Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section Two:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9

Biology: Flowering Plant Reproduction: Part Seven “Double” Fertilization” (?): The Unique Angiosperm Event Fertilization is the fusion of the cytoplasm of the egg and sperm (the gametes). In most organisms this is followed by fusion of the haploid gamete nuclei to produce a diploid zygote. Angiosperms have long been noted as having “double fertilization." However, as we shall see, this term is applied somewhat in error. Once pollination has occurred (see below), the tube cell of the male gametophyte inside the pollen grain forms the pollen tube. This tube grows through the stigma and style toward the ovules in the ovary. The germ cell inside the pollen grain divides by mitosis and releases two sperm cells, which move down the pollen tube. The pollen tube growing from a pollen grain, with haploid male gametophyte cells. Once the tip of the pollen tube reaches the embryo sac, it grows into the embryo sac through one of the synergid cells that flank the egg cell. One sperm cell fuses with the egg, producing the diploid (2n) zygote that will develop into the next-generation sporophyte. The second sperm fuses with the two polar bodies located in the center of the sac. This produces the triploid (3n) endosperm tissue that will provide energy for the embryo's growth and development. The early events in angiosperm fertilization: a) the structure of the embryo sac (female gametophyte); b) growth of the pollen tube through the micropyle end of the embryo sac. Growth of the pollen tube through the smaller synergid cell, allowing the sperm cells to enter the embryo sac (a). One of the sperm (the first one through) fuses with the egg to produce the zygote (b). The second sperm fuses with the polar bodies of the embryo sac, to form the endosperm. Stages of growth and development of the embryo. Double fertilization is a term commonly used when discussing angiosperm sexual reproduction. The application of the term is technically incorrect. There is only one fertilization—that of the egg cell with the sperm cell. Fertilization results in a diploid cell, the zygote. The “second” fertilization is not fertilization at all. Rather, it is a fusion of two polar bodies and one sperm cell to produce the endosperm, which is not a zygote. However, the endosperm is a unique tissue type among all plants. It provides a great deal of nourishment to the developing embryo. Now go on to the next part.

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