Sexual reproduction in plants
As you already know, a flower has four whorls-calyx, corolla, androecium’s and gynoecium’s. The sepals from the calyx and the petals form the corolla. The androecium’s comprises the stamens, or the male units of the flower. Each stamen bears a long, slender stalk called the filament, at the tip of which lies pollen –bearing structure called the anther. The anther produces thousands if dusty pollen grains, which give rise to the male gametes.
The gynoecium’s, also called the pistil, is the female part of a flower. It may consist of one or more carpel’s, each consisting of an ovary, style and stigma. The ovary bears ovules, which are round structures that contain the female gamete, or egg cell. At the lower end of the ovule is an opening called the micro Pyle?
Some flowers do not have a pistil while some do not have statements. Such flowers are known as unisexual flowers. Those which have both parts are called bisexual.
Pollination
The transfer of pollen grains from the anther to the stigma is called pollination. This is the first step in the sexual reproduction of flowering plants. When the anther matures, it bursts open to release pollen grains, which are transferred to the stigma of the same flower or other flowers of the same species.
When the pollen is transferred to the stigmas o the same flower or of flowers borne by the same plant, the process is called self-pollination. When it is transferred to the flowers of their plants of the same species, it is called cross- pollination.
Pollen is carried by various external agents, like the wind, insects, water and animals. Most flowers are pollinated either by the wind or insects. Those pollinated by the wind produce dusty, minute and light pollen grain, which can be easily blown by the breeze. Insect-pollinated flowers produce sticky or spiny pollen grains, which readily stick to the body insects visiting the flowers to collect nectar. When the insect’s body brushes against the stigmas of another flower, the pollen grains get transferred to that flower.
Insect-pollinated flowers usually have colorful petals, which attract insects. Thus, the corolla, which consists of the petals, plays an important role in the process of pollution. Insects are also attracted by scent. This is why lowers with white petals are usually fragrant. The same goes for very small flowers.
Artificial pollination
We use the mechanism of cross –pollination to develop new and better varieties of plants. For example, a particular variety may be resistant to a certain diseases, but may not produce a high yield. Through artificial pollination, the diseases-resistant character can be combined with that of high yield in a single variety. The same can be done in the case of other features, such as taste, color, texture, shape and nutritional value.
To begin with, two varieties with one or more desired characters are selected. They must belong to the same species. The stamens of the flowers of one variety are removed before the anthers mature, and the gynoecium’s is covered with a bag so that natural pollination cannot occur. Pollen grains from the second variety are collected and brushed on to the stigma of the first variety. The natural reproductive process is then allowed to take its own course.
The process of obtaining a new variety by artificial means is called hybridization and the variety of plant is called a hybrid. We have developed many hybrid varieties, including those of rice, wheat, maize, cotton and sugarcane.
Like it on Facebook, Tweet it or share this article on other bookmarking websites.