In this experiment I hybridized the horticultural pole-bean, or "wren's egg," with the lima bean. It proved exceedingly difficult to make this cross, but after many fruitless efforts I at last succeeded in securing a single pod containing four sound beans by using the pollen of the lima on the pistil of the horticultural bean. When these beans were planted, in the summer of 1872, a very strange result was observed--the beans themselves had in all respects the form, size, and appearance of the horticultural bean, but when their sprouts broke ground it was at once observed that the upper part of their cotyledons (varying from one-quarter to three-quarters of their length in different specimens) were indubitably those of the lima bean; while the lower part of each cotyledon was precisely that of the horticultural pole-bean. These parts were connected with serrated edges, which at last separated, allowing the lima bean part to drop away. Such separation, however, did not occur until the vines had made a foot or more of growth. The cotyledons on each side were divided uniformly in every case. Thus the influence of the pollenizing parent was very markedly shown in the young vines from the moment of their appearance. But after the cotyledons had fallen, all evidence of the paternal parentage of the plants disappeared. The vines did, indeed, show unusual vigor throughout the season, this, of course, suggesting their hybridity. But as to the appearance and characteristics in general, with this exception, they were essentially horticultural pole-beans like their maternal parent. The experiment was carried on for several succeeding generations, but the progeny showed no reversion to the traits of the lima bean. The characteristics of the pole-bean had seemingly been prepotent or dominant to an overwhelming degree.
In this odd case the pollen nuclei apparently failed to fuse with the maternal nuclei, leading to two genetically distinct endosperm regions and a "purely maternal" embryo. This, however, does not account for the unusual vigor of the plants. See Double Pollination.