Chapter 2: Sexual Reproduction in Flowering Plants

Ans: The egg apparatus of the embryo sac contains cells at three positions:
At the micropylar end, egg nucleus and synergids are located. The egg nucleus or egg cell is located at the center of these three cells. The other two cells are haploid cells lying on either side of the egg cell and are called synergid cells. These synergids constitute the filiform apparatus.

Ans: The compatibility of a pollen grain is checked or determined by the stigma part of a gynoecium or pistil. Stigma is the part where a pollen grain lands after it has been pollinated from the stamen. Mature pollens arrive at stigma, where, through a series of chemical reactions, fertilizing compatibility of these is determined. If the pollen is compatible, it makes its way towards the egg through style or stalk and if it is incompatible, it is prevented from doing so.

Ans: Cotyledons and nucellus, both are found in angiosperms and serve an important function in the life-cycle of a plant. They both serve the function of storing reserve food material and providing nutrition to the embryo of the plant formed after fertilization in a flower. Nucellus provides nourishment to the developing embryo while development of fruit is ongoing. Cotyledons provide nutrition to the embryo when the seed is germinating. There is one cotyledon in monocots and two in dicots.

Ans: Correct Ans for the blank is the generative cell. 

In a mature pollen grain, 2 kinds of cells are seen. One is a vegetative cell that functions to provide food or nutrition while developing whereas the other cell is called the generative cell which is the smaller cell and fertilizes the female egg cell during fertilization.

Ans: Meiosis is reduction division and thus, the daughter cells formed after meiosis are haploid in nature whereas mitosis leads to no change in the ploidy of cells and the daughter cells formed have the same ploidy as the mother cells. Here, lets first establish the ploidy of the cells mentioned:

Megaspore mother cell: 2n or diploid

Megaspore: n or haploid

Embryo sac: n or haploid

Egg: n

Therefore, we can see that the formation of megaspores requires meiosis and that the embryo sac involves mitosis i.e., the first division is meiosis and second is mitosis. In the third step, mitosis is occurring again as the ploidy remains the same. In the embryo sac, haploid is selected despite it having 8 nuclei as the ploidy of the cells or nuclei present in it is haploid i.e., they contain a single set of chromosomes.

Hence, 1-meiosis, 2-mitosis, 3-mitosis.

Ans: Egg apparatus is composed of a haploid egg nucleus or egg cell containing the egg with which pollen fuses to form a zygote; 

Two synergids which are also haploid cells; and

A filiform apparatus is formed by the cellular thickenings of synergids towards the micropylar end.

The pollen tube is formed as the pollen grain makes its path through the style or stalk of a gynoecium. The pollen begins its journey on the stigma where a chemical reaction determines whether the pollen is compatible or not. It then moves through the style with the help of its vegetative cell which also provides nutrition to the generative cell. This pollen then reaches the micropylar end of the ovary through a curved trajectory to meet the egg cell where the pollen nuclei i.e., the generative cell fuses with the egg nuclei to form a zygote.

Ans: The pistil after fertilization and maturation undergoes various changes. Different parts of a pistil or gynoecium form different structures thereof. The parts which develop into fruit and seeds specifically are the ovary and ovules, respectively. This is seen in true fruits and in many other fruits, various other structures help in the formation of fruit.

Ans: Polyembryony involves formation of a number of embryos i.e., more than one embryo from one fertilized egg cell.  A synergid which lies at the micropylar end is a haploid cell which means that the embryo formed from such a cell would be haploid in ploidy whereas an embryo formed from nucellus which is diploid would be diploid in ploidy.

Ans: Apomixis is a form of asexual reproduction in which an egg cell before getting fertilized develops into an embryo on itself. Therefore, the possibility of a diploid embryo from an unfertilized apomictic embryo sac is possible if the megaspore mother cell which is diploid develops into an embryo without undergoing fertilization.

Ans: When the pollen grain is shed at the three-celled stage, the three cells forming it are a vegetative cell and two generative cells. The generative cells are formed from mitosis of one generative cell which is present before maturation of the pollen grain. The three-celled stage may sometimes form after shedding of pollen grains too. The vegetative cell provides nutrition to the generative cells till they reach the gynoecium whereas the generative cells are the male gametes that later form zygote and endosperm.