Hello students, today’s part is about sexual reproduction in flowering plants. Flower the most fascinating structure is meant for sexual reproduction.

In this post we shall focus on parts of flowers directly involved in gametogenesis and reproduction.

You can have different questions on this portion in all sections of the paper and even diagram based questions as well as questions based on simple calculations.

Hope this post will help you to understand the portion and it will guide you regarding the points to be included in your answer.

FLOWER:-

Flower is highly modified and condensed shoot meant for process of sexual reproduction.

The essential floral whorls are as follows:

A) Androecium:-It is third in position and male reproductive part of flower. Its individual member is called as stamen. It is directly involved in gametogenesis i.e. gametes formation.    Typical stamen shows the presence of three parts,

Anther:-It is the terminal part and mostly bilobed, bithecus and tetra         locular.The two anther lobes produce pollen grains.

Connective:-It connects two anther lobes together and with the filament.

Filament: - It raises anther to a proper height for easy dispersal of pollen grains.

B) Gynoecium: - It is innermost in position and female reproductive part of flower. It is directly involved in gamete formation. Individual member is called as carpel or pistil. Typical carpel shows presence of flowering part.

(a) Stigma: It relives pollen grains at the time of pollination. It also helps in germination of pollen grains and development of gametophyte.

(b) Style: It is tubular part and provides passage for entry of pollen tube.

(c) Ovary: It is basal swollen part which shows the presence of ovules. After fertilization ovule develops into seed and ovary changes into fruit

INTERNAL STRUCTURE OF ANTHER

Anther is terminal part of flower. Most of the angiosperms show bilobed, bithicus, and tetralocular or tetrasporangiate anther. Anther in its transverse section shows presence of four distinct wall layers.

(a) Epidermis: It is the outermost wall layer made up of compactly arranged cells present in single layer. It is protective in function.

(b) Endothecium: It is also called as fibrous layer due to the presence of striations over its wall. These striations are due to deposition of callose over its walls. Shows hygroscopic nature, retains moisture. Under dry conditions loses water, shrinks and brings about dehiscence of anther (breaking of anther to release pollen grains.

 (c) Middle Lamella: It is made up of compactly arranged small flat cells present in two layers. It is the only double layered structure present inside the anther. It is protective in function.

(d) Tapetum: - It is the innermost layer of large compactly arranged cells with distinct nucleus. The cells secrete sporopollenin world’s toughest material. Tapetum also secretes Ubisch granules. Tapetum is nutritive in function and nourishes the pollen mother cells and developing microspores.

It also maintains the viability of pollen grains. It shows amoeboid nature or glandular nature. It also contributes in pollenkit formation.

Tapetum encloses a cavity called as pollen sac which shows the presence of   Pollen mother cells (2n) or microspore mother cells (2n).On meiosis or reduction division these cells produce pollen grains (n).This is called as Microsporogenesis.

STRUCTURE OF POLLEN GRAIN:-

Study of pollen grains is called as palynology.

Pollen grains are produced inside the anther by reduction division or meiosis in pollen mother cells (2n).

Development of pollen grains or microspores is called as Microsporogenesis.

Each pollen grain is unicellular, uninucleate, haploid structure. Pollen grains show presence of two wall layers.

A) Exine: It is outer, thick wall layer with small opening. It develops from sporopollenin, resistant material. Exine shows small openings which are called as germ pores which help pollen tube to come out. Exine is protective in function and shows ornamentation for pollination.

Artificial fossilization of pollen grains is possible due to presence of sporopollenin.

B) Intine: It is inner, thin and elastic wall layer.  After germination intine develops pollen tube which comes out of germ pore.

Pollen grain after germination develops into male gametophyte and produces male gametes (progenitor of male gametophyte.)

DEVELOPMENT OF MALE GAMETOPHYTE IN ANGIOSPERMS:-

Development of male gametophyte takes place from pollen grain (n).

The development is carried out at two different sites.

A) Before pollination / before dehiscence / inside the anther:

The development is initiated from unicellular, uninucleate haploid pollen grain inside the anther. The pollen grain shows mitosis and produces bicelled stage.

The first mitosis results in the development of two unequal cells. The larger cell with larger nucleus is called as vegetative cell or tube cell (n) and its nucleus is called as vegetative nucleus or tube nucleus (n).   The smaller cell is called as generative nucleus cell (n).  Pollen grain is released at bicelled stage in angiosperms at the time of dehiscence. (sometimes at tricelled stage).

Further development is continued over the stigma.

B) After pollination / after dehiscence / over the stigma:

The bicelled pollen grain further develops into tricelled stage. The second mitotic division takes place in generative cell producing two generative cells (n). The final stage of development is three- celled stage.  After this, cytoplasm and cell wall of all the three cells dissolve producing food material. Intine develops pollen tube which comes out through germ pore.

The matured male gametophyte shows a pollen tube with two non – motile haploid male gametes (n) and a vegetative nucleus (n).