FERTILISATION
- Fertilisation is the fusion of male and female gametes. In seed plants, i.e., gymnosperms and angiosperms the male gametes
are brought to the egg containing female gametophyte by a pollen tube (Strasburger, 1884). This phenomenon is called
siphonogamy. - The pollen tube carrying the male gametes enters the ovule either through its micropyle (porogamy, e.g., lily), chalaza (chalazogamy
e.g., Casuarina) or the sides after piercing through the integuments or funicle (mesogamy, e.g., Cucurbita). - Usually the pollen tube enters the embryo sac by passing into one of the two synergids.
- The pollen tube releases the two male gametes into the
cytoplasm of the synergid. - One of the male gametes moves towards the egg cell and fuses
with its nucleus resulting in the formation of diploid zygote. - The other male gamete moves towards the two polar nuclei
located in the central cell and fuses with them to produce a
triploid primary endosperm nucleus. - Since two types of fusion takes place (syngamy and triple
fusion) in an embryo sac, the phenomenon is termed as double
fertilisation, an event unique to angiosperms. - The primary endosperm nucleus develops into the endosperm
while the zygote develops into an embryo.
POST-FERTILISATION : STRUCTURES
AND EVENTS- Following double fertilisation, events of endosperm and embryo development, maturation of ovule (s) into seed (s) and ovary into
fruit, are collectively termed as post-fertilisation events. - The development of endosperm precedes embryo development since the cells of endosperm are
filled with reserve food materials that are used for the nutrition of developing embryo. - In the most common type of endosperm development, the primary endosperm nucleus undergoes
successive nuclear divisions to produce a large number of free nuclei without cell wall. This stage
of endosperm development is called free nuclear endosperm. - Cell wall formation occurs subsequently and the endosperm becomes cellular.
- The coconut water from tender coconut is free-nuclear endosperm and the surrounding white
kernel is the cellular endosperm. - Embryo develops at the micropylar end of the embryo sac where the zygote is situated.
- The early stages of embryo development are similar in both monocotyledons and dicotyledons.
- A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The
portion of the embryonal axis above the level of cotyledons is the epicotyl, which terminates
with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl
that terminates at its lower end in the radicle or root tip. The root tip is covered with a root cap. - Only one cotyledon is present in embryos of monocotyledons. In the grass family the cotyledon
is called scutellum. At its lower end the embryonal axis has the radicle and root cap enclosed in
an undifferentiated sheath called coleorhiza. The portion of the embryonal axis above the level
of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia
enclosed in a hollow foliar structure, the coleoptile. - The seed is the final product of sexual reproduction in angiosperms.
- Seeds are often described as fertilised ovules and are formed inside fruits.
- A seed typically consists of seed coat(s), cotyledon (s) and an embryo axis.
- As ovules mature into seeds, the ovary develops into a fruit.
- The wall of the ovary develops into the wall of fruit called pericarp.
- In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off.
- However, in few species such as apple, cashew, strawberry, etc. the thalamus also contributes to fruit formation. Such fruits
are called false fruits.
(a)Fig.: (a) A typical dicot embryo,
(b) L.S. of an embryo of grass(b)