24 ANNALS NEW YORK ACADEMY OF SCIENCES 



At any rate, certain plants of the same species, plants of other species 

 and certain seedlings of Europa parentage show little or no abortion of 

 spores when grown under the same conditions. 



6. The doctrines of plethoric sterility and of correlative sterility have 

 frequently been evoked to explain cases of sterility of plants that propa- 

 gate vegetatively. It has been claimed that there is a direct competi- 

 tion for food material between vegetative organs of reproduction and 

 the organs concerned with seed reproduction, and also that the degenera- 

 tion of sex organs may result from long-continued propagation by sexual 

 means. 



There is no conclusive evidence that any clon of plants such as 

 Europa has changed from a condition of high sexual potency to a con- 

 dition involving the degeneration of the sex cells because of extended 

 vegetative propagation. The evidence indicates that the original seedling 

 from which such a clon has been derived was itself as strongly pollen- 

 sterile and egg-sterile as are the plants now grown of the clon. This, 

 however, does not necessarily indicate that a correlative or a plethoric 

 sterility may not be operating in the Europa clon, but only that, if it is 

 operating, it was inherent in the nature of the original seedling. 



Various individuals belonging to the same species as this Europa 

 clon and also various seedlings having it as a parent are highly potent 

 as seed and pollen parents. Also many individuals of various related 

 species show little or no abortion of spores. The habit of vigorous 

 vegetative reproduction, which is well developed in the genus, does not 

 of itself necessarily induce abortion of spores and loss of sexuality. 



The [nherent and Hereditary Nature of Spore Abortion 



For various types of sterility it is frequently the rule that there is 

 an abortion of a rather definite proportion of the spores. In some cases 

 the very constant proportions of spore abortion in plants and in their 

 offspring have been interpreted in terms of hereditary factors conceived 

 to be directly concerned with sterility (see especially Belling, 1914; 

 Shnll. L927). With such a viewpoint the rather definite relation of 

 ahont twenty aborted spores to one viable spore for sporogenesis in plants 

 of the Europa clon may suggesi the presence of multiple factors operat- 

 ing in the development of the haploid generation. At any rate, the 

 rather definite degrees of abortion seen in various types of sterility 

 indicate that there is sonic condition inherent in the plant that deter- 

 mines and regulates the percentage of abortion. 



