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single megaspore with three miotic divisions producing eight nuclei; bisporic (allium type), in which two
megaspores undergo two miotic divisions, producing eight nuclei; and tetrasporic , in which four nuclei
participate in embryo formation.
Tissues discussed so far have been largely somatic and diploid. This indicates they are of parental,
non-reproductive origin and have a chromosome complement equal to that of the parental cells. In the next
stage of development the female gametophyte or embryo sac will develop from the haploid megaspore.
This process is known as megagametogenesis .
In monosporic development, which is believed to be the most common type of embryo sac formation,
megagametogenesis begins with three successive free-nuclear divisions without cell wall formation, result-
ing in eight haploid nuclei. The eight nuclei then arrange themselves within the embryo sac and cell wall
formation occurs. The mature embryo sac consists of three antipodal cells , two polar nuclei (without cell
walls), one egg cell ( ovum ) and two synergid cells (Fig. 2.3). The embryo sac is now ready to be fertilized
and the process of megagametogenesis is complete.
The developing female gametophyte is nourished by the surrounding nucellus and by vascular connec-
tions extending through the funiculus.
Figure 2.3. Stylized diagram of megagametogenesis and the development of eight haploid cells within the
expanding embryo sac (drawing by Sabry Elias).
Pollination and fertilization
Some diploid cells (microsporocytes) within the four sacs of the anther, or microsporangia, undergo meiosis
(reduction division), each forming four haploid microspores. Each microspore undergoes two mitotic divi-
sions resulting in a microgametophyte , or pollen grain . This process is called microgametogenesis . After
release from the anthers, the pollen grains germinate when they land on a receptive stigma. The resulting
pollen tube elongates through the style, grows into the locule, and enters the female ovule through the
micropyle (Fig. 2.4). Two sperm nuclei are released. In the process of double fertilization, one of the sperm
nuclei unites with the egg nucleus to form a diploid zygote that eventually develops into the embryo . The
other sperm nucleus unites with the two polar nuclei , creating a triploid (3n) nucleus that develops into the
endosperm.
After fertilization, the synergids and antipodals usually disintegrate, leaving an embryo sac with a
triploid primary endosperm nucleus and a diploid zygote.
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