Fertilisation and Embryo Development
Flowers are the reproductive organs of angiosperms. Androecium consists of stamens which represents the male reproductive organs while gynoecium consists of pistils which represent the female reproductive part. The process of transfer of pollen grains from the anther to the stigma of the flower is called pollination. Pollination is of two types, namely self pollination and cross pollination. Based on different agents of cross pollination, pollination is categorized into hydrophily (water), anemophily (wind) and entomophily (insects). The process of fusion of male and female gametes is called fertilisation that occurs inside the embryo sac. The development of pollen grains takes place in two stages that are pre pollination event and post pollination event. Microspores mature to form pollen grains. Pollen grains consist of two layers; the inner layer or the intine and the outer layer or the exine. The exine ruptures at the germ pore and the intine protrudes out to form a long tube, known as the pollen tube. Ovules are present in the ovary which consists of a stalk called funicle, protective integuments and a minute opening at one end called the micropyle. The interior of an ovule consists of a nutritive tissue called the nucellus that encloses embryo sac and provides nourishment to the embryo. The megaspore nucleus undergoes repeated mitotic divisions to form an 8-celled gametophyte or embryo sac. Fertilisation in an ovule takes place in two sets. Firstly, Between one sperm nucleus and fused polar cells and secondly, between other sperm nucleus and egg nucleus. Thus, this type of fertilisation is termed as double fertilisation. After fertilization, Zygote develops into an embryo. The endosperm nucleus undergoes repeated mitotic divisions to form a mass of triploid nuclei in the embryo sac that develops into a stored nutritional part of a seed called endosperm. Developmental patterns of endosperm may be nuclear type, cellular type or helobial type. Seeds that retain endosperm are known as albuminous seeds and those which do not retain endosperm are termed non-albuminous seeds. The development of zygotes occurs at the micropylar end of the embryo sac. Zygote forms an embryo by undergoing mitotic divisions. The collective changes occurring in the zygote till it becomes a fully developed embryo. The fruit is generally the ripened ovary. The fruits developing from a single ovary of a single flower are called true fruits while fruits which develop without fertilisation and are called as parthenocarpic fruits.
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