington Navel 

 oranges nuist be in the nature of modifying the environmental complex 

 either above ground, below frround. or. as is usually the case. both. 



If the cause underlying these water deficits lies in the asperity of 

 the atmospheric complex then practices tending to ameliorate climatic 

 conditions should work out to produce heavier crops. Such has been 

 found to be the ease. The |>hniting of windbreaks to prevent th(> 

 di.ssipation of blankets of moist air; a moderate winter i)runiug to 

 reduce the total leaf surface area ; aiul the planting of intercroi)s. such 

 as alfalfa, sweet clover, or buckwheat, which transpire large amounts 

 of water viipor; all these are methods of modifying the atmospheric 

 environiueiital complex. 



"» On the Partial Storility of Nirntiana Hvtiriils niaile with A'. siiU'rutris as a 

 Parent, III: An Accomit of tlip Mode of Floral Abscission in the F, Species 

 Hybrids, Univ. Calif. I'ubl. Bot., vol. 5 (1916), i>p. 29.V90. 



