That cases of Xenia really do occur is now established by the comprehensive and at the same time exceedingly careful experiments recently made by C. Correns with _Zea Mais_; it is only necessary to look through the beautiful figures with which his work is adorned to be convinced that heads of maize whose blossoms have been dusted with the pollen of a different kind produce more or less numerous seeds of the paternal kind, usually mingled with those of the maternal. Thus heads of the variety _Zea alba_ resulting from fertilization with _Z. cyanea_ exhibited a majority of white grains, but among them a smaller number of blue; and the converse experiment, of dusting _Z. cyanea_ with the pollen of _Z. alba_, yielded heads in which a minority of white grains appears among a majority of blue. But it is always only the nutritive layer surrounding the embryo--the endosperm--which exhibits the character of the paternal species, and even the capsule surrounding the seed shows nothing of it, but is purely maternal. Thus the heads of different species with pale-yellow capsule, when dusted with the pollen of _Z. rubra_, never have red seeds like those of _Z. rubra_, but always seeds with a pale-yellow skin, while, in the converse experiment, dusting of the red-skinned species _Z. rubra_ with pollen from _Z. vulgata_, all the seeds are red, like those of the maternal species, and the influence of the paternal species only shows when the strong red skin has been removed, so that the intense yellow colour of the endosperm, which in the pure maternal species is white, is exposed to view. Thus the mysterious influence of the pollen never goes beyond the endosperm, and the riddle of this influence is solved in the most unexpected manner, indeed was solved even before Correns had securely established the genuine occurrence of Xenia. The explanation is due to recent disclosures in regard to the processes of fertilization in flowering plants. It had long been known that the pollen-tube contains not merely one generative nucleus but two, which arise from one by division. But what had till recently remained unknown was that not only one of these penetrated into the embryo-sac to enter into amphimixis with the egg-cell, but that the other also makes its way in, and there fuses with the two nuclei which had long been designated the upper and lower polar-nuclei (Fig. 82, op. cit.). Nawaschin and Guignard demonstrated that these two nuclei fuse _with the second male_ nucleus; thus two acts of fertilization are accomplished in the embryo-sac, and one of these gives rise to the embryo, while the second becomes nothing less than the endosperm, the nutritive layer which surrounds the embryo, whose origin from the polar nuclei had been previously recognized.