(a) Explain the role of stigma in pollen-pistil interactions.

(b) Describe the post-pollination events leading to double fertilization in angiosperms, starting with a two-celled pollen grain.

OR

(a) Mention the events that lead to the development of placenta during pregnancy in human females.

(b) Explain the role of placenta during pregnancy including its action as an endocrine organ.

•Stigma is a part of male flower where pollen grains germinate because anthers ( a part of stamen that contains pollen) are closed to the stigma

•Stigma works as a pollen receptor , they are covered with waxy and sticky substance

•It has a chemical signal interaction as stigma will not allow any undesirable pollen to germinate

•It provides the rehydration to pollen

•They trap the pollen to increase their efficiency

b) After the pollen-pistil interaction

•Germination of two male gametes ( a generative cell and a tube cell ) carried out in the pollen tube

•One male gamete fuses with egg cell (syngamy) to form diploid zygote

•Other male gamete fuses with two polar nuclei to form triploid endosperm ( which become food to embryo)

•Ovules forms the seeds and ovary forms the fruit

OR

a) Placentais an organ that works as a bridge between mother and the developing fetus. In this, umbilical cord is connected to the uterine wall that allows nutrient uptake, waste elimination and gases exchange

•Humans have a chorioallantoic placentathat forms from the chorion(outermost fetal membrane) and allantois(hollow sac-like structure filled with clear fluid)

PROCESS OF PLACENTA FORMATION –

1. Placenta begins to develop upon implantation of the blastocyst into the maternal endometrium

2. Outer layer of blastocyst becomes the trophoblast (which forms the outer layer of the placenta and is divided into two further parts)

3. The placenta grows throughout the pregnancy

Development of the maternal blood supply to the placenta is completed by the end of the first trimester of pregnancy week 14

b) Role of placenta during pregnancy-

•Nutrient supply to the fetus

•Gaseous exchange between fetus and mother

•Waste elimination from the fetus

•Antibodies can be passed from mother to the fetus

•It also acts as a barrier between mother and the fetus (which means it protects the fetus form the harmful substance present in the mother’s blood)

Role of placenta as endocrine gland-

It releases and maintains the hormones such as

•Progesterone – maintains the pregnancy by thickening the cervix and promotes the mammary growth

•Estrogen – stimulate the growth of myometrium

•Human chorionic gonadotropin (hCG) – it works as a signal for delivery time

•Placental lactogen – it is related to Prolactin and growth hormone