Plants germinate, sprout, root, leaf out, and bloom mainly through the process of mitosis occurring at the cellular level. Much of the action happens in meristematic tissue containing undifferentiated cells capable of specialization.
Vascular plants, flowering plants, ferns, cacti, and mosses are among the thousands of plant groups around the world capable of perpetual plant reproduction.
The spores grow into multicellular haploid plants called gametophytes that produce haploid gametes via mitosis. Two gametes form a diploid zygote that forms sporophytes, thus completing a full life cycle.
Asexual Plant Cell Division
Plant cells that reproduce by mitosis make identical copies of themselves to sustain the local population. Rapid growth through mitosis explains how crops grow so fast in just one season.
In asexual plant cell division, there is no recombination of genes during mitosis, and intraspecies biodiversity is limited.
Asexual reproduction only requires DNA from one parent. It creates offspring that are genetically identical to the parent. Genetically identical offspring are called clones. Clones lack genetic diversity. This makes them more susceptible to disease. It also makes them less adaptable to changes in the environment.
There are different methods of asexual reproduction. They include vegetative propagation and fragmentation.
Vegetative propagation does not require seeds or spores. Instead, offspring grow from a part of the parent plant. In different plants, vegetative propagation happens in different ways. Here are a few examples.
Garlic, onions and tulip plants all reproduce using true bulbs. These short underground stems are also called scaly bulbs. They have a basal plate that is usually surrounded by modified leaves. These leaves form a papery covering called a tunic. New bulbs grow off of the parent bulb’s basal plate.
Crocuses reproduce using corms, which are similar to true bulbs. However, a corm doesn’t have as many layers. Corms are used up during the growing season and get replaced by one or more new corms.
Potato plants reproduce using tubers. These underground growths produce new plants from stems or growing points called eyes.
Ginger plants reproduce using rhizomes. These stems that grow sideways along the soil or just below the surface. They branch apart to produce new points of growth.
Strawberry plants reproduce using stolons. They look like branches growing along the ground. Stolons anchor themselves to the ground and develop roots. And these roots grow into new plants.
Plant Mitosis in Cell Division
Mitosis is the predominant process involved in plant cell division and normal growth. The cell cycle starts with interphase where the cell secures nutrients, metabolizes, enlarges, synthesizes proteins and replicates organelles.
When conditions are favorable for cell division, the cell’s chromosomes condense and line up at the middle of the cell before being pulled apart by spindle fibers. A nucleus reforms in each cell to house the chromosomes, and a cell plate separates the two cells via cytokinesis.
Plant Reproduction: Fragmentation
Spirogyra exist as unicellular organisms or as long filamentous seaweed. Filaments are comprised of plant cells lined end-to-end. If filaments break apart, each fragment can continue growing on its own.
Spirogyra is an example of a plant that reproduces asexually through fragmentation and sexually through conjugation (gamete formation).
Sexual Reproduction
Sexual reproduction requires genetic material (DNA) from two parents. The parent plants have male and female sex cells, called gametes. The genetic material from the male and female gametes combines to produce offspring. This process is called fertilization.
Seeds produced through fertilization contain genetic material from both parents. As a result, the offspring are not genetically identical to either of the parent plants. This genetic diversity can help them survive if the environment changes.
Flowering plants reproduce sexually through a process called pollination. The flowers contain male sex organs called stamens and female sex organs called pistils. The anther is the part of the stamen that contains pollen. This pollen needs to be moved to a part of the pistil called the stigma.
Plants can either self-pollinate or cross-pollinate. Self-pollination happens when a plant’s own pollen fertilizes its own ovules. Cross-pollination happens when the wind or animals move pollen from one plant to fertilize the ovules on a different plant.
The advantage of cross-pollination is that it promotes genetic diversity. Some plants have features that prevent self-pollination, such as pollen and ovules that develop at different times.
Pollinators are animals that carry pollen between plants. Many pollinators are insects, like bees, butterflies, moths and beetles. Some birds, including hummingbirds, also play a part. Likewise, certain mammals, like bats and rodents, move pollen between plants.
The colours and smells of flowers often attract pollinators. Pollen will stick to a pollinator’s body as it feeds on the flower’s nectar.
Fertilization is the next step after pollination. Once it reaches the pistil, the pollen needs to fertilize an egg inside the stigma. This egg is called an ovule.
Fertilization creates fruits that contain seeds. Some fruits are fleshy, like oranges and watermelons. Others are dry, like acorns or walnuts. These fruits are an attractive food for various animals. After digesting fruit, animals expel waste that contains seeds. This way, seeds can take root and grow in places far from the plants that produced them!
Plant Cell Reproduction: Meiosis
Plants have generational life cycles that alternate between methods of asexual and sexual reproduction. Sexual reproduction in plants occurs when a sporophyte with a full set of chromosomes divides by meiosis into haploid spores containing 50 percent less DNA than the parent cell.
The spores grow into multicellular haploid plants called gametophytes that produce haploid gametes via mitosis. Two gametes form a diploid zygote that forms sporophytes, thus completing a full life cycle.
Are There Centrioles in Plant Cells?
The centriole is a microtubule believed to play a role in spindle formation and chromosome separation. Only cells of animals and lower plants contain a centriole; higher-order plants do not have a centriole.
Instead, chromatin condenses into tightly coiled chromosomes that line up along the middle of the cell and then separate. Movement of chromosomes is assisted by microtubules and proteins in the cytoplasm that act like a spindle even though centrioles are not present.
How Does Cytokinesis Differ in Plant and Animal Cells?
The last stage of plant cell division ends with cytokinesis. Sets of vesicles line up in the middle of the cytoplasm. The new arrivals form a cell plate that will divide the large cell into two smaller cells. Then cellulose production starts, which turns the cell plate into a sturdy cell wall supporting the cell membrane.
Animal cells are flexible and do not have a cellulose wall protecting their membrane. A protein ring around the middle of the elongated, dividing cell squeezes the plasma membrane inward, forming a cleavage furrow. The parent cell divides into two daughter cells, each having their own nucleus, cytoplasm and membrane.
Adaptations of Plant Reproduction
Plant mitosis and other forms of plant cell division enable plants to live and multiply in extreme climates. For example, some types of plants shoot up during the rainy season and then die, leaving behind drought-tolerant seeds that won’t sprout until the rains return.
Some seeds and spores remain dormant for years and then come to life. In fact, researchers in Israel are successfully growing a thriving date palm tree from a 2,000-year-old seed, according to National Geographic.
References
- https://letstalkscience.ca/educational-resources/backgrounders/plant-reproduction
- https://sciencing.com/reproduction-plant-cells-7989582.html
