Dear students, today's post is for double fertilization process in Angiosperms and endosperm                 development and its types.

    DOUBLE FERTILIZATION AND ITS SIGNIFICANCE  

      The process in which haploid male and female gametes fused together to form diploid zygote is called as fertilization.

      In Angiosperms the process is termed as double fertilization because both the male gametes are used.

         Before fertilization the embryo sac, is 8 celled - 8 nucleated structure.

      At the time of fertilization the two polar nuclei fuse together to form diploid secondary nucleus. This is first fusion.

      Pollen tube carrying two non-motile male gametes (r) shows porogamy (entry of pollen tube through micropyle). Synergids (n) provides site for attachment of pollen tube inside the embryo sac. After this, pollen tube bursts and male gametes are released.

      Synergids are destroyed along with antipodal cells (n) present at chalazal end and form food material.

      One of the male gametes (n) fuses with egg cells (n) showing syngamy and diploid zygote is obtained. This is second fusion or fertilization.

Remaining male gamete fusses with diploid secondary nucleus forming triploid primary endospermic Nucleus (PEN). This is third fusion and called as triple fusion as three nuclei fuse together.

      Thus, at the end of fertilization diploid zygote and triploid primary endospermic Nucleus is obtained.

      Significance / Importance:         

      The process is unique to angiosperms.

      It results in restoration of diploid nature / sporophytic phase / dominant phase of lifecycle.

      It results in development of zygote which further develops into embryo.

      Fertilization results in the development of primary endospermic nucleus (3n) which further produces nourishing tissue endosperm.

      It provides dormancy to the seed.

      It increases germination capacity of seed.

      It brings about development of seed and fruit.

ENDOSPERM AND ITS TYPES:-

Endosperm is nourishing tissue present inside the seed of angiosperm.

It is post fertilization product in angiosperms (in gymnosperms it is pre-fertilization product).

It develops from triploid primary endospermic nucleus form by the fusion of diploid secondary nucleus and one of the male gametes (n). Hence, triploid in nature. 

Being triploid in nature, it provides better nourishment to the developing embryo. Hence, considered to be more effective.

On the basis of present or absence of endosperm at the time of maturity inside the seed, they are of two types.

(a) Endospermic seeds:

      E.g. Jowar, Maize, Rice, etc.

(b) Non-endospermic seeds.

      E.g. Pea, Bean, etc.

Endosperm is of three types:

i)    Cellular endosperm:- In these types, PEN divides showing regular karyokinesls and cytokinesis nature right from beginning.   E.g. Annona squamosa (Custard apple)

ii)   Free nuclear endosperm:- It is most common type. PEN undergoes only nuclear divisions. Development is completed as cellular. E.g. Coconut.

iii)  Helobial endosperm:- It is inter mediate type of endosperm. Micropyler chamber shows free nuclei while chalazal chamber shows nucleus only. E.g. Asphodelus.