258 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



Xenia. In concluding this discussion of the development and 

 organization of the endosperm, it seems desirable to call attention 

 to the phenomenon known as xenia. This term was coined by 

 Focke (1881) to denote the immediate or direct effect of pollen on 

 the character of the seed or fruit. In practice it is now limited to 

 the appearance of the endosperm only, and the effect, if any, on the 

 somatic tissues lying outside the endosperm has begun to be desig- 

 nated as metaxenia. 



To cite an example of xenia, it is well known that certain races of 

 Zea mays have yellow (dominant) endosperm, while others have a 

 white (recessive) endosperm. If pollen from the yellow endosperm 

 race is placed on the stigmas of the white endosperm race, one 

 might expect to obtain a hybrid embryo which would show the 

 dominant character of the yellow endosperm when it grows into a 

 mature plant and fruits in the following season. Actually, how- 

 ever, the yellow color appears in the endosperm of the same ovule. 



This was very puzzling at first but the explanation became quite 

 evident after Nawaschin's (1898) discovery of double fertilization. 

 Briefly, if a white variety (yy) is pollinated with pollen from a yellow 

 variety (YY), one of the male gametes (Y) unites with the egg 

 (y) and produces a hybrid embryo (Yy) which will behave as a 

 heterozygote for yellow endosperm in the next generation. The 

 second male gamete (Y) unites with the two polar nuclei (y, y) and 

 produces the primary endosperm nucleus (Yyy). Since the latter 

 has a factor for yellowness, the endosperm will naturally show the 

 color, although the ovule belongs to the white parent. In the recip- 

 rocal cross, i.e., when pollen from a white-grained variety (yy) is 

 used on a yellow-grained variety (YY), the grains are not white like 

 those of the pollen parent, but yellow like those of the ovule parent. 

 Here xenia would seem to be absent, but this is merely due to the 

 fact that yellowness is dominant over whiteness and therefore the 

 male gamete with a factor for whiteness has no effect on the color. 

 It is thus evident that the same mechanism operates in all cases, 

 but owing to dominance xenia appears only in certain plants and 



appear during the development of the seed, but the cells in the chalazal part of the 

 ovule, lying just above the vascular bundle, divide actively and form a very promi- 

 nent tissue which, although loose and possessed of many air spaces, soon becomes 

 filled with fat and starch and serves as a substitute for the endosperm. Fries, who 

 gave it the name "chalazosperm," suggests that it may also function as a sort of 

 food body designed to facilitate the distribution of the seed by animals. 



