FRUIT FORMATION 485 



that are nearly perfectly formed. The more defective pistils drop earliest, but 

 all flowers come into full bloom. Flowers with defective pistils always drop at 

 the pedicel base and neither the calyx tube nor the style is shed by abscission 

 because growth is not carried far enough. The immediate cause of the dropping 

 is the abortion of pistils that are structurally defective and cannot function. 



The Second Drop. — "The first drop is followed 2 weeks or so after bloom by 

 another distinct wave of falling pistils. While there are a few intergrading 

 forms between these two drops, certain features of the second drop separate it 

 distinctly from the first. Unlike the pistils of the first drop, those of the second 

 have every external appearance of being normal. Enlargement up to a certain 

 point takes place and in most cases the calyx tube breaks away at least in part 

 even though there is insufficient growth in the young plum to throw it off. The 

 style is not deciduous in the earliest pistils to fall, but, like the calyx tube, drops 

 in those which fall later. . . . Pistils which fall in the second drop, as in the 

 first, absciss at the pedicel base while the pistil is still green, although the pedicel 

 has become light yellow. Yet in the last pistils of the second drop to fall the 

 abscission layer is formed at the base of the ovary and in some instances can 

 be easily broken off at this point. . . . 



"Emphasis is placed upon the following points. . . : (a) the period of 

 abscission of the second drop extended from 17 to 30 days after bloom; (b) 

 beginning with the first pistils to fall, size differences between those persisting and 

 those which fell, gradually increased with time; (c) pistils which fell within the 

 above-mentioned time limit enlarged only up to a certain point ; ((/) those pistils 

 with the stigmas snipped before pollination, enlarged before falling, to a size 

 comparable with that of those not so treated; and (e) in each variety there was 

 a gradual increase in the size of the pistils which fell off. . . . 



"The condition found in the unfertilized series is in marked contrast with 

 that found when fertilization takes place. As early as 18 days after bloom the 

 embryo sac in which the egg has been fertilized extends the entire length of the 

 nucellus to the chalaza, and a jacket of endosperm, usually only one cell thick, 

 covers the entire area of the 'dumb-bell-shaped' sac. With the completion of 

 these changes in the embryo sac the embryo may be no larger than four cells 

 across. . . . 



"It will be seen from the above observations that all the evidence shows that 

 fertihzation has not occurred in the pistils which fall at the second drop. . . . 

 Pollination may have taken place, but tube growth was retarded to such an 

 extent that fertilization was prevented probably by the abscission of the style." 



The Third Drop or June Drop. — "Following the second drop there is still 

 another — the so-called 'June drop.' In popular usage the term June drop 

 applies primarily to the third drop of large plums because they are much more 

 conspicuous, but does not include the relatively few which fall from time to time, 

 even up to maturity ... It has been shown that time and size of dropping draw 

 a relatively sharp line between the first and the second waves of dropping. Like- 

 wise these two factors separate the second drop from the third. . . . When 

 fertilization does not take place enlargement reaches only a certain point, the 

 maximum recorded being in the 5.6 to 6.0 milUmeter class, while the mode is 

 near 3.0 millimeters. Among the last of the second drop an occasional ovule is 

 found with slight embryo development, which shows that there are connecting 



