two unequal cells; a large basal cell near the micropylar end of the embryo sac and a smaller apical cell. The apical cell divides transversely to produce a second basal cell. The two basal cells enlarge but do not divide again. The proembryo consists of these three cells. Further transverse divisions of the apical cell produce a linear embryo. Longitudinal and further transverse divisions produce a spherical embryo. By the time the embryo becomes spherical, cells within the embryo begin to differentiate to form the precursors of tissue systems (i.e. dermal, cortical and vascular). Just prior to cotyledon formation, the embryo apex becomes flattened as cell divisions form the cotyledon primordia, giving the embryo a heart-shaped appearance. At the same time, cell divisions between the cotyledon primordia give rise to the epicotyl primordium, and cell divisions near the รนร-pensor give rise to the root cap initials and root meristem. With further cell division and elongation of the cotyledons and hypocotyl the embryo becomes torpedo-shaped. During the torpedo stage the suspensor and basal cells begin to disintegrate and are absorbed by the
endosperm. The mature embryo lies straight and fills a large portion of the seed.