UNFRUITFULNESS ASSOCIATED WITH INTERNAL FACTORS 499 



is more likely to give rise to self sterile plants than is broad breeding. 

 Detjen^^ concluded from his studies with the Southern dewberry {Rubus 

 trivialis) that not only is self sterility in that species transmitted to its 

 pure offspring, but frequently to its hybrid progeny. 



Sterility and Unfruitfulness Due to Hybridity. — Unfruitfulness and 

 sterility have long been recognized as conditions frequently associated 

 with hyl)ridity. Generally the wider the crossing the greater is the degree 

 of sterility encountered. Many instances might be cited; a few will 

 suffice. Waugh^^^ describes a hybrid between the Troth Early peach and 

 the Wildgoose plum that has been named the IVIule. It bears an abun- 

 dance of flowers but they are without pistils or petals. The stamens are 

 numerous, but malformed, assuming something of the shape and appear- 

 ance of pistils. The variety is fairly constant in its flower characteristics, 

 completely sterile and also barren. He mentions another peach-plum 

 hybrid, known as the Blackman, with similar characteristics. A hybrid 

 between the pear and the quince, described under the name Pyronia, 

 flowers and fruits freely but is always seedless. ^^7 jj^ ^]^jg ^g^gg hybridity 

 is responsible for sterility alone, instead of sterility and barrenness, as in 

 the peach-plum hybrids. The Royal and Paradox walnuts, hybrids 

 between the Persian and the California and Eastern Black respectively, 

 are almost barren. In these cases, as in many other hybrids, barrenness 

 due to hybridity is associated with great vegetative vigor. The high 

 percentage of aborted pollen found in wild and cultivated blackberries in 

 New England is to be attributed mainly to a condition of hybridity. ^^ 

 A number of hybrids between Vitis rotundifolia and various species of the 

 Euvitis group have been found almost completely sterile; this is attributed 

 mainly to their hybrid condition. ^^ In describing one of these V. vinifera 



"Flowers perfect hermaphroditic and imperfect hermaphroditic; stamens 

 upright and pistils medium large in the perfect hermaphroditic ; stamens reflexed 

 and pistils well developed in the imperfect hermaphroditic flowers. . . . The 

 pollen in the perfect hermaphroditic flowers is a mixture of shriveled and plump, 

 sterile and fertile grains. The fertihty of these plump grains has been demon- 

 strated in actual hand-made cross poUinations, also by selfing some of the flowers. 

 The pollen in the imperfect hermaphroditic flowers is all shriveled and impotent. 

 The pistils in both types of flowers are mostly sterile, only two from 17 perfect 

 hermaphroditic flower-clusters having developed into berries in 1918. The 

 perfect hermaphroditic flowers are sterile because of hybridization, while the 

 imperfect hermaphroditic flowers are sterile due to the double phenomenon of 

 hybridization and intersexualism with attendant impotence." 



However, abortion of pollen and of pistils cannot always or entirely 

 be attributed to hybridity; and, conversely, hybridity is not always a 

 cause of unfruitfulness or even of sterility. Many of the cultivated 

 American varieties of the grape that are probably pure species bear some 



