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# The Angiosperm Life Cycle: An Overview Figure 38.6 provides a complete overview of the angiosperm life cycle, focusing on gametophyte development, sperm delivery by pollen tubes, double fertilization, and seed development. ## Development of Female Gametophytes (Embryo Sacs) * As a carpel deve...

# The Angiosperm Life Cycle: An Overview Figure 38.6 provides a complete overview of the angiosperm life cycle, focusing on gametophyte development, sperm delivery by pollen tubes, double fertilization, and seed development. ## Development of Female Gametophytes (Embryo Sacs) * As a carpel develops, one or more ovules form deep within the ovary. * A female gametophyte (embryo sac) develops inside each ovule within a tissue called the megasporangium. * Two integuments (layers of protective sporophytic tissue) surround the megasporangium, except at a gap called the micropyle. * Female gametophyte development begins when a megasporocyte enlarges and undergoes meiosis, producing four haploid megaspores. * Usually, only one megaspore survives; the others degenerate. * The surviving megaspore's nucleus divides by mitosis three times without cytokinesis, resulting in a large cell with eight haploid nuclei. * These nuclei are arranged in a structure called the embryo sac, containing seven cells and eight nuclei. The embryo sac also contains three antipodal cells, two synergid cells, and the egg along with two polar nuclei. ## Development of Male Gametophytes in Pollen Grains * As the stamens are produced, each anther develops four microsporangia (pollen sacs). * Within the microsporangia are many diploid cells called microsporocytes. * Each microsporocyte undergoes meiosis, forming four haploid microspores. * Each microspore develops into a haploid male gametophyte, consisting of two cells: the generative cell and the tube cell. * The male gametophyte is contained within a pollen grain formed by the spore wall of both the microspore and the anther. ## Sperm Delivery by Pollen Tubes * After the microsporangium breaks open and releases pollen, the pollen grain is transferred to a receptive surface of a stigma (pollination). * The pollen grain germinates by producing a pollen tube, a cellular protuberance that delivers sperm to the female gametophyte. * A pollen grain typically consists of a spore wall and two cells: a tube cell and a generative cell. * As the pollen tube grows through the style, the generative cell's nucleus divides by mitosis, producing two sperm. * The tube cell leads ahead of the sperm as the pollen tube grows toward the micropyle, guided by chemical attractants produced by the synergid cells. * The arrival of the pollen tube initiates the death of one synergid cell, creating a passageway; the two sperm are then released from the pollen tube near the female gametophyte. ## Double Fertilization * Fertilization, the fusion of gametes, occurs after the two sperm reach the female gametophyte. * One sperm fertilizes the egg, forming a zygote. * The other sperm combines with the two polar nuclei, forming a triploid (3n) nucleus (endosperm). * This endosperm will develop into a food-storing tissue of the seed. * Double fertilization ensures that endosperm develops only in ovules where the egg has been fertilized, preventing wasted resources. ## Seed Development * After double fertilization, each ovule develops into a seed. * The ovary develops into a fruit which encloses the seeds and aids in their dispersal. * The seed stockpiles proteins, oils, and starch, depending on the species. * Initially, carbohydrates and other nutrients are stored in the seed's endosperm, but later, depending on the species, the swelling cotyledons (seed leaves) of the embryo may take over this function. * When a seed germinates, the embryo develops into a new sporophyte.