Origin and development of the nucellar embryo and endosperm in apomictic Xanthoxylum bungeanum Maxim.

The origin and development of the nucellar embryo and endosperm in Xanthoxylum bungeanum Maxim,which is unisexual and dioecious and only appears as female plant in Beijing,were studied with cytological and histochemical methods.Before nucellar embryogenesis,50% of degeneration occurred in every stages of megasporogeness and embryo sac development.Mature embryo sac only contained egg apparatus and two closed polar nuclei.The polar nuclei autonomously developed into endosperm without fertilization and the egg apparatus ultimately degenerated.When the free nuclei of the endosperm were 16 or 32,the nucellar embryo initial (s) originated from the nucellar cells near the micropylar end of the embryo sac.More initials specialized from one or more layer of nucellar cells in the micropylar region with the ovary growth.The frequency of initial (s) origination was 27%.The initial divided and developed into multi celled proembryo and the proembryo projected into the cave of embryo sac.More than one embryos always presented side by side in the micropylar region in one seed due to more than one initials synchronic development.The nucellar embryo had a suspensor,its development passed through the similiar stages of the zygotic embryo.About 15% seeds could reach maturity.Normally,one to three mature embryos were contained in one matured seed.More than three embryos occurred in one seed in some cases.