1919] Coit-Hodgson : Abnormal Shedding of Washington X<ni I 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 these 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 placed in moist chambers and kept at room tempera- 

 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 abscission occurred 

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

 susceptible to stimuli which lead to abscission. In this connection it 

 seems desirable to call attention to the fact that other investigators 

 have found in the case of hybrids abscission is much more prevalent 

 and much more easily brought about than in the case of the parent 

 varieties. Thus Goodspeed and Kendall 66 have shown that in the case 

 of certain tobacco crosses in which only 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 flowers 

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

 which cause abscission constitute further evidence that the AA r ashing- 

 ton Navel variety is of hybrid origin? 



Methods of Amelioration 



From the preceding discussion it is obvious that all methods of 

 preventing the June drop of our present strains of Washington Navel 

 oranges must be in the nature of modifying the environmental complex 

 either above ground, below ground, 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 case. The planting of windbreaks to prevent the 

 dissipation of blankets of moist air; a moderate winter pruning to 

 reduce the total leaf surface area ; and the planting of intercrops, such 

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

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

 environmental complex. 



«e On the Partial Sterility of Nieotiana Hybrids made with .V. sylvestris as a 

 Parent. Ill: An Account of the Mode of Floral Abscission in the F, Species 

 Hybrids, Univ. Calif. Publ. Bot., vol. 5 (1916), pp. 293-99. 



