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Sexual Reproduction in Plants
Reproductive Parts of a Flower:
Flowers of some plants possess either only male reproductive part called stamen or the female reproductive part called pistils. Such flowers are called as unisexual flowers. It is likely that both male and female flower is present in the same plant or in different plants. Examples of plants containing unisexual flowers are corn, papaya and cucumber. While some plants contain both male and female reproductive part within the same flower, and hence are called as bisexual flowers. Example rose, mustard and petunia.
The stamen contains anther sitting on top of a filament. The anther contains pollen grains which produce male gametes. Likewise, a pistil contains a stalk called style, stigma and ovary containing ovules. These ovules produce female gametes called egg. In sexual reproduction, the male gametes and female gametes fuse to form a zygote.
The pollen grains present in the male flower travel across to plant containing female flower with the help of medium such as air and water. As pollens are light in weight and consist of a hard protective coating, they are not damaged easily while traveling. The pollens fall on the sticky tip, stigma of the female flower resulting in a phenomenon called as pollination. The pollens then travel through the stalk, stigma and reach the ovules where female gametes, egg is present. There are two different types of pollination occurring in nature – i) self-pollination, occurs when pollen from an anther lands on the stigma of the same plant and ii) cross-pollination, occurs when pollen from an anther lands on the stigma of a different plant (Figure 7.5.1). Cross-pollination results in diversity of the traits present in a plant.
The process of fusion of male and female gametes is defined as fertilization. Once the pollen reaches the ovules of a female flower, it results in the fusion of egg and the pollen resulting in the formation of a zygote. The zygote then develops into embryo.
Fruit and Seed Formation:
As the embryo develops, it is enclosed within a hard protective coating. This hard protective coat is called as seed . The ovary of the flower develops into a fruit and other parts of the flower fall off gradually. The fruit ripens eventually and is plucked off. In some plants, the ovary develops into a fleshy fruit like mango and apple, while some develop into hard fruit like almonds and walnuts.
The distribution of seeds in to different places is called as seed dispersal. Seed dispersal is essential to grow healthy plants otherwise; it would result in a competition for necessary elements such as sunlight, water and minerals. Hence, the medium through which seeds are dispersed are water, air, birds. By this way, plants grow in different habitats and adapt themselves to the environment. Some seeds are light and have wings-like structure to be carried off by wind, for example maple seeds. While some seeds are enabled to float in water for example coconut. Birds and other animals also contribute to the dispersal of seeds by eating the fruit and throwing away the seeds in a place where they migrate. In the case of castor plants, it is observed that there is a sudden burst of the fruit resulting in scattering of seeds distantly from the parental plant. In this way, the seeds are dispersed and result in growth of a new plant.