Question

How does double fertilization in a flowering plant occur?

Answer 1

Double fertilization in Angiosperms involves fusion of both the male gametes in pollen tube to form diploid zygote and a triploid endosperm mother cell.

Explanation:

              Double fertilization involves fusion of both the male gametes in pollen tube, growing through ovule to reach the embryo-sac. 
              The embryo sac (Female gametophyte) has egg-apparatus towards the micropyler end and three antipodals towards the chalazal end. 
              The egg apparatus consists of central egg or oosphere flanked by two synergids. 
              In the middle of the embryo-sac is the diploid nucleus.  
              One of the male gametes fuses with egg or oosphere to form diploid oospore (zygote) that gives rise to embryo.
              The other male gamete fuses with secondary diploid nucleus to form triploid cell that acts as endosperm mother cell. 
               This phenomenon of fusion of both the male gametes is termed double fertilization. It is unique to Angiosperms.
              Double fertilization is advantageous as the endosperm is formed only if embryo is formed. It thus avoids any unnecessary expenditure of energy on the part of the plant to form endosperm as in Gmnosperms in which female gametophyte is multicellular that acts as endosperm. 
              The energy spent in forming elaborate female gametophyte goes waste in Gymnosperms, if there is no fertilization.
              However. in Agiosperms endosperm is formed as a special tissue as a consequence of double fertilization, whereas the female gametophyte consists of only seven cells.