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Class 12th Biology 2016 Set1 Delhi Board Paper Solution

Question 24

(a) Explain the post-pollination events leading to seed production in angiospersms.
(b) List the different types of pollination depending upon the source of pollen grain.


(a) Briefly explain the events of fertilization and implantation in an adult human female.
(b) Comment on the role of placenta as an endocrine gland.


(a) As a result of pollen-pistil interaction, germination of pollen tube takes’ place carrying two male gametes. One male gamete fuses with the egg cell (syngamy), while the other fuses with two polar nuclei to form primary endosperm nucleus (PEN). The zygote develops into an embryo while the PEN develops to form endosperm. After double fertilisation, the ovule matures into a seed while the ovary matures into a fruit.

(b) Different types of pollination depending upon the source of pollen grain are :

 Autogamy: Transfer of pollen grains from the anther to the stigma of the same flower.  Geitonogamy : Transfer of pollen grain from the anther to the stigma of another flower of the same plant.  Xenogamy : Only types of pollination which brings genetically different types of pollen grains to the stigma.




(a) Fertilization : A sperm comes in contact with the zona pellucida layer ovum and induces changes to block entry of additional sperms. The entry of sperm induces completion of meiosis II leading to the formation of anootid and second polar body. The haploid nucleus of the sperm and that of the ovum fuse to form a dipolid zygote.
Implantation : The trophoblast layer of the blastocyst attaches to the endometrium of the uterus. The uterine
cells divide rapidly and cover the blastocyst which becomes embedded in the endometrium and implantation is completed.

(b) Placenta acts as an endocrine tissue and produces several hormones like :

 human chorionic gonadotropin (hCG) human placental lactogen (hPL)  Estrogens, progestogens, etc.