SPOROGENESIS AND GAMETOGENESIS

          1. SPOROGENESIS:- The process of formation of male reproductive unit (microspore or pollen grain) and female reproductive unit (megaspore or egg) is known as sporogenesis. Sporogenesis is of two types –

            (a) Microsporogenesis:- The process of formation of male reproductive unit (microspore or pollen grain) is known as microsporogenesis.

            Microsporophyll or the male reproductive organ of a flower bears two anther lobes at the tip. The transverse section of a young anther shows a mass of homogenous, meristematic cells limited by an epidermis. A hypodermal layer having larger cells with dense cytoplasm and prominent nuclei forms below the epidermis. This layer is called archesporial layer. In archesporial layer four cells become very much prominent, one in each developing lobe. These cells are called archesporial cells. Each archesporial cells by a periclinal division forms a primary wall cell and an inner primary sporogenous cell. The primary wall cellby periclinal and anticlinal division forms 2-6 layers of the wall of microsporangium. The primary sporogenous cell by repeated division forms a mass of sporogenous cells. (Of the concentric layers of the microsporangium wall – the outermost one is called epidermis or exothecium, the innermost is the tapetum and the middle layer is the endothecium).

            The primary sporogenous cells are diploid (2n) which may undergo first by meiotic division to produce a large number of diploid nuclei which may directly behave as microspore mother cell (MMC). Each diploid nucleus or each MMC by meiotic division gives rise to four haploid microspores. Initially all the four microspores remain enclosed in a common wall. This four celled structure is called microspore tetrad. A mature microspore is called pollen grain.

              (b) Megasporogenesis:- The process of formation of female reproductive unit (megaspore) is known as megasporogenesis.

            Megasporophyll or the female reproductive organ of a flower comprises three basic parts – ovary, style and stigma. Attached inside the ovary lies the ovule, which is surrounded by one or two protective layer of wall, called the integument. Inside the wall, the ovule is occupied by a kind of tissue called nucellus.

            One of the hypodermal cells of the nucellus of an ovule becomes more prominent because of its large size and denser cytoplasm. This is the primary archesporial cell. This archesporial cell divides to form the outer primary parietal cell and an inner primary sporogenous cell. Primary sporogenous cell act as megaspore mother cell (MMC) and the primary parietal cell forms the wall of the MMC.

            The MMC is diploid (2n). It undergoes reduction division (meiotic) at first to produce two haploid (n) cells. These two haploid cells again divide by transverse (mitotic) division to produce a row of four haploid cells. Of these four cells, one cell of the chalazal end behaves as functional megaspore while the remaining upper three cells degenerate.

            The functional megaspore gives rise to female gametophyte or the embryo sac.

          2. GAMETOGENESIS:- The process of formation of male and female gametes i.e., sperm and egg respectively is known as gametogenesis. Gametogenesis is of two types –

            (a) Microgametogenesis:- The process of formation of male gamete or sperm is known as microgametogenesis.

            Microspore or the pollen grain is the first cell of the male gametophyte. It consists of a single nucleus and covered by two walls – outer wall is called exine and an inner wall is called the intine. The formation of the male gametophyte starts with the division of microspore nucleus while the microspores are still within the microsporangium. At first the microspore nucleus divides transversely to form two cells – the larger one is called the vegetative cell and the smaller one is called the generative cell. Further development of male gametophyte takes place after pollination.

            After pollination the pollen tube germinates. The exine breaks and the intine protrude out of the exine to produce long pollen tube. The vegetative nucleus now enters the pollen tube and is called tube nucleus. The generative nucleus also enters the pollen tube and divides to form two nuclei. These two nuclei are called male gametes.

          (b) Megagametogenesis:- The process of formation of female gamete or egg is known as megagametogenesis.

            Functional megaspore is the first cell of the female gametophyte. The nucleus of the functional megaspore first divides into two nuclei (n) which move apart to the opposite poles. Each nucleus then again divides to form four nuclei (n). Further division takes place to form total of eight nuclei. Of these four nuclei assemble at the chalazal end and the other four nuclei assemble at the micropylar end.

            One nucleus from each end now moves towards the centre of the embryo-sac and fuses with each other to form secondary nucleus (2n) or definitive nucleus. The three nuclei remaining at the micropylar end form the egg apparatus. Of, the three nuclei, the middle one is the largest and is known as egg or oosphere or ovum. The other two nuclei are called the synergids. The three nuclei at the chalazal end are known as the antipodal cells.

            Synergids help in the process of fertilization by guiding the pollen tube to the egg cell. The antipodals have no function and soon degenerate. During fertilization one of the male gamete fuses with the egg to form an embryo (2n), while the other fuses with the secondary nucleus (triple fusion) to form endosperm nucleus (3n).

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