Modes of Reproduction by Single Organisms
Cell division or fission is the process by which a cell, called the parent cell, divides into two cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. In meiosis however, a cell is permanently transformed and cannot divide again.
Cell division is the biological basis of life. For simple unicellular organisms such as the Amoeba, a single cell division gives rise to an entire organism. On a larger scale, cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. Cell division also enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by cell division from gametes. And after growth, cell division allows for continual renewal and repair of the organism.
The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information, which is stored in chromosomes, must be replicated, and the duplicated genome separated cleanly between cells. A great deal of cellular infrastructure is involved in keeping genomic information consistent between "generations".
Fragmentation is a form of asexual reproduction where an organism is split into fragments. The splitting may or may not be intentional. Each of these fragments develops into mature, fully-grown individuals that are a clone of the original organism. If the organism is split any further the process is repeated.
Fragmentation is seen in many organisms such as molds, some annelid worms, and starfish. Binary fission of single-celled organisms such bacteria, protozoa and many algae is a type of fragmentation.
Molds, yeast, and mushrooms, all of which are part of the Fungi kingdom, produce tiny filaments called hyphae. These hyphae obtain food and nutrients from the body of other organisms to grow and fertilize. Then a piece of hyphae breaks off and grows into a new individual and the cycle continues. Fragmentation is observed in non-vascular plants as well, e.g., liverworts and mosses.
Regeneration is a specialised form of asexual reproduction; by regeneration some organisms can replace an injured or lost part, and many plants are capable of total regenerationâ€”i.e., the formation of a whole individual from a single fragment such as a stem, root, leaf, or even a small piece from such an organ. Single phloem cells from a carrot plant, when grown on an agar medium, would form a complete carrot plant by tissue culture. Among animals, the lower the form, the more capable it is of total regeneration; no vertebrates have this power, although clones of mammals have been produced in the laboratory from single somatic cells. In some plants structures that form on the leaves give rise to young plantlets. Rhizomes, bulbs, tubers, and stolons are other forms of vegetative reproduction.
Plants propagate by means of seed or by means of vegetative sprouts. Perennial weeds may be divided into two classes, those, which propagate vegetatively, and those, which do not. The first class may propagate by two distinct methods, roots and stems.
Plants, which propagate by means of creeping roots, have the power to produce buds upon the roots at indefinite points. The buds called adventitious buds develop into stems and these stems may become independent plants by the decay of the connecting root.
The creeping stem may be above ground when it is called a stolon or it may be below the surface when it is called a rootstock or more technically a rhizome. The only plant among weeds, which propagates by stolons, is Buckbush. More commonly the propagation is by means of rootstocks. A rootstock can be distinguished from a root by the presence upon its surface of leaf surface scales at definite intervals, the nodes. The new stems spring from buds at the nodes and the roots usually occur at the nodes though in some cases they are scattered all along the rootstock.
A mode of reproduction resembling multiple fissions which breaks the organism up into a number of pieces, or spores, each of which eventually develops into an organism like the parent form. Vegetative thallus cells showing single chloroplast. In the transition to the reproductive state, 8-16 zoospores are developed within each cell. Sporulating thallus cells show cells containing 8-16 zoospores. Some cells are empty having discharged their spores.