54 MTBIOLOGY TODAY | JULY ‘15female gametophyte. This type of embryo sac is called
monosporic.development of female gametophyte
• The functional megaspore is the first cell of female
gametophyte or embryo sac.
• The nucleus of megaspore undergoes free nuclear mitotic
divisions to form 8 nuclei and gives rise to embryo
sac or female gametophyte, the process is called
megagametogenesis.
• Out of 8 nuclei, 4 nuclei are arranged at each pole. One
nucleus from each pole migrates to the centre to form
two polar nuclei which further fuse to form a diploid or
secondary nucleus.
• Three nuclei at the base of embryo sac form antipodal cells.
The remaining 3 nuclei at the micropylar end get surrounded
by cytoplasm and are called egg apparatus.
• Thus a typical angiosperm embryo sac, at maturity, though
8-nucleate is 7 celled with 3 micropylar, 3 chalazal and
one central cell. It is formed by one meiosis and three
mitosis.
• In egg apparatus, one cell is larger and is called the egg
or oosphere and the other two are called synergids.
• Synergids are short-lived (one of them degenerates long
before fertilisation and second after entry of pollen tube
into embryo sac).
• These synergids help:
- In growth of pollen tube towards egg by secreting
chemotropically active substances. - In nutrition of embryo sac by absorption and
transport of food from nucellus through their filiform
apparatus.
• Filiform apparatus in the form of finger like projections
from cell wall is present in upper part of each synergid.
The filiform apparatus is useful for the absorption and
transportation of materials from the nucellus to the
embryo sac. Hook like structures help in easy penetration
of pollen tube and liberation of male gamete from the
pollen tube.
• Egg is the constant feature of embryo sac. It fuses with one
male gamete to form zygote (syngamy) and hence embryo.
The central cell consists of two polar nuclei. Two polar
nuclei fuse to form single diploid secondary nucleus
or definitive nucleus.
• The typical 8 nucleate 7 celled embryo sac, found in 80%
flowering plants is called Polygonum type of embryo
sac.
• Oenothera type of monosporic embryo sac develops
from single functional megaspore of micropylar end (not
chalazal). Oenothera embryo sac is four nucleated having
egg apparatus and a single polar nucleus in centre.
Antipodals are absent. This type is characteristic of the
Family Onagraceae.
• Tetrasporic embryo sac is of different types viz.
Peperomea, Penaea, Fritillaria, Plumbagella, Plumbago,
Drusa and Adoxa.