X. — The basic number of chromosomes in a polyploid series. 



Xj, Xj, Xg. — First, second and third generations following 

 irradiation with X-rays. 



X-chromosome. — The sex chromosome which is associated 

 with an identical homologue in one sex (the homozygous sex) 

 and with a dissimilar partner in the other sex. In most 

 animals the homozygous sex is the female (XX) the male 

 being XY. Some biologists reserve X and Y for this rela- 

 tionship using W and Z for the heterosomes where the 

 homozygous sex is the male (see W-chromosomes). The X- 

 chromosome may be unpaired in certain organisms. See Com- 

 pound X-chromosomes. 



Xenia. — The effect of pollen on the embryonic and maternal 

 tissues of a fruit. (Some writers limit xenia to effects on 

 embryo and endosperm using the term metaxenia for any 

 effects produced by pollen on maternal tissues of the fruit). 



Xenogamy. — Cross-pollination within a species. 



Xenoplastic Graft. — A heteroplastic graft (i.e. an inter- 

 specific or inter-generic graft) which "takes" successfully. 



Xerocleistogamy. — Self pollination taking place within flow- 

 ers whose perianths have been prevented from opening by 

 excessive dryness. 



