1919] Coit-Hodgson : Abnormal Shedding of Washington Navel Orange 321 



the normal fruits average somewhat lower in temperature than the 

 air, and in turn those destined to drop are somewhat higher in tem- 

 perature than the air. Fruits permitted to suffer for lack of water 

 show a temperature approximately that of the air surrounding them. 

 It may be that increase in temperature due to water deficits is the 

 ultimate stimulus to abscission, still it should be pointed out that the 

 increases in temperature as recorded by us are of a much smaller 

 magnitude than the daily range in temperature changes. We are 

 fully aware, of course, that strictly accurate temperatures of plant 

 tissues can only be obtained by thermo-electric means, the mercury 

 thermometer being too subject to fluctuation and variation for very 

 delicate work. 



Factors Operative in Causing Water Relation Strains 



It is of course obvious that, given a plant transpiring a certain 

 amount of water vapor daily, unless there be a sufficient water supply 

 in the soil within reach of the absorbing roots to make up for that lost 

 by the plant and in addition supply enough for its metabolic processes, 

 water deficits of the kind mentioned must eventually occur. That 

 under these conditions such do occur and that they are followed by an 

 abnormally severe .shedding of the young fruits when in the critical 

 period, is the observation of the authors and the experience of many 

 growers. In the season of 1916 the junior author had under observa- 

 tion a ten-acre block of orange trees in the Oroville district which 

 had been top worked to the Washington Navel variety five years 

 previously. They bloomed very heavily and set an excellent crop. 

 Through an accident to the irrigation system preventing a sufficient 

 supply of water these trees were allowed to suffer for lack of water 

 at the time when the young fruits were about one centimeter in 

 diameter. At the time of irrigation several days later the fruits had 

 not fallen and it was hoped that the crop could be saved. Within a. 

 week practically every fruit was shed, although the trees looked well 

 and had entirely recovered from the drought. 



Observations, confirmatory in every respect to those given above, 

 were made on a row of trees at the Kellogg place in 1917. These trees 

 were permitted to suffer for lack of irrigation. Although the only 

 trees in the row which at the time bore fruits in the critical stage were 

 of the Valencia variety, which variety is much less subject to shedding 

 than the Washington Navel, still within a week after the application 

 of the water many of the young fruits had fallen. The desirability of 



