The life cycle of plants alternates between a diploid and a haploid generation. In flowering plants the haploid gametophytes are sexually dimorphic and produce the gametes, which fuse to produce the diploid sporophyte of the next generation. The megagametophyte of maize follows the Polygonum-type pattern of development:one of the four meiotic products, the functional megaspore, undergoes three free nuclear divisions to produce a polarized, eight-nucleate syncytium. Cellularization produces

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... cells that differentiate into four cell types:two syn-ergids, three antipodals, and the two female gametes, the egg cell and the central cell. The position of the nuclei in the syncytial phase and the position and differentiation of cell types after cellularization follow stereotypical patterns, suggesting a tight genetic regulation of the cellular processes involved. Recent genetic evidence demonstrates that many of these cellular processes are regulated by the activity of the haploid genome of the megagametophyte itself, rather than the parental diploid genome from which it originates. The functions performed by the megagameto-phyte includes both basic cellular functions and functions that unique to the meg-agametophyte, such as pollen tube guidance and reception, as well as processes associated with double fertilization and the maternal control over seed development. In this chapter we describe the development and functions of the megagame-tophyte, and what is known the regulation of the underlying processes.