STOUT AND SUSA, CHROMOSOME IRREGULARITIES 25 



In hybridity the abortion is assumed to be determined by the inter- 

 relations of the chromosomes. In apogamy the abortion is obviously 

 due to a very different condition or at least a further development, for 

 there is loss of sexuality with the substitution of vegetative methods of 

 seed formation, which maintains the nuclear organization of the somatic 

 cells. In intersexes the chromosomal complex is of itself in no way 

 directly responsible for the abortion that operates in only one of the 

 two kinds of sex organs (pistils or stamens). 



The Immediate Causes of Spore Abortion 



A study of the fundamental nature of the cell processes involved in 

 the destructive activities of spore abortion shows that they fall into a 

 few simple categories whose essential characteristics and etiological rela- 

 tions can be viewed in the light of the known facts of cell growth and 

 reproduction. 



Spore abortion is largely predetermined by the events of fertilization. 

 In fertilization there is the fusion of two nuclei, which may possess and 

 very often do possess cytoplasm and germ plasm of very different 

 hereditary and physiological properties. Also polyploidy, arising within 

 a strain, may produce gametes that are quite different in quantitative 

 as well as qualitative properties. That the interactions in fertilization 

 may be destructive is well shown in embryo abortion. In some cases such 

 abortion is so definite that the theory of balanced lethals may be applied. 

 As Davis (1915, 1923) has pointed out, there may be a high degree of 

 abortion of zygotes or embryos in hybrids and in such plants as certain 

 of the Oenotheras, which are also somewhat characterized by irregu- 

 larities in sporogenesis. Much embryo abortion is no doubt due to a 

 decided lack of harmony in the interactions between protoplasmic units 

 that become associated in the same nucleus through fusion. Fertiliza- 

 tion is a critical point in ontogeny. It initiates an association of differ- 

 ent elements of protoplasm that prevails and interacts throughout somatic 

 divisions and culminates in the more complicated relations in sporo- 

 genesis. 



Certain grades or degrees of differences in gametes will yield nuclear 

 organizations in fertilization that apparently live more or less in harmony 

 until sporogenesis is reached, when the abortion of spores becomes a 

 conspicuous feature. The abortion is associated with the more intimate 

 pairing of chromatin units in synapsis and with the return to the haploid 

 generation. It is characterized by irregularities in chromosome behavior. 



