CBSE 12th Term-I Biology 2021-22 


SEXUAL REPRODUCTION 

IN FLOWERING PLANTS

BASIC CONCEPTS

CBSE 12th Term-I Biology 

1. A typical microsporangium appears circular and is

surrounded by fom walls:

(i) Epidermis: It is the outermost single layer of cell which is protective in name.

(ii) Endothecium: It is the second layer with thick cells. help in dehiscence and is protective in nature. 

(iii) Middle layer: It is the third layer composed of 1-3 layers of cells, help in dehiscence and is protective in nature.

(iv) lapelum: It is the fourth and innermost layer of cell with dense cytoplasm and many nuclei. It provides nourishment to the developing pollen grains.


2. The process of formation of microspore from a pollen mother cell by meiosis is called microsporogenesis. Pollen mother cell (PMC) or microspore mother cell undergoes meiotic divisions to form chister of four cells called microspore tetrad. On maturity, the auther dehydrates and the microspores separate from each other to form pollen grains with two layered wall-outer hard exine and inner intine,

3. The outer wall layer possess exine, which is a hard layer made of sporopollenin which is one of the most resistant organic material present in nature. It can withstand high temperature and strong acids and alkalis. Even enzymes cannot degrade sporepollenin and hence pollens arr preserved as fossils. The inner thin layer of intine is made up of cellulose and pectin


4. The process of formation of haploid megaspores from the diploid megaspore mother cell (MMC) is called megasporogenesis. MMC first divides transversely into two tells called dyad. These two cells again clivide transversely, as a result a linear row of four haploid cells is produced which is called megaspore letrad or linear tetrad. Out of these four megaspores, only one remains fictional while the other three degenerate.


5. The formation of female gametophyte (embryo sac) is called megagametogenesis. Female gametophyte is 7 celled, 8 nucleate as it consist of 7 cells but 8 nuclei (2 nuclei in polar cell).


6. The transfer of pollen grains from anther and their deposition over stigma of the pistil is termed as pollination. Depending upon sources of pollen graius, pollination is of three types.


Pollination

(i). Autogamy (ii). Geltonogamy (Iii). Xenogamy/Allogamy

(i). Autogamy

1. The transfer of pollen grains is from anther to stigma of the same flower, 8.g., pea, nice, wheal, ele It is self-pollination

2. Brings polen with similar genetic constitution on sligma.

(ii). Geltonogamy

1. The transfer of pollen grains is from anther of one flower to the sbgma of another flower of same plant, eg Cucurbita..

2. It is functionally cross-pollination but genetically similar to Butogamy

(Iii). Xenogamy/Allogamy

1. The transfer of pollen grains is from anther of one flower to the stigma of another flower of different plant, eg. Papaya maize.

2 Brings genetically different types of pollen grains to stigma.


7. On reaching synergid, pollen tube releases the two male gametes into cytoplasm of synergid.

One of the male gamete fuses with egg nucleus to form a diploid cell called zygote. This event is called syngamy.


Other male gamete fuses with polar nuclei at the centre to produce a triploid primary endosperm nucleus (PEN). This is termed as triple fusion.


8. As syngany and triple fusion take place simultaneously in the embryo sac, it is termed as double fertilisation. The central cell after triple fusion forms primary endosperm cell (PEC) which later develops into endosperm. The zygote later develops into an embryo.


9. Endosperm develops first, followed by an embryo. Endosperm develops from PEN. PEN undergoes

successive nuclear divisions to give rise to free nucles and this stage of endosperm development is called free nuclear endosperm


10. Embryo develops from zygote at the micropylar end of embryo sac. The nutrition for development is provided by endosperm. The zygote divides mitotically to form proembryo. The different stages of the developing proembryo are globular and hears-shaped embryo, which finally forms mature embryo


11. The phenomenon of asexual reproduction that imitates sexual reproduction by formation of seed without fertilisation is called apomixis or agamospermy. Ways of development of apomictic seeds:

(i). A diploid egg is formed without reduction division and develops into embryo without fertilisation. 

(ii). Some cells of the nucellus, which are diploid in nature, start dividing and without fertilisation develop into embryo, e.g., ritrus fruits and mangu.

12. The occurrence of more than one endaya in a seed is called polyembryony, e.g., orange. Ways of forming polyembryonic seeds.

(i). Development of cells like synergals, cells of cellus, cells of integument, into embryo. 

(ii). Formation of more than the embryu sac in an ovule

(iii). Formation of more than one egg in an embuyu sar.