UNFRUITFVLNESS ASSOCIATED WITH INTERNAL FACTORS 495 



developing pistils and stamens of manj^ species and is responsible for 

 many failures in fruit setting. 



Sometimes degeneration takes the form of an abortion of the entire 

 pistil. This may occur early or comparatively late in the course of its 

 development; consequently in certain species there are pistils in all 

 stages from those very rudimentary and plainly not functioning to those 

 that apparently are perfect in structure and ready for fertilization. 

 Goff^' records this condition as very common in many varieties of our 

 native plun>s, and Hodgson^^ states that the same thing is found in the 

 pomegranate. It occurs more frequently in the ornamental types of the 

 pomegranate than in those varieties cultivated primarily for their fruit; 

 in either case it is one of the main causes of the failure of the fruit to set. 

 Waugh/^* in a rather extended study of the occurrence of defective 

 pistils in plums, found striking differences in various groups. His 

 findings are summarized in Table 1. 



Table 1. — Percentage of Defective Pistils in Different Groups of Plums 

 {After Waugh'^*) 



Domestica group 4.3 Wayland group 10.5 



Japanese group 11.2 Wildgoose group 19 . 8 



Americana group 21 . 2 Chicasaw group 10. 5 



Nigra group 17.0 Hybrids group 18.1 



Miner group 1.9 



In a number of species and varieties the pistils attain their usual size 

 and they contain ovules that to the unaided eye appear entirely normal. 

 However, examination shows partial or complete degeneration in the 

 embryo sac just prior to its maturing; therefore fertilization is impossible. 

 Embryo sacs of the orange showing degeneration at various stages in 

 their development are pictured in Figs. 2 to 4 of Plate III. Sometimes 

 these degenerative processes set in early in the development of the ovules 

 and their abortion is so complete that it is evident to the unaided eye 

 at the time for fertilization. In the Unshu and Washington Navel 

 oranges, however, the fruits may develop in spite of that defect, though 

 they are seedless. Embryo sac abortion thus becomes in certain instances 

 a cause of seedlessness rather than unfruitfulness. Pistil abortion, 

 apparently at a comparatively late stage in development, has been found 

 to explain the failure of many strawberry blossoms to set fruit and the 

 production of "nubbins" from many others. ^^^ One of the two ovules 

 in the ovary of the plum" and other stone fruits is often much sma^er 

 than the other at the time of flowering, showing that at least a part of the 

 almost universal failure of one of the ovules to develop into a seed is 

 due to processes operating before the time of fertilization. It should be 

 noted in this case, as in many other fruits, that the abortion of a part 

 of the ovules of the flower does not lead necessarily to unfruitfulness. 



