STRUCTURE AND DEVELOPMENT OF MALE GAMETOPHYTE

STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

STRUCTURE AND DEVELOPMENT OF MALE GAMETOPHYTE

• Pre - Pollination Development 

• Pollen grain or microspore is the first cell of male gametophyte and represents immature male gametophyte . 
• Development of male gametophyte is precocious , that is , it begins inside the microsporangium or pollen sac . 
• When first formed the microspore or young pollen grain has a centrally placed nucleus embedded in dense cytoplasm covered by plasma membrane .
• It grows in size with the inflow of nutrients . Vacuoles develop and bring about rapid growth of pollen grain . 
• The vacuoles push the pollen grain nucleus to one side near the wall . 
• The protoplast then divides mitotically to form two unequal cells- small generative cell and large tube or vegetative cell . 
• A layer of callose develops around the generative cell which separates the cell from the pollen grain wall . Later on , callose dissolves and the naked generative cell comes to lie freely in the cytoplasm of the tube cell . 
• The tube cell has a vacuolate cytoplasm which is rich in the food reserve ( starch protein , fat with mostly unsaturated fatty acids ) and cell organelles . 
• Its nucleus becomes large and irregular . The generative cell is spindle shaped to spherical in outline with thin dense cytoplasm surrounding a prominent nucleus . 
• In some species the generative cell divides into two nonmotile male gamates prior to the dehiscence of anther and release of the pollen grains 
• Therefore , at the time of pollination , the pollen grain is either 2 - celled ( tube cell + generative cell ; found in over 60 % of flowering plants , mostly dicots ) or 3 - celled ( tube cell + two male gametes ) .

• Post - Pollination Development . 

• On the stigma the compatible pollen grain absorbs water and nutrients from the stigmatic secretion through its germ pores . 
• The tube or veg etative cell enlarges . 
• It comes out of the pollen grain through one of the germ pores or germinal furrows to form a pollen tube . 
• The pollen tube is covered over by intine . 
• The pollen tube absorbs nourishment from the cells of the style for its growth . 
• The tube nucleus descends to the tip of the pollen tube . 
• The generative cell ( or its products ) also passes into it . 
• It soon divides into two nonmotile male gametes if it is not already divided . 
• Each male gamete is lenticular to spherical in outline . 
• It has a large nucleus which is surrounded by a thin sheath of cytoplasm . 
• Each male gamete is considered to be one cell . 
• The tube nucleus may degenerate completely The pollen tube is glandular secretory ? absorptive . It has ( 1 ) Growth zone at tip ( cap block ) with dense active cytoplasm rich in vesicles . ( m ) Nuclear zone containing tube nucleus and male gametes . ( iii ) Vacuolization zone rich in vacuolated cytoplasm between active and inactive cytoplasm with a series of callose plugs to senarate older inactive cytoplasm . 
• Pollen tube not only carries male gametes but also secretes hormones and absorb food from style A pollen grain with pollen tube carrying male gameles represents mature male gameto phyte . 
• It is 3 celled ( one tube cell + 2 male gametes ) and 3 nucleated structure formed by two mitotic divisions .

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