THE MODIFICATIONS OF THE CHROMOSOME CYCLE 



57 



3. Endosperm Characters 



Few genes, except simple lethals, are known affecting the female 

 gametophyte of higher plants. A peculiar situation arises, however, in 

 the formation of endosperm, and associated tissues such as the aleurone, 

 which are produced (by "double fertilization") by fusion of one of the 



polar 

 nucleus 



syncr$idae 



ovum 



ertjlt'red 



endosperm 

 nuclei 



Fig. 21. Double Fertilization and the Formation of Endosperm in higher 

 plants. In h two sperm nuclei have entered the embryo sac from the pollen tube. 

 One unites with the egg nucleus to give the diploid embryo, while the other 

 unites with two embryo sac nuclei ("polar nuclei") to give the endosperm, the 

 nuclei of which (shown in telophase in c) are therefore triploid. 



(From Wilson.) 



pollen tube's haploid nuclei with two of the haploid embryo-sac nuclei 

 and are therefore triploid in constitution. Several endosperm characters 

 are known; they appear to the unsophisticated eye as characters of the 

 seed. Their inheritance depends on the dominance relations in the 

 triploid tissue; in some cases an endosperm with one dose of the 

 dominant and two of the recessive shows the dominant character (e.g. 

 Su starchy dominates over two doses of su sugary in maize grains), in 

 others the recessive (e.g. two doses of / floury show in the presence of 

 F flinty in maize). The immediate apparent effect of pollen on the 

 characters of the seed in a cross between sugary female and starchy 

 male, for instance, is spoken of as xenia, a phenomenon which may 



