UNFRUITFULNESS ASSOCIATED WITH INTERNAL FACTORS 501 



as due to incompatibility, using that term in its broader sense signifying 

 that the normal processes of fertilization fail somewhere between the 

 production of functional gametes and the fusion of the sex cells. 



I nterfruitf Illness and Inter fertility. — Just as the terms self fruitfulness 

 and self fertility refer to the ability of a plant or a variety to mature 

 fruits or seed with pollen from its own flowers, so interfruitfulness 

 and interfertility indicate the abihty of two plants or two varieties to 

 mature fruits and seed with each other's pollen. Varieties that are 

 self unfruitful because of dioecism, such as for instance pistillate flowered 

 strawberries, figs of the Smyrna type and the date palm, have long been 

 known to be interbarren as well. Other fruit varieties, such as many 

 of the grapes, that are self barren, or partly so, because of impotent 

 pollen, have been recognized as interbarren for the same reason. ^^ Until 

 comparatively recently, however, it has been the rather general belief 

 that most fruit varieties are interfertile, or at least interfruitful, even 

 though they might be self sterile, provided that they bear good pollen. 

 That is, it was assumed that any variety of apple can successfully 

 pollenize and fecundate any other apple variety, the only precaution 

 necessary in planting being to choose varieties blossoming at approxi- 

 mately the same season. .Occasional instances of interunfruitfulness 

 were encountered in experimental studies^"^ but later work with the same 

 varieties in the same or in a different place often proved them interfruitful 

 and the first results were regarded as due to accident or experimental 

 error. However, Whitaker and Milton, which are open pollinated 

 seedlings of the Wildgoose plum, have been reported intersterile and 

 though both are fertile when pollinated with Sophie, that variety is 

 sterile to their pollen.^" 



In 1913, Gardner^'' reported the three leading varieties of the sweet 

 cherry grown on the Pacific Coast as intersterile and interunfruitful 

 in Oregon and a little later the same condition was reported for two of 

 these varieties in California. '^^ At the same time all three varieties 

 were found to have perfectly good pistils and potent pollen. This is 

 clearly an instance of intersterility due to incompatibility. More 

 recently several varieties of the almond have been shown to be inter- 

 sterile in California. ^-^ Stout^-" has found cross incompatibility occurring 

 sporadically in his pedigree cultures of chicory and it has been recorded 

 in tobacco. ^^ In summarizing their observations on cross incompati- 

 bihty in tobacco. East and Parks state :^^ "Cross-sterility in its nature 

 identical with self-sterility was found in every population of self-sterile 

 plants tested. The percentage of cross-sterility in different populations, 

 based in each case on numerous cross matings, varied from 2.4 per cent, 

 to 100 per cent. " 



Cross-stei'ility is much less common than self-sterility but apparently 

 is to be expected in all those groups in which self-sterility exists. Data 



