junei6, I9I9 Relation of Weather to Fruttfulfiess in Plum 107 



ANALYSIS OF WEATHER AT BLOOMING TIME 



With weather apparently having such an important bearing upon the 

 setting of fruit, as is indicated in the spraying experiment and in Table I, 

 a more detailed analysis of weather has been made during blooming time 

 and for 10 days after, with the object of determining whether there are 

 certain conditions each season which can be singled out as prohibiting 

 a set of fruit. At the outset it should be stated that there are factors 

 which operate beyond the 20-day period to reduce the crop. Neverthe- 

 less, there are influences entering during blooming time which do not 

 operate in the same manner anywhere else in the life cycle. As a result 

 of the sum total of these influences a sufficient number of pistils have or 

 have not set, as the case may be, at the 5- or 6-week period to determine 

 definitely the prospect of a crop. 



WIND 



The experiments of Waugh (16) show that no fruit set from wind- 

 carried pollen when insects were excluded by a covering of coarse mos- 

 quito netting. Further tests (18) with microscopic slides covered with 

 vaseline, to which pollen adheres, showed that when the slides were 

 placed at various heights and distances from trees in full bloom on bright 

 sunny days even a direct wind did not carry sufficient pollen to bring 

 about effective pollination at a distance equal to that from one tree to 

 another. Wind pollination, therefore, may be regarded as insufficient, 

 even under the most favorable conditions. 



Pollination under orchard conditions is affected by windy weather, 

 however, especially when prolonged, if insect visits are prevented. Dur- 

 ing a strong wind, rain, cold, or cloudy weather, conditions are such that 

 insect activity is reduced to a minimum. Waugh {16, 17) shows that 

 honey bees, of the 30 or more species of insects found to visit the plum, are 

 (16, p. 247) "nearly always the most active workers, and the ones which, 

 by the character of their operations in the flower, may be held chiefly 

 responsible for the proper distribution of pollen." These results are con- 

 firmed by Backhouse (j). Wind, therefore, may be regarded as having 

 more of an indirect than direct bearing upon the setting of fruit, since 

 pollen is not wind-carried in quantities sufficient for ample pollination. 

 The influence upon bee flight, however, may be serious at certain times. 



The curve for wind in Plate 15 runs through the point of hourly wind 

 movement from 6 a. m. to 6 p. m. While the average wind movement 

 considered aside from sunshine and the character of the day is of little 

 significance, it shows what may be expected at this time of year in Min- 

 nesota. The average wind movement per hour, within the above limits, 

 for 7 years was approximately 15 miles, while the average of the extreme 

 wind movement recorded, within the same limits, for the 7-year period 

 was near 19. The extreme movement recorded was 38 miles. Assum- 

 ing that a wind of 25 miles per hour approaches a condition where bee 



