Tapetum is the innermost wall layer of the microsporangium.
It nourishes the developing pollen grains.
Characteristics of tapetal cells:
(i) They possess dense cytoplasm.
(ii) They have more than one nucleus.

1.Emasculation is necessary to prevent the self-pollen grains falling on the stigma.
2. Emasculation is not needed when the female parent chosen bears unisexual,  i.e. female flowers.

(i) Certain nucellar cells around the embryo sac divide, protrude into embryo sac and develop into embryos, e.g. Citrus, mango, etc.
(ii) The diploid egg cell of the embryo sac, formed without meiosis develops into embryo without fertilisation.

(i) Certain nucellar cells around the embryo sac divide, protrude into embryo sac and develop into embryos, e.g. Citrus, mango, etc.
(ii) The diploid egg cell of the embryo sac, formed without meiosis develops into embryo without fertilisation.

(a) Globular embryo of a dicot plant.
(b) Zygote

Triple fusion:
It is  the process of fusion of three nuclei, two polar nuclei (fuse to form a diploid secondary nucleus) and a male gamete.
It occurs in the central cell of the embryo sac of the ovule of angiosperms.
Then nuclei involved are two polar nuclei and a male gamete.

Vallisneria is pollinated by water.
Water lily is pollinated by insects.
In Vallisneria, the male flowers (later pollen grains) are released on the surface of water and keep floating.
The female flowers reach the surface of water by uncoiling of the long stalk
When the passively carried male flowers/pollen grains meet the female flowers, pollination is effected.
The stalk of the female flower coils and the flower is drawn inside the water column.

Development of Female Gametophyte:
- The megaspore mother cell undergoes meiosis to form four megaspores.
- Three of them degenerate and the functional megaspore enlarges to form the embryo sac.
- Its haploid nucleus undergoes a mitotic division and the two daughter nuclei move towards the poles.
- Each of them undergoes two successive mitotic divisions.
- Cell wall formation starts in the 8-nucleate stage.
- Three of them at the micropylar end form the egg apparatus, consisting of a female gamete and two synergids.
- Three of them at the chalazal end form the antipodal cells.
- One nucleus from each of the poles moves to the centre; they are called polar nuclei.
- The two polar nuclei fuse in the centre (now the central cell) to form a diploid secondary nucleus.
- Thus, the mature embryo sac is 7-celled and 8-nucleate.

(i) The process of formation of megaspores from Megaspore Mother Cell or MMC (diploid) is called megasporogenesis.
It occurs inside the nucellus of developing ovule of angiosperms. The process begins very early when nucellus is not conmpletely surrounded by the integuments. The MMC (a large cell containing dense cytoplasm and a prominent nucleus) enlarges in size and divides by meiosis. It first divides transversely into two cells called megaspore dyad. 
These two cells apgain divide transversely as a result, a linear row of four haploid cells megaspore tetrad is produced.

Out of the four megaspores one situated at the chalazal end remains functional and the rest three gradually degenerate. Such a development of embryo sac is called monosporic development, e.g. Pobygonum.
(ii)

(a) A develops into the embryo.
    B develops into the endosperm.
(b) - The endosperm development starts from the primary endosperm cell containing the primary endosperm nucleus.
- In the most common type of endosperm development, the primary endosperm nucleus undergoes successive free nuclear divisions to give rise to a number of free nuclei; at this stage, it is called free nuclear endosperm.
- Subsequently wall formation takes place from the periphery and proceeds towards the centre and the endosperm becomes cellular.
- This type of endosperm development is seen in coconut; the water of the tender coconut represents the free nuclear endosperm with thousands of nuclei in it and the white kernel around it represents the cellular endosperm.
- In albuminous seeds,some amount of endosperm persists in the mature seed, as the developing' embryo does not consume it fully, e.g. wheat/maize.
-In exalbuminous seeds, the endosperm is completely consumed by the developing embryo before seed maturation, e.g. groundnut and pea.
Development of primary endosperm nucleus:
- The PEN undergoes successive nuclear divisions to give rise to free nuclei, this stage of endosperm development is known as free-nuclear endosperm.
- Subsequently cell wall formation occurs and the endosperm becomes cellular .

- After entering one of the synergids,the pollen tube discharges the two male gametes into the cytoplasm of that synergid.
- One of the male gametes moves towards the female gamete/egg and fuses with it; this fusion is called syngamy and it results in the formation of a zygote.
- The second male gamete moves towards the two polar nuclei in the central cell.
- The two polar nuclei just fuse to form a secondary nucleus and the second male gamete fuses with the secondary nucleus in the central cell; this fusion is called triple fusion and, it results in the formation of a triploid primary endosperm nucleus.
- Since two fusions, syngamy and triple fusion occur in an ovule, the phenomenon is known as double fertilisation in angiosperms. 
Post-fertilisation events:
- The central cell becomes the primary endosperm cell after triple fusion; the primary endosperm nucleus develops into the endosperm.
- Zygote develops into the embryo.
- Antipodals and synergids degenerate.
- The integuments develop into the seed coat.
- The ovule becomes transformed into a seed.