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12th Biology 2012 Set1 Outside Delhi Board Paper Solution

Question 29

The following is the illustration of the sequence of ovarian events “a” to “i” in a human female:

(a) Figure ‘f’ illustrates the corpus luteum and the pituitary hormones responsible for are luteinizing hormone (LH) and follicle stimulating hormone (FSH).
(b) The corpus luteum secretes progesterone, which is a steroid hormone responsible for the development of the endometrium and maintenance, respectively. The endometrium of the uterus gets thickened and blood supply to the endometrium increases. It is essential for implantation of the fertilized ovum and other events of pregnancy.
(c) ‘d’ illustrates the developing follicle whereas ‘e’ illustrates the mature Graafian follicle. The difference is that primary follicles in the ovary grow to become a fully mature Graafian follicle.

Structure of Graafian Follicle


(a) At the time of fertilization, pollen tube brings two male gametes and the processes that take place in the embryo sac are-

(i) Syngamy- It is the fusion of both haploid male and female gamete to form a diploid zygote which further develops to form embryo.
Male Gamete + Egg (female gamete) ---> Zygote --->Embryo
Syngamy is also called generative fertilization.
(ii) Triple fusion- The other male gamete moves to the antreal cell to fuse with the two polar nuclei to form a triploid primary endosperm nucleus (PEN) which grows to form primary endosperm cell (PEC) and which divide mitotically to from endosperm.
Two polar nuclei + Male Gamete ---> PEN ----> PEC ---> Endosperm
Endosperm provides nutrition to the developing embryo. Thus triple fusion is also called vegetative fertilization.
Since this process involves two types of fertilization, it is referred to as double fertilization.
L.S of an endospermous monocot seed