Why meiosis is essential at the time of sporogenesis in angiosperms? Explain the process of formation of male gametophyte in angiosperms with suitable diagrams.

Sporogenesis s the process of formation of spores. It takes place by the process of meiosis so as to maintain the ploidy level of the zygote and maintain the number of chromosomes in the individual species. The anther in the male flowers of the angiosperms contains two lobes and each lobe has two pollen sacs or microsporangia. Each microsporangium has many microspore mother cells in it. The pollen sacs produce pollen grains. Each pollen grain has two cells: large generative cell and one small tube cell. This pollen grain is transferred to the stigma of the flower. The secretion by the stigma directs the pollen grain and the intine comes out of the pollen grain and the pollen tube formation is initiated. The tube cell has stored food and energy.The tube cell has stored food and energy. -The tube cell, generative cell and cytoplasm all move to the pollen tube which is directed by the ovary’s chemical stimuli. This pollen tube consisting of two male gametes and a degenerating tube nucleus represents the male gametophyte. The process of formation of the male gametophyte has been shown diagrammatically:

What is double fertilization? Describe the process with diagrams from the stage of megaspore formation to zygote formation.

Double fertilization is the process in which one of the two male gametes fuses with the egg cell producing diploid zygote that develops into embryo or new sporophyte and other male gamete fuses with the two polar nuclei resulting in the formation of triploid endosperm. The primary sporogenous cell functions directly as diploid megaspore mother cell or megasporocyte. The megaspore mother cell (MMC) undergoes meiosis and forms a linear tetrad of 4 haploid megaspores. The process of meiotic formation of haploid megaspores from diploid megaspore mother cell is called megasporogenesis. Commonly the chalazal megaspore remains functional while the other 3 degenerate The functional megaspore is the first cell of female gametophyte. The cell enlarges and undergoes three free nuclear mitotic divisions. The first division produces two nucleate embryo sac. The two nuclei shift to the two ends and divide there twice forming four nucleate and then eight nucleate structure. One nucleus from each side moves to the middle. They are called polar nuclei. The remaining three nuclei form cells at the two ends. 3 celled & apparatus at the micropylar end and three antipodal cells at the chalazal end. The middle binucleate part organises itself into central cell.

Why grafting is not possible in monocot plants?

Monocot plants have scattered, closed vascular bundles that lack the cambium tissue that is present in dicot plants. Cambium is the meristematic tissue that has a regeneration capacity due to which it can form new tissue at the time of grafting. The lack of cambium in monocots makes it impossible to graft. It only works with dicots because of the presence of cambium and not with monocots.

Name the edible part of an apple.

In most of the plants, fruits are formed from the fertilized ovary and other floral parts degenerate and fall off. Apple is a false fruit, that developed from other parts of the flower as well as the ovary. The fleshy edible part of an apple is its thalamus.

Explain any two outbreeding devices that have developed in flowering plants to encourage cross-pollination.

Cross-pollination is the transfer of pollen grains from the anther to the stigma of the pistil of flowers of different plants. It is an important phenomenon to create variability among species. Hence, nature prefers cross-pollination and different adaptations are made by the plant to encourage this. These are called outbreeding devices. Some of them are as follows: Self-incompatibility: The mature pollen falls on the receptive stigma of the same flower but fails to produce a pollen tube due to the presence of a similar self-sterile gene. Dichogamy: Stigma and pollen grains of the flower mature at different times. Hence, the receptivity of stigma and release of the pollen grain is not synchronized. The stigma becomes receptive much before pollens are released or after they are released to avoid self-pollination.

What is triple fusion? Name the nuclei involved in triple fusion.

Triple fusion is the fusion of two polar nuclei with one male gamete to form a triploid primary endosperm nucleus (PEN). PEN develops to form triploid endosperm. The endosperm provides all the nutrition to the developing embryo. Triple fusion takes place in the embryo sac that is the female gametophyte of the flowering plants. Triple fusion involves one haploid male gamete and two haploid polar nuclei or a diploid secondary nucleus'(formed by the fusion of polar nuclei).