Double fertilization & triple fusion

Double fertilization and Triple fusion in angiosperm

1. Double fertilization and Triple Fusion Dr. Vasanta I. Kahalkar Dept. of Botany M. G. College, Armori

2. Entry of Pollen Tube into the Ovule: After reaching the ovary, the pollen tube enters the ovule through micropylar end. It then enters one of the synergids through the filiform apparatus present at micropylar end. The synergids direct the growth of pollen tube by secreting some chemical substances. The tip of the pollen tube enters one synergid , then the penetrated synergid starts degenerating. After penetration, the tip of the pollen tube enlarges and ruptures by releasing the contents, including the two male gametes and vegetative nucleus.

3. Entry of Pollen Tube into the Ovule:

4. Entry of Pollen Tube into the Ovule: After reaching the ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by one of the folowing routes:- Porogamy :- When the pollen tube enters the ovule through micropylar end, it is called porogamy . Eg .- Lily Chalazogamy : The entry pollen tube into the ovule from chalazal region is known as chalazogamy . Eg - Betula Mesogamy :- The pollen tube enter the ovule through its middle part i.e. Integuments or through funicle .

5. Double fertilization and Triple Fusion The process of fusion of one male gamete with the egg along with the union of the second male gamete with the two polar nuclei or the secondary nucleus is called double fertilisation. Out of the two male gametes, one fuses with the egg to carry out generative fertilization or syngamy . It gives rise to a diploid zygote or oosphere . The nucleus of the second male gamete fuses with two haploid polar nuclei or diploid secondary nucleus of the central cell to form a triploid primary endosperm nucleus (PEN). The central cell is called primary endosperm cell (PEC). This is called vegetative fertilization or triple fusion

6. Double fertilization and Triple Fusion

7. Significance of Double Fertilization Fusion of male and female gamete are store diploid condition. There are chances of polyembryony , and the plant has better chances of survival. Double fertilization gives rise to an endosperm that provides nourishment to the developing embryo. It increases the viability of the seeds of angiosperms.

8. ENDOSPERM AND ITS TYPES Endosperm is the nutritive tissue for the developing embryo and also the seedling. In angiosperm the endosperm develops from triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilization, triple fusion or fusion of a male gamete with secondary nucleus of the central cell.

9. Type of endosperm: on the basis of development endosperm are three types:- Nuclear endosperm :- In the nuclear types of endosperm development the primary endosperm nucleus divides by repeated mitotic free nuclear divisions without the formation of walls. It result in the formation of a large number of free nuclei in the central cell of the embryo sac. A big central vacuole develops in the embryo sac pushing all the nuclei to the peripheral cytoplasm. Finally cell wall formation takes place from the periphery of the embryo sac towards the centre leading to the formation of cellular endosperm tissue. In the Coconut, the endosperm is multicellular in the outer part and free nuclear in the centre. Nuclear endosperm is the most common type of endosperm and mostly found in polypetalae . Eg .- Cotton.

10. CELLULAR ENDOSPERM In the cellular type of endosperm development the 1 st nuclear division of the endosperm nucleus is immediately followed by wall formation. The 1 st division result in the formation of two equal sized chamber, chalazal and micropylar chambers. The subsequent division are fallowed by regular cell wall formation. This types of endosperm formation is common in gamopetalae . Eg.-Datura .

11. Helobial endosperm In the Helobial type of endosperm development, the endosperm is intermediate between cellular and nuclear types. The division of primary endosperm nucleus is followed by wall formation and as a result two chamber micropylar and chalazal chamber, are formed. Generally the chalazal cell divides by repeated free nuclear divisions and further development takes places in the same way as the nuclear endosperm. Helobial type of endosperm development is prevent in Helobiales order of monocotyledonds . Eg - Erumirus .

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