Pollination in plants

41. Write a note on the need and significance of pollination in plants ?
Ans. (i) Pollination is essential for the formation of fruits and viable seeds.
(ii) Pollination adds a bit of auxin which stimulates the ovary to grow and mature into fruits.
(iii) By cross, pollination new varieties of plants can be prepared.
42. What do you understand by double fertilization ?
Ans. Pollen tube carries two males cells. On reaching the embryo sac t bursts.
One male gamete fuses with the egg to from a diploid zygote. it is called syngamy while the other male gamete fuses with the diploid secondary nucleus to form a triploid primary endosperm nucleus. It is called triple fusion. Syngamy + triple fusion is called double fertilization.
43. Describe the strcture of atypical monosporic embryo sac found in angiosperms ?
Ans. An embryo sac represents the female gametophyte. It has 8 haploid nuclei to begin with. Three nuclei become arranged in egg apparatus towards the micropylar' end. A central large °egg is surrounded by two synergids. Similarly threee nuclei at the opposite or chalazal end form three antipodal cells. Two nuclei come in the middle of embryo sac and are called polar nuclei. They fuse to form a diloid nucleus called secondary nucleus.
44. Give the important characters of emtomophilous flower ? Ans. (i) They produce large, coloured and showy corollas.
(ii) They may develop fragrance to attract the insects.
(iii) Some flowers develop [special structures so as to allure insects to them.
45. Explain the developed. embryo sac ?
Ans. The embryo sac develops from the single surviving megaspore in the nucellus. It increases in size. The nucleus divides mitotically into two nuclei which go to different poles. Each nucleus divides twice to form four nuclei each. Out of them, the three, at the -micropylar end be come organised into an egg apparatus (An egg and two synergids); the three at the chalthal end become arranged into antipodal cells. The remaining two nuclei come in the middle and are called polar nuclei. They fuse to form a diploid nucleus called secondary nucleus. This seven celled structure is called the female gametophyte or embryo sac.
46. Explain the significance of double fertilization ?
Ans. (i) It is necessary for the formation of viable seeds.
(ii) Endosperm develops only after fertilization. So, in case fertilization fails. there is no need for any endospherm and so no energy is wasted.