Abstract: The development of embryo sac in Prunus americana Marsh var. is often abnormal. The number of egg apparatus cells, polar nuclei, antipodal cells, and their situations in the embryo sac may be abnormal. The structer and behaviour of the nuclei of the egg apparatus cells, polar nuclei, antipodal cells may be abnormal. The nuclear substances may migrate out.Fertilization is often abnormal. The number, size, morphology, and structure of the spermatids entering the embryo sac may be variable. The spermatidsmay be tad-pole shaped, probably helpful for spermatid active motion. There may be polysperms entering egg apparatus, mono-fertilization or single polar nuclear fertilization. The chromatic substances of the spermrnatids may migrate out.The abnormal development of embryo sac, abnormal spermatids and abnormal fetilization induce degeneration. Thus the nuts decay and the fruits drop early. For controlling we suggest apply methods of changing genetic characters.