UNFRUITFULNESS ASSOCIATED WITH EXTERNAL FACTORS 515 



well known and in the section on Temperature Relations is a some- 

 what detailed account of the more important factors in frost occurrence 

 and their bearing upon fruit production. However, temperatures well 

 above the freezing point often are important in determining the setting 

 of fruit. Darwin^s calls attention to the rather common failure of 

 European vegetables to develop fruits and seeds when grown in India and 

 attributes this failure to the hot climate of that country. In some of 

 these instances the influence of temperature may be more directly upon 

 the formation of flower buds and flower parts than upon the processes of 

 fruit setting. 



Goff^^ has shown that though pollen of most deciduous "fruits, like 

 the plum, cherry, apple and pear, germinates freely at temperatures of 

 50°F. or above, the process is practically inhibited by temperatures of 

 40°F. or lower. In a number of plums the stigma is receptive for a 

 period of only 4 to 6 days. The abscission of the style occurs in from 

 8 to 12 days after bloom and it is not influenced to any great extent by 

 temperature.^^ On the other hand, the period required for the germina- 

 tion of the pollen grain and its penetration of the style may depend on 

 temperature and may be as short as 4 days and as long as 12. A period 

 of cool, but frostless, weather during blossoming, therefore, may prac- 

 tically prevent fertilization and thus very materially limit the set of fruit. 

 Presumably similar conditions are found in many other fruits, though 

 the relative importance of this factor varies greatly with different 

 species and varieties. 



In this connection mention should be made of the indirect influence 

 of temperature on fruit setting through its effect on the activity of pollen- 

 carrying insects. Evidently the temperature at which bees and other 

 pollen-carrying insects will work depends on conditions, for 40°F. has 

 been given as the lowest temperature at which the honey bee will take 

 flight^^ though normally they do not leave the hive until the temperature 

 reaches about 60°F., except after a considerable period of confinement. 

 Whatever the exact temperature may be, it is evident that should all 

 other conditions be favorable a continued period during blossoming well 

 above freezing but still too low for much activity of the pollen-carrying 

 insects may account for many failures in fruit setting. 



An interesting example of the influence of temperature on fruit setting is 

 furnished by the papaya. Though usually a strictly monoecious plant, the 

 "male" form sometimes bears fruit in cool climates. In commenting on the 

 change of sex here involved Higgins and Holt remark: ^^ "This 'fruiting of the 

 male papaya' takes place most freely in cool climates outside the tropics or at 

 high altitudes. In Hawaii it may be seen that these trees fruit more abund- 

 antly on the mountains than near the sea level. Information received by cor- 

 respondence with experiment stations and botanic gardens in many parts of the 

 world, in reply to direct inquiry, have confirmed this conclusion. In torrid cli- 



