122 Journal of Agricultural Research voi. xvn, no. 3 



The time of pollination is uncertain^ but bees were present in large 

 numbers on May 20. On a single stigma of Minnesota No. 35 there were 

 162 pollen grains, mostly embedded in the stigmatic fluid. There 

 were fewer grains on the stigmas of Minnesota No. 21. In the field 

 records, made at the time of fixing this material, it was stated that the 

 "stigmas were brown in all cases and dead in some." From this it will 

 be seen that the receptive period was much shorter than is common in 

 the plum. The condition, then, in these two varieties was as follows: 

 (i) Pollination had taken place, (2) on the third day after bloom no 

 tubes had formed in the stigmas examined, and (3) the end of the recep- 

 tive period had been reached. 



On each variety the dying back in the styles averaged 5 mm. by 

 May 31, and by June 2, 13 days after bloom, the abscission layer was 

 fully formed and disintegration of the cells in it had started. On this 

 date additional pistils were collected and fixed, and in these pollen tubes 

 could not be found in the micropyle, nor had embryos formed in any 

 of the six which were sectioned. This is not surprising when it is noted 

 that under the favorable conditions of the greenhouse, Surprise pollen 

 tubes required six days to grow the full length of the style. 



These trees under observation were" 8 years old from planting and were 

 under clean cultivation. On Minnesota No. 21, 25 per cent of the 

 buds were winterkilled and only 5 per cent of the flowers set fruit; on 

 Minnesota No. 35, 10 per cent were winterkilled and the percentage of 

 fruit to set was 10. On each there was a light crop of ripe fruit. 



In the case of these two varieties, then, the small percentage of fruit 

 to set is not necessarily due to a lack of pollination, but apparently to 

 the delay in tube formation, during which the stigmas turned brown 

 and some died, conditions which either prevented or delayed tube 

 growth. According to this, in those fruits which set, tube growth had 

 either started on the 20th, before the rain, or was sufficiently rapid 

 after it to pass the abscission layer before the style fell. The weather 

 conditions for this season are analyzed in Plate 15. 



From Table III it will be further seen that under the favorable grow- 

 ing conditions of the greenhouse, the rate of tube growth is so slow that 

 the abscission layer is passed dangerously near the time of dehiscence. 

 In the orchard, however, during the most suitable conditions, fully as 

 many fruits set as in the greenhouse, and it is very probable that the tube 

 extension is even more rapid. 



The bearing of low temperatures upon the status of tube growth 

 noted above warrants further discussion. The lower temperature limit 

 of pollen germination in the plum was determined by Goff (5), as pre- 

 viously noted, to be approximately 40° F. At 70° F. there was an 

 abundance of tube growth, and at 51° F. the rate of growth was inter- 

 mediate between the two extremes. Entering the factor of humidity in 

 relation to temperature, his experiments further show that there was 



