Figure 1.

Schematic diagram of wild-type ovule development. (A) Along the proximal–distal axis of ovule primordia three domains can be distinguished: the distal nucellus that harbors the megaspore mother cell (mmc), the central chalaza, and the proximal funiculus. (B) While the mmc divides meiotically to give rise to a tetrad of haploid cells, integuments are initiated from the chalaza that subsequently grow to enclose the nucellus. (C) The three distal cells of the tetrad die and the functional megaspore undergoes three rounds of mitotic division. (D) The mature embryo sac is composed of three antipodal cells at the chalazal end, two synergids at the micropylar end, one egg cell, and two polar nuclei, which eventually fuse to give the diploid central cell. (a) Antipodal cell; (c) chalaza; (ec) egg cell; (es) embryo sac; (f) funiculus; (i.i) inner integument; (m) micropyle; (mmc) megaspore mother cell; (ms) megaspore; (n) nucellus; (o.i) outer integument; (pn) polar nuclei; (s) synergid; (t) tetrad.