29-i University of California Publications in Agricultural Sciences [Vol. 3 



Navel orange which would interfere with the germination of pollen or 

 the normal extension of the pollen tube. The exclusion of pollen by 

 the bagging method has shown that in setting fruit the Navel orange 

 is entirely independent of pollen. This experimental evidence is borne 

 out by the practical experience of growers who secure as abundant 

 crops from large isolated plantings of Navels as from mixed plantings. 

 It is therefore entirely safe to conclude that lack of pollination and 

 fertilization of the Navel orange does not result in the stimulus leading 

 to abscission. 



Relative Position on Stem 



There is some variation in the relation borne by orange fruits to 

 the main supporting axis. As it has been suggested that with some 

 other plants this relation largely determines whether a given fruit 

 will be able to persist, it was thought worth while to investigate the 

 importance of this point in connection with oranges. A large number 

 of fruits were examined and divided into two classes : those which 

 terminated the axis, and those which did not. These two classes are 

 well illustrated in plate 33. It seems reasonable to suppose that in the 

 case of the non-terminals, an organ of limited secondary thickening 

 (the pedicel) being in competition with one of unlimited secondary 

 thickening (the main axis) might suffer from an increasing prejudice 

 to its water supply. It was found by counts of large numbers of 

 fruits that the ratio of terminals to non-terminals was 5 to 6. The 

 new current season's growth which bore terminal fruits averaged 3.8 

 leaves per shoot, while the non-terminals averaged 3.95 leaves per 

 shoot. In the latter case 1.85 leaves were below and 2.1 leaves above 

 the fruits. Counts of fruits which had successfully survived the 

 abscission period showed on one tree 16 terminals to 31 non-terminals, 

 but on another tree 25 terminals to 14 non-terminals. Counts of 

 dropped fruits also failed to support the above supposition, and it is 

 evident from our examination of large numbers of specimens that 

 abscission in this case is quite independent of such differences in the 

 relation of fruit to axis as is shown in plate 33. 



The Gas Factor 



It has long been recognized that the subjection of certain plants to 

 an atmosphere containing traces of various narcotic or poisonous gases 

 is sufficient to cause abscission of leaves and other plant parts. One 

 of the first indications of smelter fume injury is the shedding of the 

 leaves of certain plants due to the presence of sulfur dioxide, which is 



