Types of Asexual Reproduction
Some protozoans and many bacteria, plants and fungi reproduce via spores. Spores are structures naturally grown as part of an organism’s life cycle and designed for separation from the organism and dispersal via a medium such as air or water. When conditions are correct, the organism will release its spores, which are each then considered entirely separate and autonomous organisms. Given an environment suitable for life, the spores will then develop into fully grown organisms and eventually grow their own spores, repeating the cycle.
Prokaryotes and some protozoa reproduce via binary fission. Fission occurs at the cellular level when a cell’s contents are replicated internally and then subjected to division. The cell then forms into two distinct entities and separates itself. Each partial cell then reconstitutes the missing parts of its internal structure. At the end of the process, the single cell has become two new fully developed cells, each with identical genetic properties.
Many plants have evolved specialized genetic features that allow them to reproduce without the aid of seeds or spores. Examples include the prostrate aerial stems of strawberries, the bulbs of tulips, the tubers of potatoes, the shoots of dandelions, and the keikis of orchids. This form of specialization is most common in environments with seasonally harsh conditions; it allows plants to survive and thrive in situations where the traditional seeding process is subject to frequent interruption.
Organisms like proteins, yeast, and some viruses reproduce via budding, a process by which an entirely new organism grows on an existing one. Unlike fission, this is not brought about by the separation of an existing organism into two partial entities. The developing organism begins its life as an entirely separate life form from its “parent”, separating into an autonomous entity only when it has fully matured. As the “child” organism proceeds through life, it will produce its own buds.
Segmented worms and many echinoderms such as starfish reproduce asexually via fragmentation. In this process, an organism physically splits and develops new, genetically identical organisms out of each segment. The segments rapidly grow new cells to constitute their muscle fiber and internal structure through mitosis. This split can be either intentional or unintentional on the part of the organism.
Parthenogenesis, a reproductive strategy that involves development of a female (rarely a male) gamete (sex cell) without fertilization. It occurs commonly among lower plants and invertebrate animals (particularly rotifers, aphids, ants, wasps, and bees) and rarely among higher vertebrates. An egg produced parthenogenetically may be either haploid (i.e., with one set of dissimilar chromosomes) or diploid (i.e., with a paired set of chromosomes). Parthenogenic species may be obligate (that is, incapable of sexual reproduction) or facultative (that is, capable of switching between parthenogenesis and sexual reproduction depending upon environmental conditions). The term parthenogenesis is taken from the Greek words parthos, meaning “virgin,” and genesis, meaning “origin.” More than 2,000 species are thought to reproduce parthenogenically.
Grafting is the process of joining two plants together (an upper portion and a lower portion) to grow as one. The upper portion of the plant is known as the scion, which is attached to the lower portion known as the rootstock. This is most often done for fruit trees, and virtually all trees in orchards are grafted. Grafting in the orchard is done because the seeds of a fruit tree cannot reproduce true to their genetics. Therefore, the branch of a desirable tree is grafted to a suitable rootstock.
Layering is a means of plant propagation in which a portion of an above-ground stem grows roots while still attached to the parent plant and then detaches as an independent plant. Layering has evolved as a common means of vegetative propagation of numerous species in natural environments. Layering is also utilized by horticulturists to propagate desirable plants.
sexual reproduction in plants
Reproduction in plants takes place sexually and asexually as well. But the majority of the flowering plants reproduce sexually. The flower is the reproductive part of a plant i.e., both male and female gametes are produced by flowers. Sexual reproduction in plants takes place in flowers. The complete flower typically consists of four parts:
- Stamen (male reproductive part)
- Pistil/Carpel (female reproductive part)
Stamen (male reproductive part) consists of anther and filament.
- The anther is a sac-like structure that produces and stores pollen.
- The filament supports the anther.
Pollination & Fertilization
In order to form a zygote, male gametes in pollen grains have to fuse with egg in the ovule. This is achieved by the process called pollination. Pollination is the process of transferring pollen grains from the anther – male part of a flower, to the stigma – female part of a flower. Depending on the pollen landing, pollination can be classified into two types-
Self-Pollination: A pollination where the pollen transfer takes place between the anther and stigma of the same flower.
Cross-Pollination: A pollination where the pollen transfer takes place between the anther and the stigma of different flowers of the same plant or different plants of the same species.
Generally, pollination takes place with the help certain agents so called pollinators. They include insects, water, birds, the wind, etc. Once pollen gets transferred to stigma the male gametes from pollen grains release and fuses with egg in the ovule to form a zygote. This process of fusion of gametes is called fertilization. The zygote thus formed, divides and develops into an embryo, and later into a seed. The ovary develops into a fruit.
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