question_answer

(i) What is triple fusion? What is the product of this process? Where and how does it take place? Give the name of nuclei involved in triple fusion.

(ii) Which of the following, an endosperm or an embryo, develops first in an angiospermic seed? Give reason for the precedence.

(iii) State the role of endosperm in mature albuminous seed

(iv) Draw a labelled diagram of a mature dicotyledonous embryo.

Or

(i) With the help of a flowchart, explain the events that occur during fertilisation in humans. Also, mention the site of its occurrence.

(ii) Successful fertilisation leads to absence of menstruation in females. Give reason.

Answer:

(i) Triple fusion refers to the process of fusion of three haploid nuclei. It takes place in embryo sac. The three nuclei that fuse together are nucleus of the male gamete and two polar nuclei of the central cell. They produce a triploid primary endosperm nucleus.

(ii) The development of endosperm is followed by the development of an embryo in angiospermic seeds. It is because the endosperm cells provide nutrition to the developing embryo.

(iii) In mature albuminous seeds, endosperm exists as the storage tissue. It stores starch and fat.

(iv) Diagram showing mature dicot embryo.

Or

(i) Flowchart showing events of fertilisation in humans is as follows

Discharge of semen in female vagina

\[\downarrow \]

Movement of sperms to reach at ampullary - isthmus junction of Fallopian tube

\[\downarrow \]

Release and reach of ovum at the same junction

\[\downarrow \]

Contact of sperm with zona pellucida of the ovum

\[\downarrow \]

Secretion of acrosome makes a passage for sperm

\[\downarrow \]

Initiation and completion of meiotic division of secondary oocyte and fusion of sperm with ovum

\[\downarrow \]

Formation of zygote

The site of occurrence of fertilisation In humans is the ampullary - isthmic junction of Fallopian tube.

(ii) Successful fertilisation leads to development of foetus, its implantation and formation of placenta. The developing placenta secretes the hormone hCG, which maintains the corpus luteum and stimulates it to secrete progesterone and oestrogen. It causes growth of endometrium and checks its breakdown. This, leads to the absence of menstruation in females.