It is important to know the structure of the plant reproductive system because only then you can understand the terms and the role of different parts of the plants. In a typical flowering plant, reproductive parts are located within the flowers. In unisexual plants, the male and female reproductive parts are located within the same flower while in bisexual plants they are located in separate flowers. The male reproductive organ is known as androecium and the female is known as gynoecium. The androecium is composed of stamens which is made up of a filament and an anther. The anther contains the pollen sac which produces pollens. Male gametes are contained within the pollens.
The female reproductive part or the gynoecium is composed of one or more pistils. Pistil contains carpels, which shelter the ovary and ovules. Female gametes are located inside ovules. A pistil has three parts namely stigma, style and filament. The stigma is supported by stalk and it forms the receptor of pollen grains. The pollen grains pass through the style, filament and finally reaches the ovule. Each pollen grain has a vegetative nuclei and a generative nuclei, which play a significant role during fertilization.
• Difference in Mechanisms
Pollination is the process by which pollen grains, containing the male gametes are transferred to the ovules, bearing the female gametes for sexual union. The pollen grains get deposited to the sticky tip of the pistil, known as stigma and travels down to the filament in order to reach the ovules.
After pollination has taken place, there is fusion of male and female gametes. The pollen grains start germinating in response to a sugary substance secreted by the stigma. Now, from each pollen a pollen tube grows that tries to penetrate the ovary for fertilization. The pollen tube penetrates the ovule through a tiny opening known as the micropyle. Once it has burst into the ovules in the embryo sac, fertilization takes place. The vegetative nuclei or the cytoplasm is digested by the hydrolytic enzymes released by the pollen tube, while the generative nucleus divides to form two separate haploid sperm nuclei. One of the two sperm cells now fertilizes the egg cells inside the ovules to form a diploid zygote.
In double fertilization, the remaining male nuclei fuses with the diploid secondary nucleus to produce triploid primary endosperm nucleus. This forms the endosperm, the nutrient rich tissue inside the seed. It is most commonly found in angiosperms.
• Types of Pollination
Pollination is broadly classified as self pollination and cross-pollination.
Self Pollination: It is the transfer of pollen grains from the anther to stigma of the same flower or to another flower borne on the same plant. Self pollination takes place in both unisexual and bisexual flowers. It is further sub divided into two types:
- Autogamy - Pollination in same flower in same plant.
- Geitonogamy - Pollination in two different flowers in the same plant.
- Anemophily - Wind
- Entomophily - Insects
- Hydrophily - Water
- Ornithophily - Birds
- Cheiropteriphily - Bats
Depending upon the point of entry of the pollen tube into the ovule, fertilization is of three types.
Mesogamy: The pollen tube enters the ovule through integuments. E.g. Cucurbita
Porogamy: The pollen tube enters the ovule through the micropyle. It is most commonly found in angiosperms.
Chalazogamy: The pollen tube enters the ovule through chalaza. E.g. Casuarina
From the aforementioned content we can infer that pollination is simply the transfer of pollen grains from the anther to the stigma of the same/different flower while fertilization is the actual reproductive process wherein the sperm nuclei fuses with the ovary to form the zygote. In short, a successful pollination leads to fertilization. And post fertilization, embryogenesis (formation of new embryo) occurs. The ovule becomes the seed and the ovary becomes the fruit.