Plants reproduce sexually through the fusion of male and female gametes in the flower. Asexual reproduction is through stems, roots and leaves.
Plant reproduction comes in two types: sexual and asexual. Sexual reproduction is similar to human reproduction, which involves the fusion of the male (pollen) and female (ovule) gametes to form a new organism that inherits the genes of both the parents. The sexually reproductive part of a plant is the flower. Asexual reproduction, on the other hand, involves vegetative reproduction through stems, roots and leaves. Essentially, the parent plant regenerates itself by using one of its parts (roots, stems or leaves).
Both sexual and asexual methods of reproduction have their own set of advantages. In sexual reproduction, the new plant formed is a combination of genes, giving it an advantage in new ways concerning adaptation in changing environments. It can also avoid the transmission of certain diseases, due to some genes being dominant and others being recessive. Asexual reproduction is faster and perhaps the only manner of reproduction in species that do not bear flowers. Since asexual reproduction is basically the cloning of the parent plant, farmers can ensure that there is no genetic abnormality by selecting a healthy plant for reproduction.
Parts of a flower
The flower consists of four whorls or parts (calyx, corolla, androecium and gynoecium). The first whorl is the calyx, which contains the green sepals. The second whorl is the corolla, which contains the petals. The petals are brightly colored to attract the agents of pollination (bees and other insects) that aid in reproduction. The petals also protect the inner two whorls, which are directly involved in reproduction. The third whorl is the Androecium—the male reproductive part of the plant. The innermost whorl is the gynoecium—the female reproductive part of a plant.
The androecium contains a bundle of stamens that consist of a tube called a filament and the swollen end called the anther. The anther contains the pollen grains. Once matured, the pollen grains burst out in order to reach the female reproductive part of the flower.
The gynoecium contains the pistil, which is composed of the tube (style) that reaches the ovary. The swollen tip of the style is the stigma, which receives the pollen grains. The ovary contains the ovules, which turn into the seeds.
Unisexual and bisexual flowers
Unisexual or monosexual flowers contain either of the reproductive parts (stamen or pistil). Papaya, corn and cucumber are examples of unisexual flowers. The same plant can have both male and female unisexual flowers. Bisexual or complete flowers contain both the male and female gametes—the stamen and pistil. Examples of bisexual flowers are roses, petunias and mustard plants.
The transfer of the pollen from the anther to the stigma of a flower is known as pollination. The pollen (male gamete) and the egg (female gamete) unite to form a zygote during fertilization. A zygote then turns into an embryo, which finally becomes the seed. The seed then germinates into a new plant. There are two types of pollination—self-pollination and cross-pollination.
Self-Pollination: If the pollen grain from the anther lands on the stigma of the same flower or another flower of the same plant, it is known as self-pollination.
Cross-Pollination: If the pollen grain of an anther falls on the stigma of a flower of another plant, but of the same species, it is called cross-pollination.
Agents of pollination: Birds, insects, animals, water and wind are all called “agents of pollination”, as they assist in plant reproduction. Wind and water carry the pollen grains to other plants. Birds and insects are attracted to the color and scent of the flower. When they flit from one flower to another, pollen grains get stuck in their body and are then transported to other flowers. Animals and birds eat the fruits and disperse the seeds or the seeds remain undigested and are thus excreted from their body in full form.
Asexual reproduction is the means by which we can clone the best plant of a species. The Bartlett pear (1770) and the Delicious apple (1870) are still being asexually reproduced to get a product of the exact same quality. Vegetative propagation is a form of reproducing through the leaves, stems or roots of the parent plant. Asexual reproduction can also be done artificially by cutting, grafting and layering.
Sweet potato, Dahlia and Asparagus are all reproduced through tuberous roots. The roots of such plants contain buds that can produce leafy shoots under favorable conditions. Potato and ginger are reproduced through stem tubers, which are the small buds present on the vegetable. Bryophyllum reproduces through leaf margins. If a leaf falls on moist soil, it can give rise to a new plant. Runners like strawberry reproduce through stems. Plants like cacti reproduce when a part becomes detached from the parent plant. The detached part then starts a life of its own. The new plants produced by vegetative reproduction are an exact copy of their parent plants.
Sexual and asexual reproduction are different adaptations plants have used to perpetuate their species even during unfavorable conditions and manage to survive. Plants with flowers utilize sexual reproduction by attracting various agents of pollination. Plants without flowers rely on stems, leaves and roots that grow buds and reproduce identical copies!