Mode of Pollen Tube Growth in Pistils of Eucommia ulmoides (Eucommiaceae, Garryales

In Eucommia ulmoides, the only extant species of Eucommiaceae, fertilization is delayed for several weeks after pollination. Based on a developmental study of pollen tube growth in the E. ulmoides pistil, we show that during days 11–13 after pollination, pollen tubes grow intermittently in three steps, in close association with the development of ovules: (1) from the stigma to within the placental tissue, (2) from within the placenta through the funiculus into the tissue of an enlarged integumental tip, and (3) from the integumental tip to the mature embryo sac. The number of pollen tubes decreased from many to several during the second step and from several to only one in the third step. Thus, the three-step pollen tube growth mode plays a role in pollen selection for fertilization. Occasionally, a long micropylar canal promoted the reduction in the number of pollen tubes from two or three to one. A comparison with the diverse modes of pollen tube growth in pistils of species of Fagales, which also demonstrate delayed fertilization, showed that the mode of E. ulmoides is distinct from these modes. This unique mode might have contributed to the survival of this species from the early Eocene.