Explain the germination of pollen grain with the help of a diagram.

Germination of Pollen Grain. Germination of the pollen grain starts within the pollen sac. The cell divides to form two cells. One of these is smaller in size and called generative cell and bigger one the tube cell. At this two celled stage the pollen grains are transferred to the stigma of the carpel and the pollen germinates further. The surface of the stigma is moist. The pollen grains swell due to absorption of moisture. The intine grows out through the germ pore in the form of a tube called pollen tube. It grows into the tissue of the style towards the ovary. As the pollen tube grows in length, the tube nucleus and generative cell moves into it. The tube nucleus keeps ahead of the generative nucleus. After reaching the ovary, the pollen tube makes its way towards the ovule and enters it through the micropyle. In certain cases it may enter from the chalaza or even from the sides. Ultimately, it passes through nucellus and reaches the egg apparatus. By this time the tube nucleus, has degenerated and the generative nucleus divides to form two male gametes which now move to the tip of the tube. The pollen tube bursts open either due to absorption of moisture from the synergid cell or by the tip of these cells causing a mechanical injury to the tube. The male gametes are released and also synergid disintegrate. Thus fusion takes place between one of the gametes with egg nucleus. It is termed fertilization. The cell thus formed is called oospore.


Germination of Pollen Grain. Germination of the pollen grain starts w 
Germination of pollen grain and formation of male gametophyte in an angiosperm.

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Explain the development of female gametophyte. Illustrate the answer with suitable diagrams.

Development of female gametophyte

1. The functional megaspore is the first cell of female gametophyte of angiosperms.

2. It enlarges to form the female gametophyte, also called embryo sac.

3. Its nucleus undergoes a mitotic division and the two nuclei move to the opposite poles, forming the 2-nucleate embryo sac.

4. Two successive mitotic divisions in each of these two nuclei result in the formation of an 8-nucleate embryo sac.

5. Cell wall formation starts at the eight-nucleate stage, resulting in the formation of a typical female gametophyte.

6. Three cells are grouped together at the micropylar end to form the egg apparatus, consisting of two synergids and a female gamete/egg cell.

7. Three cells are grouped together at the chalazal end; they are called antipodal cells.

8. The remaining two nuclei are called polar nuclei; they move to the centre of the embryo sac (now called central cell) and fuse to form a diploid secondary nucleus.

9. Thus, a typical angiosperm embryo sac is 8-nucleate and 7-celled.


Development of female gametophyte
1. The functional megaspore is

 Development of embryosac.
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The innermost layer of anther is tapetum whose function is :
  • dehiscence
  • mechanical
  • nutrition
  •  protection.

C.

nutrition
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What do you mean by development of embryo? Support the answer with diagram.

Development of embryo. 1. The zygote or oospore divides by a transverse wall into an upper suspensor cell and lower embryonal cell.

2. The suspensor cell which lies towards the micropylar end, divides by transverse divisions to constitute 7-10 cells called suspensor.

3. The upper cell of the suspensor filament towards micropylar end is called haustorial cell, whereas the cell lying above the embryo cell is called hypophysis.

4. The haustorial cell enlarges in size and attaches the suspensor to the tip of embryo sac.

5. The embryonal cell divides by second longitudinal division at right angle to the first and then by transverse division to form an octant or eight-celled embryo.

6. Out of these eight cells, the lower four cells of octant away from the suspensor give rise to the plumule and the two cotyledons, while the above four cells of octant near the suspensor forms the hypocotyl and stele of radicle.

7. Now this octant divides by a periclinal division to form outer single layered dermatogen from which arises the epidermis layer. The inner cells further divide to form periblem below the dermatogen and the central plerome.

8. The periblem forms the cortex while the plerome gives stele of embryo.

9. The lowermost cell of suspensor, which is lying just above the octant cells is known as hypophysis. The hypophysis divides to give rise the dermatogen and periblem of the radicle.

10. A fully developed embryo of dicotyledons has an embryonal axis differentiated into plumule, two cotyledons and radicle.


Development of embryo. 1. The zygote or oospore divides by a transver

 Stages of embryo development in a dicot angiosperm.

11. In the beginning embryo is globular. When two cotyledons differentiate from the sides with faint plumule in the centre, embryo becomes heart-shaped. Now the embryo undergoes rest and ovule is transformed into seed.

Monocot embryo. Unlike the dicots, where the embryonal mass is formed of eight cells, the anterior cells forming the plumule and the cotyledons, and the posterior forming hypocotyl, the development is much variable in monocots i.e. grass family. In some cases, suspensor does not develop at all. Only one cotyledon appears in monocots as a terminal structure. The plumule always appears laterally from it.

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In a bisexual flower when the gynaecium matures earlier than anther, it is called as :
  • protandry
  • protogyny
  • herkogyny
  • None of the above.


B.

protogyny
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