Embryology of Pisum sativum

Abstract

1. Pollination takes place in Pisum sativum between 24 and 36 hours before the open flower stage. Fertilization has occurred and division of the zygote is under way by the time the flower is fully opened. 2. The pollen tube enters the embryo sac between the synergids, neither of which disintegrates until later. 3. One male gamete nucleus fuses with the two polar nuclei. In the division of the primary endosperm nucleus three separate groups of chromosomes are recognizable on the equatorial plate. 4. The primary endosperm nucleus divides before the zygote nucleus. Free nuclear division occurs in the endosperm in the early stages of development. Cell formation later takes place in the micropylar portion of the endosperm, the chalazal portion remaining multinucleate. The endosperm is resorbed in the course of embryo development. 5. During the division of the zygote nucleus, the chromosomes from the two gamete nuclei remain in more or less distinct groups until the equatorial plate stage. 6. The zygote divides transversely to form a two-celled proembryo. The basal (micropylar) cell then divides longitudinally to form two suspensor cells, and the apical cell divides transversely to form an apical embryo mother cell and a middle cell. The middle cell divides longitudinally to form a two-celled middle piece. The nuclei of the two basal and two middle cells divide to form multinucleate suspensor cells having respectively sixty-four and thirty-two nuclei each. 7. The embryo develops in a typical manner from the apical embryo mother cell. 8. The two multinucleate basal cells elongate and push the embryo and middle piece to the region of the bend of the campylotropous ovule. 9. The suspensor cells disintegrate shortly after differentiation of the cotyledons, epicotyl, and hypocotyl. 10. The cotyledons act as storage organs in the mature seed, the endosperm being completely assimilated in the course of embryo development.