I20 Journal of Agricultural Research ' voi. xvii, No. 3 



base becomes very distinct in some varieties, as Winnipeg (P. nigra), as 

 early as 8 days after blooming. In this variety at the 8-day period the 

 cells in the abscission layer had reached an advanced stage in their 

 disintegration, and while the style was still persistent, it was much 

 lighter in color above the point of abscission, a condition which suggests 

 the cutting off of food material. If the pollen tube had not passed the 

 abscission layer by this time, it is probable that it would not have done so, 

 since it would have had to grow through a region of partly disintegrated 

 cells. Consequently all tubes which had not passed the abscission layer 

 by the time of the abscission of the style (Pi. 14, F) would be definitely 

 eliminated as far as fertilization is concerned. Tube growth from the 

 stigma to the abscission layer, therefore, must take place between the 

 beginning of receptiveness and the shedding of the style. 



If pollination occurs late in the receptive period, the condition of the 

 stigma begins to change so rapidly that only favorable growing conditions 

 for the tubes will enable them to pass the abscission layer before the 

 style drops. In this way the abscission of the style sets a definite time 

 limit to a certain minimum of tube growth which may be as short as 4 

 days and as long as 1 2 . It will be clear then that the later in the receptive 

 period pollination takes place and the more tube growth is retarded, the 

 more uncertain fertilization becomes. 



RATE OB' POLLEN-TUBE GROWTH 



It will be seen from the above that the rate of pollen-tube growth 

 becomes an important factor in fertilization, especially during unfavorable 

 weather accompanied by rain and low temperatures. In order to de- 

 termine the rate pollen tubes advance down the style, this point has been 

 studied in fixed preparations of pistils taken under orchard conditions 

 and also from controlled crosses in the greenhouse where the time of 

 pollination could be determined definitely. The greenhouse temperature 

 during this experiment was not recorded, but varied from 55° to 65° F. 

 Pistils from the orchard in all cases were collected after a period of vari- 

 able weather of rain and low temperatures. The results showing the 

 extent of tube growth under different conditions are presented in Table 

 III. 



Sandsten (14), in determining the time required for the pollen tube to 

 reach the ovary, cut the pistils off controlled crosses at interv^als of 48 

 and 60 hours, respectively. From the data he presented it appears 

 that the plum is fertilized at the 60-hour period. It should be stated, 

 however, that this shows that tube growth had merely extended below 

 the point at which the style was cut in that time. The 7-day period, 

 at which time the final observations were made, is too soon to determine 

 certainly whether fertilization has taken place judging from size alone. 



