Seeds and Seedlings - Seed Testing - page 15

Figure 2.4. Stylized diagram of pollination and fertilization (drawing by Sabry Elias).
Embryo development
After sexual fusion, or syngamy , a brief period of reorganization occurs during which the large vacuole
adjacent to the zygote gradually disappears with the zygote cytoplasm becoming more homogeneous and
the nucleus larger. The duration of this period varies greatly among species and may take weeks or even
months, but is usually about four to six hours before the zygote begins to divide. Lines of polarity in
preparation for future division and growth already exist in the embryo sac, having been established in the
unfertilized egg. The still undivided zygote typically elongates along the horizontal axis and small vacuoles
become evenly distributed through the cytoplasm.
Embryo development, or embryogeny , starts with the i rst few cell divisions of the zygote forming
the proembryo . Plant species may be classii ed according to the pattern of cell division, which results in
different embryogeny types. The i rst division almost always occurs at right angles to the longitudinal
axis, resulting in a terminal cell next to the micropyle and a basal cell at the chalazal end. The apical cell
divides many times, forming all or most of the embryo. Concurrently, a suspensor develops from divisions
of the basal cell. Further divisions of the suspensor serve to push the embryo upward into the interior of
the embryo sac in contact with the nutritive supply of the nucellus (Fig. 2.5). Depending on the pattern of
subsequent divisions, embryogeny types are classii ed as crucifer , asterad , solanad , caryophyllad , cheno-
podiad , or piperad as follows:
I. The i rst division of zygote is transverse.
A. Terminal cell of proembryo divides by a longitudinal wall
1. Crucifer (Onagrad) - basal cell plays only a minor role (or none) in embryo development.