● Majority of flowering plants produce `color{Violet}"hermaphrodite flowers"` and pollen grains are likely to come in contact with the stigma of the same flower.

● Continued `color{Violet}"self-pollination"` result in `color{Violet}"inbreeding depression"`.

● Flowering plants have developed many devices to `color{Violet}"discourage self pollination"` and to `color{Violet}"encourage cross-pollination"`.

● In some species, pollen `color{Violet}"release"` and stigma `color{Violet}"receptivity"` are not `color{Violet}"synchronised"`.

● Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen.

● In some other species, the anther and stigma are placed at `color{Violet}"different positions"` so that the pollen cannot come in contact with the `color{Violet}"stigma of the same flower."`

● Both these devices `color{Violet}"prevent autogamy"`.

● The third device to prevent inbreeding is `color{Violet}"self-incompatibility"`.

● This is a `color{Violet}"genetic mechanism"` and prevents `color{Violet}"self-pollen"` (from the same flower or other flowers of the same plant) from fertilising the ovules by `color{Violet}"inhibiting pollen germination"` or pollen tube growth in the pistil.

● Device to prevent self-pollination is the production of `color{Violet}"unisexual flowers"`.

● If both male and female flowers are present on the same plant such as `color{Violet}"castor and maize"`( `color{Violet}"monoecious"`), it prevents autogamy but `color{Violet}"not geitonogamy"`.

● In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female ( `color{Violet}"dioecy"`).

● This condition prevents both autogamy and geitonogamy.

● Artificial hybridisation is one of the `color{Violet}"major approaches"` of `color{Violet}"crop improvement"` programme.

● In such crossing experiments it is important to make sure that only the `color{Violet}"desired pollen grains"` are used for pollination and the stigma is protected from `color{Violet}"contamination"` (from unwanted pollen).

● It is achieved by `color{Violet}"emasculation and bagging"` techniques.

● If the female parent bears `color{Violet}"bisexual flowers"`, `color{Violet}"removal of anthers"` from the flower bud before the anther dehisces using a pair of forceps is necessary. This step is referred to as `color{Violet}"emasculation"`.



● Emasculated flowers have to be covered with a `color{Violet}"bag of suitable size"`, generally made up of `color{Violet}"butter paper"`, to prevent contamination of its stigma with unwanted pollen.

● This process is called `color{Violet}"bagging"`.

● When the stigma of bagged flower attains `color{Violet}"receptivity"`, mature pollen grains collected from anthers of the male parent are `color{Violet}"dusted on the stigma"`, and the flowers are `color{Violet}"rebagged"`, and the fruits allowed to develop.



● If the female parent produces `color{Violet}"unisexual flowers"`, there is `color{Violet}"no need"` for emasculation.

● The female flower buds are `color{Violet}"bagged before the flowers open"`.

● When the stigma becomes `color{Violet}"receptive"`, pollination is carried out using the `color{Violet}"desired pollen"` and the flower `color{Violet}"rebagged"`.

● After entering one of the `color{Violet}"synergids"`, the pollen tube `color{Violet}"releases"` the two `color{Violet}"male gametes"` into the cytoplasm of the synergid.

●One of the male gametes moves towards the `color{Violet}"egg cell"` and fuses with its `color{Violet}"nucleus"` thus completing the `color{Violet}"syngamy"`.

● This results in the formation of a `color{Violet}"diploid cell"`, the `color{Violet}"zygote"`.

● The other male gamete moves towards the `color{Violet}"two polar nuclei"`located in the `color{Violet}"central cell
"` and fuses with them to produce a `color{Violet}"triploid primary endosperm nucleus"` (`color{Violet}"PEN"`).

● As this involves the fusion of three haploid nuclei it is termed `color{Violet}"triple fusion"`.

● Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed `color{Violet}"double fertilisation"`, an event `color{Violet}"unique to flowering plants"`.

● The central cell after triple fusion becomes the `color{Violet}"primary endosperm cell (PEC)"` and develops into the `color{Violet}"endosperm"` while the zygote develops into an `color{Violet}"embryo"`.