Angiosperms, or flowering plants, are the most advanced form of plants. They have been around for 130 million years! We see them on grasslands and mountains, in forests and meadows, and in freshwater marshes. Angiosperms include the grains and fruits we eat along with the colorful flowers we see in a garden. In this lesson, we will look at how angiosperms reproduce.
Life Cycle of Angiosperms
The angiosperm life cycle begins with the development of the flower. The anthers and the ovary are the reproductive parts. Each cell inside the anther undergoes meiosis and produces four haploid spore cells. Each of these cells becomes a single pollen grain. Within each pollen grain, the nucleus undergoes one mitotic division to produce two haploid nuclei. The pollen grain, which is now the male gametophyte, stops growing until it is released from the anther and deposited on the stigma.
The female gametophyte develops within the ovules of the ovary. A single diploid cell undergoes meiosis to produce four haploid cells. Three of these cells disintegrate while one undergoes mitosis to produce eight nuclei, which, along with the surrounding membrane, constitute the embryo sac. The embryo sac is the female gametophyte. At its base lies the egg nucleus, which is also called the female gamete. When fertilized, this cell becomes the zygote, which develops into a new plant.
Reproduction in flowering plants begins with pollination. This process involves bringing together the male and the female parts of a flower. Pollen grains from the anthers of a flower are transferred to the stigma of the same flower or of another flower either by self-pollination or cross-pollination. Self-pollination occurs when anther and stigma are either in the same flower or in different flowers on the same plant. Certain legumes, such as the peanut plant, self-pollinate. Cross-pollination occurs when the two flowers involved are on different plants. The willow tree is an example of plants that cross-pollinate. Pollination is carried out by pollinators, which include wind, insects, birds, and bats. Animal-pollinated plants are often brightly colored or have a strong fragrance or sweet nectar to attract their pollinators.
Once pollen grains are transported from the anthers to the stigma, fertilization takes place.
During fertilization, the sperm cells in the pollen grains must reach the eggs. When a pollen grain lands on a stigma, a tube grows from the pollen grain through the style of the flower. Sperm cells in the pollen grain move down the pollen tube and toward the ovary. Inside the ovary are ovules, each of which contains an egg. Fertilization occurs when one of the sperm cells fuses with an egg inside an ovule. When the egg is fertilized, a diploid zygote is produced, which will grow into the new plant embryo. See the parts of the plant involved in fertilization in this cross section. Remember that fertilization cannot take place if a pollen grain from just any flower is transferred to any stigma. A pollen tube forms only when a pollen grain lands on a flower of the same species.
Angiosperms are unique in that they can reproduce by double fertilization. Two distinct fertilizations take place in the embryo sac. One sperm nucleus fuses with the egg nucleus to form the diploid zygote. The other sperm nucleus fuses with the two polar nuclei to form a triploid cell, which is a cell containing three sets of chromosomes. This triploid cell becomes the endosperm, which is a nutrient-rich tissue that provides nourishment to the developing embryo. Double fertilization is believed to be one reason angiosperms have thrived for millions of years. During this process, the endosperm tissue is formed only when the ovule is fertilized, which means that the plant's food resources are not wasted if fertilization doesn't happen.