juneie, I9I9 Relation of Weather to Fruitjulness in Plum 109 



Pollen taken from flowers in which the pistils were killed appeared 

 normal in color and in content when observed in a mount of lactic acid. 

 Its viability, however, was not tested, but judging from appearances 

 this freeze injured pollen much, less than pistils. 



Goff (5) shows that plum pollen was not destroyed by a short exposure 

 to freezing temperatures. Sandsten {14) tested this point further and 

 found that when plum pollen was exposed to a temperature of 29.3° F., 

 56 pef cent germinated, compared with 62 per cent in the check, Vv^hich 

 was not exposed to the freezing temperature. He also found that the 

 time required for germination was increased one-half as a result of the 

 influence of the low temperature. On the other hand, 21 plum pistils 

 exposed to the same temperature for six hours were all killed except two. 



The action of low temperatures in retarding pollen-tube growth is no 

 doubt one of the primary factors in the failure of fruit to set. The exper- 

 iments of Goff (5) show that plum pollen does not germinate at tem- 

 peratures below 40° F., and even at temperatures as high as 51° F. that 

 there is slow pollen-tube growth. A dotted line is drawn through the 

 graph for each year in Plate 1 5 at these two points. The extent to which 

 the curve for the minimum temperature extends below the line where 

 pollen-tube growth does not take place shows that in some seasons, as in 

 1 91 5, a prolonged cool period following blooming may be the principal 

 cause of the failure of fruit to set. 



With reference to the influence of temperature upon insect flight, it 

 appears that a definite point can not be selected below which activity 

 ceases. Furthermore, temperature can not be considered separate from 

 wind, rain, and sunshine. Recent investigations upon the honey bee, 

 which is the chief pollinizer of the plum, however, show something of its 

 response to temperature. Phillips {12) states that 57° F. is "the lowest 

 temperature which normal bees ever experience in the hive." At air 

 temperatures below this immediately surrounding the bees in cold 

 weather, they begin to cluster. Kenoyer {10) in reporting the data col- 

 lected over a 29-year period by J. ly. Strong at Clarinda, Iowa, shows that 

 only I per cent of the total honey produced for that period was collected 

 when the temperature was below 70° F. compared with 53 per cent when 

 the temperature ranged between 80° and 90° F. Nevertheless, this does 

 not deal directly with the point as to what temperature prevents the 

 pollinating activity of bees on plums in early spring. 



The opinions of two bee men regarding the lower temperature in which 

 bees will fly are as follows: 



The normal temperature for bees to take flight is 46° F. This temperature is i 

 degree to 2 degrees lower for Camiolan races and up to 3 degrees lower after long 

 confinement. ^ The individual bee can continue muscular movement only so 

 long as the temperature of the body does not fall below 45° F., but at this 

 temperature it loses its power of movement. {12, p. 59.) In general bees will not 



' Letter from Prof. Frances Jaeger, University Farm, Dec. 31, 1918. 



