APPENDIX I 



Xenia, the effect c f more distantly related, as contrasted with more 

 closely related, pollen on the maternal tissue of a fruit. Originally 

 applied {I'ocke 1881) to die effect on the endosperm, now known 

 to be genetic and direct. Metaxenia has been used for true xcnia 

 in contrast to the xenia of Focke. 

 X-RAY Breakage, the following terms are used in describing the results 

 of X-ray breakage. They have also been applied to cases of spon- 

 taneous breakage: — 

 Chromosome Break, one which occurs in the chromosome before its 

 division into the two chromatids observable at the following mitosis, 

 when the break is recorded. As opposed to — 

 Chromatid Break, one which follows this division, and therefore 

 breaks only one of the chromatids at any one point. Mather and 

 Stone 1933. 

 Isochromatid Break, chromosome break in a cell otherwise apparently 



characterized by chromatid breaks. Lea and Catcheside 1942. 

 Reunion, the joining of broken ends of chromosomes or chromatids. 



Classified by Darlington and Upcott 1941 into 

 ' Restitution, where the pre-breakage structure is restored. 

 Chromosome Reunion, of the ends of chromosomes following 



chromosome breakage. 

 Chromatid Reunion, of the ends of chromatids following either 



chromosome or chromatid breakage. 

 Sister Reunion, union of the identical broken ends of sister chroma- 

 tids following chromosome breakage. 

 Minute (Fragment), a fragment less than the diameter of the 

 chromatid and hence spherical. May be double, or, following 

 sister reunion, single. Midler 1940. 

 Empirical Coefhcient of Reunion (E.C.R.), the proportion of 

 the recognizable breaks in the chromosomes or chromatids, of a 

 sample of nuclei which have undergone recognizable reunion. 



Y Chromosome, with diploid sex differentiation, the sex chromosome 

 that is present and pairs with the X chromosome in the sex 

 heterozygote (Heterogamctic Sex). With haploid sex differentiation 

 (Bryophyta, etc.) the sex chromosome of the Male. 



z, the natural logarithm of the ratio of the two estimated Standard 



Deviations, v. Variance Ratio. 

 Z Chromosome, sometimes used for the Y chromosome where the female 



is the Heterogamctic Sex. 



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