Question

Answer the following questions:

[(i)] Explain the structure of a mature embryo sac of a typical flowering plant.

[(ii)] How is triple fusion achieved in these plants?

OR

[(i)] Describe the changes in the ovary and the uterus as induced by the changes in the level of pituitary and ovarian hormones during menstrual cycle in a human female.

Hint:

Double fertilisation involves both syngamy (fusion with egg) and triple fusion (fusion with central cell). Menstrual cycle is hormonally regulated through a feedback mechanism.

Answer 1

1. • [(i)] A mature embryo sac in angiosperms is an 8-nucleated, 7-celled structure. It consists of:
     • Three cells at the micropylar end: one egg cell and two synergids.
     • Three antipodal cells at the chalazal end.
     • Two polar nuclei in the central cell which later fuse to form a diploid secondary nucleus.
   • [(ii)] Triple fusion occurs when one male gamete fuses with the diploid secondary nucleus (formed by two polar nuclei) in the central cell. This forms a triploid primary endosperm nucleus (PEN), leading to the formation of endosperm. Along with syngamy, this is called double fertilisation, a unique feature of angiosperms.
2. • [(i)] During the menstrual cycle:
     • In the follicular phase, FSH stimulates growth of ovarian follicles, leading to increased estrogen production.
     • Around the 14th day, a surge in LH induces ovulation.
     • In the luteal phase, the corpus luteum secretes progesterone, which prepares the endometrium for implantation.
     • If fertilisation does not occur, corpus luteum degenerates, progesterone levels drop, and menstruation begins.