52 University of California Publications in Agricultural Sciences [Vol. 3 



duced by rapid transpiration rather than a mere difference in osmotic 

 concentration of the solutions involved. Moreover, negative pressure 

 produced by transpiration would seem a more plausible explanation 

 of the results obtained by Chandler 11 with tomato plants and by him 

 attributed to osmosis. For as Dixon 12 has pointed out "it is quite pos- 

 sible for the solvent, water, to be in a state of tension, i.e., at a negative 

 pressure, while the dissolved substance may be at a positive pressure 

 and be active as a distending force in the cell. ' ' 



These experiments, it is believed show that, during the afternoon 

 at least, strong negative pressures exist in the water columns of citrus 

 trees under the climatic conditions here considered and that the young 

 developing fruits are deprived periodically of a part of their water 

 supply by excessive transpiration from the leaves. 



During the June-drop period of 1916 a number of holes were dug 

 in various parts of the Edison orchard to a depth of six feet and 

 moisture determinations made at various depths and on all sides of 

 the tree. The moisture content was found to range between 5%-6% 

 just before irrigation and between 10%-12% soon after irrigation. A 

 fruit grower observing this soil and the vigorous growth of the trees 

 would hardly conclude that the trees were suffering from lack of 

 water. However, the specific effect of variations in the moisture con- 

 tent of the soil on the transpiration rate and water content of orange 

 leaves has not yet been carefully determined. While there is little 

 room to doubt that the above-ground complex is more important in 

 influencing transpiration than the below-ground complex, still it is 

 entirely possible that this abnormal water deficit in the leaves and 

 fruit may be more easily induced by sudden changes in the climatic 

 complex under conditions of a deficient moisture supply in the soil or, 

 what amounts to the same thing, an inhibition of the normal absorption 

 due to lack of sufficient aeration or excessively high soil temperatures. 

 Nevertheless evidence is not lacking that marked changes in air tem- 

 perature and humidity may be sufficient to cause abscission of young 

 fruits even though the soil moisture conditions be ideal. Such appar- 

 ently is the explanation of the heavy drop of Washington Navels 

 already referred to, which occurred over most of the citrus districts 

 immediately following the excessive temperatures of June 15-17, 1917, 

 when the mercury reached 110-120° F in many parts of the citrus 

 district south of the Tehachapi mountains. 



ii Mo. Sta. Ees. Bull. 14, pi. 13. 

 12 Loc. cit. p. 140. 



