(a) (i) Explain the role of stigma in pollen pistil interaction. (ii) Describe the post pollination events leading to double fertilization in…

Stigma has the ability to recognise the right or compatible and wrong or

incompatible type of pollen

Compatible pollen grain germinates on the stigma to produce pollen tube that grows through the tissues of stigma and style and reaches to the ovary

Generative cell divides into two male gametes while moving through style and Pollen tube enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus

The pollen tube releases the two male gametes in the cytoplasm of Synergid and one male gamete moves towards the egg cell and fuses with its nucleus forming a zygote (syngamy)

The other male gamete moves towards the two polar nuclei present in the central cell and fuses with them to form Primary Endosperm Nucleus / PEN

(triple fusion).

(Since syngamy and triple fusion both occur in embryo sac so it is termed as

Double Fertilization)

During menstrual phase in the menstrual cycle of human female endometrium lining of the uterus and its blood vessels which forms liquid comes out through vagina and flow lasts for $\displaystyle 3$-$\displaystyle 5$ days.

As the regeneration of endometrium through proliferation takes place.

Graafian follicle ruptures to release secondary oocyte, remaining part of graafian follicle transforms into corpus luteum