Biology XII:2 Sexual rep in flowering plants(1 Pre Fertilization)

Pre-Fertilisation Events Several hormonal and structural changes result in the development of a flower. Inflorescences bear the flower buds, and then the flowers. Flowers are the reproductive parts of a plant. In the flowers, the androecium (male reproductive part) and the gynoecium (female reproductive part) develop. Androecium The androecium consists of whorls of stamen. The stamen consists of the filament (long and slender stalk) and anther (bilobed structure). Filament is attached to the thalamus or to the petal. Anther: A typical anther is bilobed and each lobe is dithecous (consists of two theca). Theca are separated by a longitudinal groove running lengthwise. The microsporangia are located at the corners, two in each theca. They further develop to form pollen sacs, which contain the pollen grains. Structure of microsporangium The microsporangium is surrounded by four wall layers (epidermis, endothecium, middle layers, and tapetum). The outer three layers are protective and help in dehiscence of anther to release the pollen grains. The tapetum provides nourishment to the developing pollen grains. In the young anther, the sporogenous tissue forms the centre of each microsporangium. Microsporogenesis It is the process of formation of microspore from PMC (Pollen Mother Cells). As development occurs in the anther, the sporogenous tissue undergoes meiosis to form microspore tetrad. Each cell of sporogenous tissue has capacity to give rise to a tetrad. Hence, each cell is a potential pollen or PMC. As the anther matures, the microspores get detached from each other and develop into pollen grains. Pollen grains Represent the male gamete and are spherical, having a two-layered wall: Exine (outer) − Hard layer made of sporopollenin, which is extremely resistant and can withstand high temperatures, acidic and alkaline conditions, and enzymes Intine (inner) − Thin and continuous layer made up of cellulose and pectin Mature pollen grain contains two cells: Vegetative cell − Large with irregular nucleus, contains food reserves Generative cell − Small and floats in the cytoplasm of the vegetative cell In 60% of the angiosperms, pollen grains are shed at 2-celled stage while in others generative cell undergoes mitosis to form two male gametes (3-celled stage). The viability of pollen grains after they are shed depends upon temperature and humidity. It ranges from 30 minutes to few months.