Abstract
After controlled pollination of Helianthus annuus L. florets, the whole course of fertilization is described and documented on 24 microphotos. The timing of events is evaluated. The average cell cycle in the proembryo is 2 hours and the nuclear cycle in endosperm - 60 minutes.Plasmoptysis is suggested as the mechanism of pollen tube opening in the synergid. The structure of the thread-like sperm nucleus is interpreted as an end to end union of chromosomes, and the morphological changes of the sperm nuclei are explained as folding and coiling, based on a spiralization mechanism of chromosomes. Cytochemical observations indicating ribosome formation in the course of the nuclear cycles in the endosperm are described. The mechanisms accelerating nuclear cycles in the endosperm are discussed.