Embryological studies on Spiranthes sinensis (Pers.) Ames

• Developments of gametophyte and embryo of S. sinensis observed and recorded.
• Reproductive calendar for phase relation and dependency of embryogeny compiled.
• Unlike other orchid, pollinations advance but not a must for embryo sacs’ maturation.

Spiranthes sinensis (Pers.) Ames is one of the two rare native terrestrial orchids in Taiwan. In this work, we aimed to characterize its reproductive development. The male gametophytes (pollen) mature earlier than the female gametophytes (embryo sacs). Meiosis of pollen mother cells begins 15–16 days before anthesis and pollen develop into the bi-cellular stage 7–8 days later. Pollen reach maturity at the time of anthesis, while the embryo sacs remain at the megaspore mother cell stage. The embryo sacs mature 5 days after flower opens. The pattern of embryo sac development is of the Polygonum type. There are 5 nuclei in a mature embryo sac. Pollination can hasten embryo sac maturation by approximately 3 days, compared to a non-pollinated flower. In a successful fertilization event, pollen tubes enter the embryo sacs 2 days after pollination and the zygotes form soon after. Globular embryos are evident on the 5th day post-pollination. There is no endosperm development in mature seeds. A developmental reproductive calendar is presented showing key reproductive events leading to embryo formation for future comparative studies.