作 者 ：Yan Hua, Yang Hong-yuan and W. A. Jensen
Keywords:Helianthus annuus, Embryo sac, Female germ unit, Ultrastructure,
Abstract:This paper deals with the ultrastructural changes in sunflower (Helianthus annum L.) embryo sacs before and after fertilization with particular emphasis on the wall change events. At early stage, the embryo sac component cells are separated with each other by complete walls. During maturation, a common wall zone bordering the egg cell, synergids and the central cell becomes thinner and eventually disappears, leaving only two layers of plasma membrane for the future target zone of double fertilization. The egg nucleus and the secondary nucleus move toward each other near to the plasma membrane. After pollination one of the synergids deposits electrondense materials on its tonoplast, which is the first sign of degeneration; the other synergid has no such changes. The pollen tube penetrates the degenerated synergid and the discharged )contents, including two sperms, move along a gap between the two plasma membranes. Each sperm fuses with the egg or the central cell, respectively. After fertilization, the fertilized egg and secondary nuclei move away from each other. A new wall regenerates along the previously naked zone,separating the zygote, primary endosperm cell and persistent synergid. The zygote soon enters into mitosis, even while wall formation continues. Multilayered endoplasmic reticula and dictsomes, which are located in the central cell and along the new wail, secrete numerous vesicles into the forming wail. Based on present observations, and previously reported facts in other plant materials, it is concluded that the egg, synergids and the central cell make up a female germ unit. This unit is formed prior to fertilization by degradating a portion of the common wall, which, together with other important features, facilitates gametic fusion. Once fertilization is finished, the unit is dismissed and the zygote and persistent synergid form a wall over their entire surface. The antipodal cells, a member of embryo sac also, are not a participant of the female germ unit.
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