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



ing between 50-100 per square millimeter as compared to 300-450 per 

 square millimeter on the leaves. Measurements of the leaves situated 

 within six inches of the fruit showed that, in addition the leaf area 

 immediately behind the young growing fruit is larger than the area of 

 the fruit until it reaches approximately two inches in diameter, after 

 which falling of the fruit is comparatively rare. Therefore, it seems 

 highly probable that the transpiration of the fruit as compared to 

 that of the leaves situated immediately behind it is an almost negligible 

 factor and it appears reasonably certain that either water is actually 

 drawn back or that the normal supply is decreased. 



Considering these two possibilities, the first merits more considera- 

 tion as it is supported by proof which, though not absolute, is at least 

 presumptive evidence of a strong enough character ; while on the other 

 hand the second possibility, agreeing though it does with the most 

 recent theory on sap movements in plants as put forth by Dixon, is 

 still a theoretical consideration. According to this theory, which pos- 

 tulates strong tensions existing in the ascending water columns, no 

 assumption of an actual reversal of the current is necessary in order 

 to explain a decrease in moisture content. During normal conditions 

 the relation between the tensions existing in the water columns leading 

 to the fruits and those leading to the leaves is such that both organs 

 receive an adequate water supply. The tension existing in any one 

 of these water columns is a function of the transpiring force existing 

 in the transpiring plant organ as modified by atmospheric conditions. 

 Therefore, as these transpiring forces vary, the tensions vary. Trans- 

 piration from the leaves, for reasons pointed out above, is subject to 

 much greater variation than that from the fruits. Therefore during 

 periods when evaporation is greatly accelerated the tensions in those 

 water columns leading to the leaves are greatly increased and as a 

 consequence more water is drawn to them. As the source of supply 

 in the conducting system is practically constant, the amount in the 

 fruits is thereby reduced and this results in a decrease of relative 

 water content of a magnitude conditioned by the transpiration of the 

 fruit. 



However, it should be noted that the data in table 1 show a 

 decrease in absolute water content of the fruit of 15% to 20%, a loss 

 of considerable magnitude. There are only two ways in which such 

 a decrease in absolute water content can take place: (1) the water 

 is lost by transpiration from the fruit, or (2) it is drawn back by 

 the leaves. But since the fruit possesses a very small stomatal area 



