UNFRUITFULNESS ASSOCIATED WITH INTERNAL FACTORS 497 



defective pollen.'' This dropping of grape blossoms or of the partly 

 developed berries in those of the Vinifera varieties is commonly known 

 as "coukire." 



Dorsey^*^ has made a study of the cj^tological changes within the 

 developing pollen grain of the grape leading to, or associated with, 

 its impotence. Figm-es 9 to 11 in Plate II show something of the nature 

 of these degenerative changes. He distinguishes between what he terms 

 sterile pollen and aborted pollen. In the former after true pollen grains 

 are formed degeneration occurs in cither their generative or vegetative 

 nuclei or in both. Aborted pollen results from a development arrested 

 at an earlier stage. The following quotation from his report brings out 

 the more important details of his investigations: 



"In the formation of the sterile and fertile pollen of the grape the hetero- 

 typic and homotypic divisions and the divisions of the microspore nucleus take 

 place normally. Sterile pollen in the grape results from degeneration processes 

 in the generative nucleus or arrested development previous to mitosis in the 

 microspore nucleus. Where degeneration begins early after the division of the 

 microspore nucleus, both the generative and vegetative nucleus may be affected. 

 If the generative cell is well organized before disintegration begins the vegetative 

 nucleus may remain normal. . . . 



"Aborted microspores occur in various percentages in the native forms, as 

 well as in the cultivated varieties. While in the end the result is the same, a 

 distinction should be made between aborted and sterile pollen. The former occurs 

 in both sterile and fertile forms and seems to be due to arrested development soon 

 after being Uberated from the tetrad, while the latter results from disintegration 

 processes subsequent to mitosis in the microspore nucleus, and occurs associated 

 with the reflex type of stamen and the absence of the germ pore. . . . 



"The amount of aborted pollen which occurs in the grape varies much in 

 different vines. In the 52 cultivated varieties the average per cent, of aborted 

 pollen is 22.83, compared with 4.08 in 121 wild staminate vines of V. vulpina 

 and 3.70 in 50 wild pistillate. ... Of the 52 cultivated varieties only 10 

 have less than 5 per cent, of aborted pollen. . . . 



"The difference between the percentage of aborted pollen in known hybrids 

 and the pure forms, among the cultivated varieties, is only slight. The average 

 percent of aborted pollen from 10 vines, of varieties generally regarded to be 

 pure V. labrusca, is 23.10, while that for 38 of the hybrid varieties is 24.60. 

 There are some instances, however, among the hybrids, as in Black Eagle, where 

 the amount of aborted pollen is small. . . . 



"Since aborted pollen occurs in much the same relative amounts in the self 

 fertile and self sterile varieties, from the standpoint of fertilization and the setting 

 of fruit it would seem that the aborted pollen is unimportant in the grape, be- 

 cause in the fertile forms there is still an abundance of potent pollen.'"'^ 



It should not be inferred, because the discussion thus far has been 

 limited to the grape, that sterility or unfruitfulness due to pollen abortion 

 does not occur in other fruits. Pollen abortion is a common occurrence 



32 



