INBREEDING AND CROSSBREEDING IN SEED DEVELOPMENT 85 



upon and competition for an outside nutrient supply in the latter, require 

 that the tissue shall share in the advantages of sexuality. The advantage 

 gained is not that of amphimixis in general, as in the embryo, but solely the 

 extra vigor of growth associated with the union of unlike nuclei in the mother 

 cell. Thus hybrid vigor in the endosperm has some claim to uniqueness. The 

 sole object gained by entry of a sperm into the nuclear makeup of this sterile 

 tissue is the added vigor of growth thus acquired. Some of the evidence by 

 which the validity of this point of view may be tested will now be considered. 



INBREEDING AND CROSSBREEDING EFFECT ON 

 SEED COLLAPSE IN MEDICAGO SATIVA 



Two classes of matings on seven alfalfa plants were carried out under 

 favorable growtli conditions in a greenhouse. After removal of the anthers 

 from the flowers used, a part of the flowers were pollinated with pollen from 

 the same respective plants. This constitutes the self-fertilized series. Other 

 flowers on the same plants were cross-pollinated, the pollen being derived in 

 each case from an unrelated plant within the group. These matings comprise 

 the cross-fertilized series. 



Since alfalfa is regularly cross-fertilized, the second series of matings is 

 designed to maintain the level of heterozygosity normal to the endosperm 

 and embryo in this species. The enforced self-fertilization, on the other 

 hand, would be expected to reduce heterozygosity in the endosperm mother 

 nucleus and the zygote by 50 per cent. It is proposed to review the conse- 

 quences for seed development of this sharp reduction in heterozygosis. 



Following the above two series of matings, the pistils were collected at 30, 

 48, 72, 96, 120, and 144 hours and imbedded in paraffin. After sectioning and 

 staining, data were taken on fertility of the ovules, frequency of fertile ovules 

 collapsing, number of cells in the embryo, and number of nuclei in the endo- 

 sperm. Detailed observations were made subsequently on growth of the 

 integuments. 



Alfalfa was known previously to be partially self-incompatible. It was 

 not unexpected, therefore, to find that only 15 per cent of the ovules became 

 fertile after selfing in contrast to 66 per cent after cross-pollination. The new 

 fact which emerged was the much higher incidence of collapse of ovules sub- 

 sequent to fertilization in the selfed than in the crossed group. The data are 

 summarized in Table 5.1. Fertilization occurred within about 30 hours after 

 pollination under the prevailing conditions. It was somewhat delayed after 

 selfing. Little evidence of breakdown of the seeds was found at 48 hours. In 

 the 72 hour and subsequent collections, however, the phenomenon was com- 

 mon. The results presented in the table cover the period from 72 hours to 

 144 hours, inclusive, and are based upon 433 seeds and 1682 seeds in the 

 selfed and crossed series, respectively. 



Growth of the young seed at this stage appears to be quite independent 



