Characteristics of Angiosperm
These are also called the Flowering Plants. Male and female gametes are developed in specialized structures called flowers. The seeds are enclosed by fruits. They are a very large group of plants and are found in a wide range of habitats. The angiosperms can be microscopic (Wolfia) to over 100 metre tall trees. They are highly important for us because we get food, fodder, fuel, medicines, and many other useful products from them.
Angiosperms are divided into two classes, viz. dicotyledon and monocotyledon.
Reproduction In Angiosperms
Male Sex Organ: Stamen is the male sex organ in the flower. A stamen is composed of a slender filament and an anther at the top. The anthers produce pollen grains through meiosis.
Female Sex Organs: Pistil or carpel is the female sex organ in a flower. The pistil consists of an ovary. The ovary may enclose one or many ovules. The embryo sac is formed by meiosis. Each embryo sac has three main parts, viz. egg apparatus, polar nuclei and atnipodal cells. The egg apprataus is composed of three cells, including the egg and two synergids. Apart from that, there are three antipodal cells and two polar nuclei. The polar nuclei finally fuse to produce a diploid secondary nucleus.
Pollination: Transfer of pollen grains to stigma is called pollination. Pollination is facilitated by various agents; like wind, water, animals, insects, etc.
Fertilization: Pollen grain germinates on the stigma and produces a pollen tube. The pollen tube pierces through the stigma and style and reaches the ovule. The male gametes are discharged near the embryo sac. One of the male gametes fuses with the egg cell and forms a zygote. The other male gamete fuses with the diploid secondary nucleus to produce the triploid Primary Endosperm Nucleus (PEN). This event is called Double Fertilisation because of the involvement of two fusions. Double fertilization is unique to angiosperms.
Embryo: The zygote develops into an embryo and the PEN develops into endosperm. Endosperm provides nutrition to the developing embryo. Synergids and antipodals degenerate after fertilization. Ovules develop into seeds and ovaries develop into fruits.
Alternation of generations
Both haploid and diploid cells in plants can undergo mitosis. Due to this unique ability in plants, formation of haploid and diploid plant bodies becomes possible. The haploid plant body produces gametes and it represents a gametophyte. After fertilization, the zygote undergoes mitosis and produces a diploid sporophytic plant body. The diploid sporophytic plant produces haploid spores by meiosis. The spores then undergo mitosis and form a haploid plant body once again. The haploid and diploid plant bodies can be of variable lenghts in different divisions of Plant Kingdom. They alternate with each other. This phenomenon is called alternation of generation.
Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between gametophyte and sporophyte. This phenomenon is called alternation of generations.
But different groups of plants differ in relative length and size of haploid or diploid generations. Such variations are as follows:
Haplontic: In such plants, a free-living gametophyte is the dominant photosynthetic phase. In this case, the sporophytic generation is represented only by the one-celled zygote. There is no free-living sporophyte. The zygote undergoes meiosis to form haploid spores. The spores then undergo mitosis to form the gametophyte. Examples: Volvox, Spirogyra and some species of Chlamydomonas.
Diplontic: In this case, the diploid sporophyte is the dominant photosynthetic phase of the plant. The gametophyte is represented by the single to few-celled haploid gametophyte. Gymnosperms and angiosperms show this pattern.
Haplo-diplontic: In this case, both phases are multicellular and free-living. Bryophytes and pteridophytes follow this pattern.