Cell biology, a core discipline within Biology Topics, investigates the structure and function of cells.
Different Methods of Asexual Reproduction in Plants
Budding is an asexual method of reproduction. In budding, a small part of the body of the parent plant grows out as a bulb-like projection called ‘bud’ which then detaches and becomes a new plant. We will now describe the reproduction in yeast plant by the process of budding. Yeast is a non-green plant (which is a fungus). Yeast is a single-celled plant (or single-celled organism). Each single cell of yeast is a complete plant in itself. Yeast cells are so small that they can be seen only through a microscope.
Yeast reproduces by budding. Figure (a) shows a parent yeast cell (which is a complete plant). In yeast, first a bud appears on the outside of the cell wall [Figure (b)]. The nucleus of parent yeast cell then divides into two parts and one part of the nucleus moves into the bud [Figure (b)], Ultimately, the bud separates off from the parent yeast cell and forms a new yeast cell (or new yeast plant) [Figure (c)],
The budding in yeast, however, often takes place so fast that the first buds start forming their own buds and all of them remain attached to the parent yeast cell forming a chain of yeast cells [Figure (d)]. After some time, all the yeast cells of the chain separate from one another and form individual yeast plants. The new yeast cells grow, mature and produce more yeast cells. If this process continues, then a large number of yeast cells (or yeast plants) are produced in a short time.
We can study the process of asexual reproduction in yeast by budding in the laboratory as follows : Take 100 ml. of water in a conical flask and dissolve 10 grams of sugar in it. Then add 5 grams of yeast powder (or yeast granules) to this sugar solution and stir it well with a glass rod. Put a cotton plug in the neck of the conical flask. This conical flask containing sugar solution and yeast mixture is kept aside in a warm place for 3 to 5 days. When froth is observed in the flask, the yeast culture is ready for examination.
Take out a small quantity of the yeast culture solution from near the bottom of the conical flask with the help of a dropper and place a drop of this culture solution on a clean slide. Add a very little of iodine solution over the culture solution drop to stain it. Place a coverslip over the slide. Keep the slide under the microscope and observe it first under low power and then under the high power of the microscope. Note the formation of buds on the yeast cells and how they separate from the parent cell.
Some of the plants having relatively simple body structure can break up easily into smaller pieces (or fragments) on maturing. These pieces or fragments can then grow and form new plants. This is another method of asexual reproduction in plants called fragmentation which can be defined as follows : The breaking up of the body of a plant into two (or more) pieces on maturing, each of which subsequently grows to form a new plant, is called fragmentation. The breaking up of the body of the parent plant in fragmentation to form new plants occurs naturally (on its own) when the parent plant matures.
Many times we see slimy, green patches in ponds or in other stagnant water bodies such as lakes, etc. (see Figure). The green patches are due to the growth of plant-like organisms called ‘algae’. An alga reproduces by the asexual method of fragmentation. When an alga matures, it breaks up into two (or more) fragments. Each fragment of alga then grows to form new algae. This process of reproduction of alga by fragmentation continues due to which a large number of algae are produced in a short period of time which cover a large area of water in the pond. So, when water and nutrients are available, algae grow and multiply rapidly by the process of fragmentation. An example of common alga is Spirogyra.
Spirogyra is a green, filamentous alga plant which is found in ponds, lakes and slow moving streams. Spirogyra reproduces by the asexual method of reproduction called fragmentation. This happens as follows : When Spirogyra becomes mature, its filament (body) simply breaks into two or more fragments
on its own, and each fragment then grows into a new Spirogyra. This break up of the filamentous body of Spirogyra brings about its asexual reproduction. In Figure 8, a mature Spirogyra filament is undergoing fragmentation to produce three new Spirogyra. These three Spirogyra will mature in due course of time and break again to produce even more Spirogyra. And this process of reproduction in Spirogyra goes on and on.
Spore formation is an asexual method of reproduction in plants. Spores are the asexual reproductive bodies. Spores are not seeds. In the spore formation method of reproduction, the parent plant produces hundreds of tiny spores in spore cases’. When the spore case of the plant bursts, then the spores spread into air. As the spores are very light, they keep floating in air and carried over large distances by air.
Each spore is covered by a hard, protective coat to withstand unfavourable conditions such as high temperature and low humidity. Due to this, spores can survive for a long time. When the air-borne spores land on food (or soil) under favourable conditions (like damp and warm conditions), they germinate and produce new plants.
Most of the fungi, and flowerless plants (such as ferns and mosses) reproduce asexually by means of spores. The common bread mould plant which grows on a piece of stale bread is a fungus (whose scientific name is Rhizopus). The common bread mould plant (or Rhizopus fungus) reproduces by the method of spore formation. This is described below.
The tiny spores of bread mould plant (or Rhizopus fungus) are almost always present in the air. If we keep a moist slice of bread aside for a few days, then the spores of bread mould plant present in air settle on the moist slice of bread and germinate to form new bread mould plants.
The bread mould plants first look like a white cottony mass covering the bread slice which later on turns black. If we observe the surface of this slice of bread through a magnifying glass, then the bread mould plant growing on it will appear to be like that shown in Figure. The common bread mould plant consists of fine, thread-like projections called hyphae and thin stems having knob-like structures called, sporangia at the top (see Figure) (The singular of sporangia is sporangium). Each sporangium contains
hundreds of minute spores enclosed in a spore case. When the spore case bursts, the tiny spores are dispersed in air (see Figure). These spores are the asexual reproductive bodies of bread mould plant which can produce more bread mould plants under suitable conditions. Actually, it was one such spore present in the air which grew on the moist slice of bread kept aside by us for a few days.
The plants such as ferns and mosses also reproduce by means of spores. A fern is a flowerless green plant which has feathery leaves (see Figure). Fern plant reproduces by spores released from the underside of its leaves. The clusters of spore-containing sporangia on the underside of a fern leaf are called ‘sori’ (see Figure) (singular of sori is sorus). The special type of leaf of a fern plant is called ‘frond’. Moss is a very small flowerless green plant without roots which grows in damp places. It forms a mat of vegetation. Moss plants also reproduce by means of spores. We will now describe the sexual reproduction in plants.