Hypophysis (plant embryogenesis)

Plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germination. The zygote produced after fertilization, must undergo various cellular divisions and differentiations to become a mature embryo. An end stage embryo has five major components including the shoot apical meristem, hypocotyl, root meristem, root cap, and cotyledons. Unlike animal embryogenesis, plant embryogenesis results in an immature form of the plant, lacking most structures like leaves, stems, and reproductive structures

Embryogenesis occurs naturally as a result of single, or double fertilzation, of the ovule, giving rise to two distinct structures: the plant embryo and the endosperm which go on to develop into a seed. The zygote goes through various cellular differentiations and divisions in order to produce a mature embryo. These morphogenic events form the basic cellular pattern for the development of the shoot-root body and the primary tissue layers; it also programs the regions of meristematic tissue formation. The following morphogenic events are only particular to eudicots, and not monocots.

Following fertilization, the zygote and endosperm are present within the ovule. Stage I, in the photograph on this page, represents what the ovule looks like after fertilization occurs. 1 indicates the endosperm and 2 indicates the single celled zygote. After fertilization, the zygote undergoes an asymmetric transverse cell division that gives rise to two cells - an apical cell and a basal cell. These two cells are very different, and give rise to different structures, establishing polarity in the embryo. Polarity is an important property of plant embryogenesis that is necessary for the rest of the plant life cycle. The apical cells lies above the basal cell and is much smaller. The important aspect of the apical cell, is that it contains most of the cytoplasm from the original zygote.Cytoplasm is the substance that contains all of the organelles; which indicates that the apical region gives rise to more advanced strucutres, like the hypocotyl, shoot apical meristem, and cotyledons. The basal cell resides underneath the apical cell and is much larger. While the apical contains more cytoplasm, the basal cell consists of a large vacuole. This region gives rise to the supporting structures, like the hypophysis and the suspensor complex.

...
Wikipedia