junei6, I9I9 Relation of Weather to Fruit fulness in Plum 119 



The factors, then, which place a time limit upon the mutual functional 

 period and which have a direct bearing upon the setting of fruit are (i) 

 the longevity of the pollen, (2) the length of the receptive period and life 

 of the stigma, (3) the abscission of the style, (4) the rate of the pollen- 

 tube growth, and (5) the influence of low temperature upon pollen 

 germination. 



THE LONGEVITY OF PLUM POLLEN 



The results of Sandsten (14) showed that plum pollen collected from 

 such widely separated sources as Washington, Missouri, Tennessee, and 

 Minnesota retained its germinating power for six months when subjected 

 to the normal humidity and temperature changes incident to the period 

 of the test. There was a gradual decline, however, in the percentage of 

 germination from an average of 54 per cent at the end of the first month 

 to about 8 per cent at the end of the sixth. Furthermore, relatively 

 adverse conditions do not affect the longevity of the pollen, since short 

 exposures to water do not kill it and freezing temperatures only retard 

 germination. Under favorable conditions, therefore plum pollen retains 

 its viability considerably longer than it is functional under orchard 

 conditions. 



LENGTH OF RECEPTIVE PERIOD AND LIFE OF THE STiGMA 



As has been noted, the plum stigm.a is receptive under orchard con- 

 ditions for a maximum of one week but begins to turn brown at the end 

 of approximately three to five days. Adverse weather conditions may, 

 however, extend the functional period somewhat, particularly when 

 accompanied by low temperatures. The delay in pollination up to a 

 certain point does not prevent tube growth. Crosses were successful in 

 the greenhouse on stigmas which were receptive four days previous to 

 the application of pollen. Under these conditions, however, drying and 

 browning does not take place as quickly as in the orchard where the active 

 period of secretion is over at the end of three to five days and is followed 

 by a period of partial inactivity of the stigma. 



Furthermore, the stigma is more easily dried by the wind late in the 

 receptive stage than immediately after becoming receptive. Tube for- 

 mation would undoubtedly be more uncertain if pollination were delaved 

 until late in the receptive period, as would be the case during a prolonged 

 rain. Pollen germination, as well as considerable tube growth, must, 

 therefore, take place if fertilization is to be effected within a relatively 

 short time and before the conditions of the stigma prohibit tube growth 

 or before dying back in the style overtakes tubes which have been formed. 



ABSCISSION OF THE STYLE 



The styles do not begin to absciss until about two weeks after blooming 

 (PI. 14, E), although the abscission layer at the point of abscission near the 



