No. 464] STUDIES ON PLANT CELL.— VII. 585 
endosperm nucleus. Or, the two polar nuclei and the sperm 
nucleus may all three fuse together practically simultaneously. 
And again the sperm nucleus may unite first with one of the 
polar nuclei and the second be drawn later into the triple fusion. 
But no cases seem to have been reported in which but one polar 
nucleus unites with the sperm leaving the other free although 
such a combination may be expected. Also, no one has ob- 
served an independent division of the sperm nucleus within the 
endosperm, although as we shall see, there are reasons for believ- 
ing that such a development may sometimes take place. 
We have already given in Section IV the reason why these 
triple nuclear fusions may be kept apart from sexual phenomena 
since we have no knowledge of the phylogenetic history of the 
processes involved. It seems best at least for the present to 
regard the phenomenon as a special development associated with 
the peculiar and highly specialized conditions within the embryo- 
sac. This detailed and highly difficult problem of phylogeny has 
no especial bearing on the physiological features of xenia with 
which we are at present concerned. 
The best understood examples of xenia are found in the 
hybrids of maize and are clearly described in the very interest- 
ing paper of Webber (:00). As is well known, some of the 
varieties of corn are distinguished among other characters by 
the color of the kernels, which are blue, red, yellow, and white, 
and also by the surface, which is smooth in the starchy corns 
(flint or dent) and wrinkled in the sugary sweet corns. When 
well marked pure races are grown out of reach of chance cross- 
pollination, the offspring remain true to their seed characters 
but it has long been known that the varieties of corn hybridize 
very readily so that when grown close together the ears will very 
frequently present seeds mixed as to color and texture. Thus 
when exposed to cross-pollination a corn which is characteris- 
tically yellow or white may bear blue or red kernels or a form 
with wrinkled and starchy kernels may develop smooth starchy 
corn if varieties with these characters are in the vicinity. The 
color character is known to lie in these examples in the outer 
` layer of the endosperm (aleurone layer) and of course the food 
material whether prevailingly starch or sugar, which gives the 
