Final answer:
The mature female gametophyte in seed plants is known as the embryo sac, which forms from one surviving megaspore that undergoes mitosis to create an eight-nucleate, seven-cell structure during megagametogenesis.
Step-by-step explanation:
The mature female gametophyte in seed plants is called a(n) embryo sac. During the process of megasporogenesis, a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, of which only one survives. The second phase, called megagametogenesis, involves the surviving haploid megaspore undergoing mitosis to produce an eight-nucleate, seven-cell female gametophyte.
The embryo sac consists of seven cells: one egg cell, two synergids, three antipodal cells, and a central cell containing two polar nuclei, which will later fuse to form the triploid endosperm after fertilization. The antipodal cells degenerate, while the synergids play a crucial role in guiding the pollen tube to the egg cell for successful fertilization.
The ovule contains the megasporangium and is protected by two layers of integuments. Following fertilization, the resulting diploid zygote develops into the embryo, with the fertilized ovule forming the seed's additional tissues.