io8 Journal of Agricultural Research voi. xvii, no. 3 



flight is hindered, it will be seen from Plate 15 that wind alone is not gen- 

 erally prohibitive of bee blight, but that at certain critical times, as on 

 April 28 and 29, 191 5, following a period of cloudy weather with fiequent 

 rains, it may become important — more so, in fact, from the standpoint 

 of insect flight than from that of mechanical injury to flowers. 



In addition to the considerations noted above, wind has a general dry- 

 ing effect upon the flower parts. Dehiscence is quickened and petals 

 drop earlier. There is, however, no marked drying noticeable in the 

 stigma during early receptiveness, but late in the receptive period stigmas 

 can be found which appear distinctly dry even before the stigmatic cells 

 are dead. Since the absorption of stigmatic fluid is no doubt the first 

 act in germination the dr3ang effect of wind upon stigmas may be re- 

 garded as much more critical late in receptiveness than earlier, especially 

 in view of the more unfavorable conditions for tube growth, if pollination 

 has been delayed. 



TEMPERATURE 



Temperature is primarily of interest in this connection from three stand- 

 points: (i) Its direct effect upon pollen or pistil, (2) its influence upon 

 pollen-tube growth, and (3) its interference with bee flight. Krom Plate 1 5 

 it will be seen that there are many periods of low temperature during 

 blooming time which are occasionally accompanied by frost. With ref- 

 erence to direct injury, it will be interesting to record here the damage 

 to flowers at two distinct stages of growth. 



On the night of April 19, 191 8, a freeze occurred at the Fruit-Breeding 

 Farm, when the petals were just showing in the earliest blooming varie- 

 ties. There was no injury to pollen or pistil, but as many as one-half of 

 the petals were killed on some of the varieties. These bloomed, however, 

 at the usual time, and the small dead petals persisted, while those not 

 killed underwent the usual enlargement. 



This freeze was follovv^ed by another on May 12, one week after bloom- 

 ing, when the flowers were further advanced. But this time all stigmas 

 were dead on the varieties which had bloomed earlier. The calyx tube 

 was still persistent, as there had not as yet been sufficient pistil growth 

 to break it except in two varieties of Prioius nigra. Although generally 

 there was little injury to pistils at this stage, different varieties showed 

 considerable differences in the degree of injury. On Stella, growing in a 

 low location, approximately 65 per cent of the pistils were killed, and on 

 Minnesota No. 21 (Burbank X Wolf), adjacent, there was less than i per 

 cent. Where injury occurred the entire pistil was killed, and in two days 

 it turned black, dried rapidly, and dropped a few days later at the pedicel 

 base. On the higher locations there was no injury to any of the varieties. 

 Compared with the region in Utah in which -Ballantyne (2) studied frost 

 injury, frosts do not appear to bear such a vital relation to fruitfulness in 

 Minnesota. 



