向日葵胚囊的超微结构和雌性生殖单位问题

本文对向日葵胚囊中卵细胞、助细胞与中央细胞开花前和传粉后的超微结构变化进行了研究。着重报道了不同发育时期这三种细胞之间特定区域的界壁的消长动态。在此基础上结合现有文献资料探讨了由三者共同组成“雌性生殖单位”以适应受精作用的问题。

ULTRASTRUCTURE OF SUNFLOWER EMBRYO SAC IN RESPECT TO THE CONCEPT OF FEMALE GERM UNIT

This paper deals with the ultrastructural changes in sunflower (Helianthus annuus 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 me- mbrane 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 syner- gids 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 fertiliza- tion,the fertilized egg and secondary nuclei move away-from each other.A new wall regene- rates 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 dictyosomes,which are located in the central cell and along the new wall,secrete numerous vesicles into the forming wall.Based on present ob- servations,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 import- ant 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.