INTERNAL CAUSES OF VARIATION 183 



not equally developed. One of the twin pair of oats is more or 

 less undeveloped. Is this difference in size due to season, food 

 supply, room, or to some peculiarity in the germ ? It may be lack 

 of room in pod-bearing plants, but it cannot be that in the case 

 of corn. The strong presumption is, in the opinion of the writer, 

 that these differences in size are partly due to differences in food 

 supply but more largely to inherent differences in the germs. 



SECTION VI XENIA, OR FERTILIZATION OF THE 

 ENDOSPERM, DOUBLE FERTILIZATION 



If one kind of corn be fertilized by another, the mixture will 

 show the first year. For example, if white and yellow corn be 

 planted side by side, the white ears will have many yellow grains, 

 showing at once the effects of cross fertilization. These " off" 

 kernels are the mixed seeds, but, reasoning from analogy, we 

 should not expect the mixture to appear until the grains are 

 planted and the generation of mixed breeding is at hand. The 

 visible part of the kernel is not the germ ; it is the " endo- 

 sperm," or surrounding portion, which serves as food for the 

 sprout until the young plant has established itself. It is related 

 to the germ much as the white of egg and its shell are related 

 to the yolk. Fertilization is of the germ. How, then, do these 

 outside parts become affected ? 



It will be remembered that in the animal the female germ 

 gives rise to one mature functional cell, the ovum, and three 

 non-functional, the polar bodies ; that the male cell gives rise 

 also to four mature cells, the spermatozoa, all functional, and 

 that the nucleus of the one unites directly with that of the 

 other without intervening nuclear divisions. 



In plants, however, it is found to be substantially different. 

 The mature female cell, corresponding to the ovum, instead of 

 awaiting fertilization, continues its activity, undergoing gener- 

 ally two (sometimes more) divisions of the nucleus, giving rise 

 to eight, or some other corresponding number of " sub-nuclei," 

 which remain floating within the cytoplasm. It will be remem- 

 bered that of these eight sub-nuclei only one is capable of func- 

 tioning as an egg nucleus ; also that two others remain near 



