What exactly are potatoes, onions, garlic, or ginger? These and other many important plant-based foods are not like fruits, they do not have seeds. They are not roots like a carrot, either. Plants can reproduce by other means, besides producing seeds and fruits. We will describe the types of asexual reproduction in plants, including vegetative reproduction with special structures that the plant develops, artificial methods of reproduction, and the advantages of the different types of plant reproduction.
What is asexual reproduction in plants?
Plants can reproduce sexually, through the fusion of two haploid sexual gametes coming from two parents, or asexually (which means “non-sexually”), from only one parent and without the fusion of haploid gametes. The result of asexual reproduction is technically a clone of the parent, as no mixing or recombination of genetic material with another individual happens.
Many plants can reproduce both sexually and asexually and can shift from one method to the other depending on the circumstances. Asexual reproduction is more common in plants than animals, but a few animals can reproduce asexually too if external conditions are not ideal for sexual reproduction.
Sexual reproduction is common among eukaryotes (plants, fungi, animals, and protists) and, while some can reproduce both sexually and asexually, exclusive asexuality is rare among eukaryotes (although the life cycles of most single-cell eukaryotes, or protists, are not well studied). On the other hand, most prokaryotes (bacteria and archaea) reproduce asexually.
Types of asexual reproduction in plants
Plants naturally present two main types of asexual reproduction:
Fragmentation
In this type of asexual reproduction, a new plant forms from a part or fragment of the original plant. It is commonly called “vegetative reproduction or propagation” because the fragment originates from a vegetative organ of the plant (stems, roots, or leaves) and not from a reproductive organ (flowers in angiosperms).
Fragmentation is the most common type of asexual reproduction in plants and the fragments are usually modified stems, roots, or leaves. This type of asexual reproduction is based on the presence of meristems in these parts of the plant and the ability of parenchymatic tissue to differentiate into other types of tissue when needed. Moreover, plants are able to develop roots in other parts of the plant beside the main root, known as adventitious roots, which also aid in asexual reproduction.
Some of the most common vegetative parts that plants use for asexual reproduction are:
- Stolon: strawberry.
- Bulb: onion.
- Tuber: potato.
- Corm: taro.
- Plantlet: kalanchoe.
Apomixis
Some plants evolved a different type of asexual reproduction using sexual structures but without undergoing fertilization. In apomixis, a diploid cell in the ovule originates an embryo, and the ovule develops into a seed (and the ovary into a fruit). Thus, no haploid gametes are produced, and the embryo is a clone of the parent plant.
Apomixis comes from the Greek and means “away from mixing” because there is no fertilization of a female gamete, and then no mix of genetic material from two parents occurs.
Diagram of asexual reproduction in plants
The below shows diagrams and images of different structures used by plants for vegetative asexual reproduction.
Figure 1: Types of structures for vegetative reproduction in plants. A) tuber, B) rhizome, C) stolon, D) corm, E) bulb, and F) plantlets. Source: A) MartinThoma, CC0, B-D) Pearson Scott Foresman, Public domain, E) RoRo, CC0, all via Wikimedia Commons, F) Udik_Art, free use, pixabay.com.
Examples of asexual reproduction in plants
Asexual reproduction type/vegetative fragment | Description | Plant examples |
Vegetative: Rhizomes | Modified underground stems that grow horizontally. They serve as storage for proteins and starch. Roots and shoots can develop from a growing rhizome and form new plants. | Grasses, lilies, irises, ginger, turmeric, banana, and orchids. |
Vegetative: Stolons | Commonly called runners, stolons are modified stems that also grow horizontally above the ground. As in rhizomes, roots and shoots can grow at the tips of the stolon, or at nodes along the long stolon, and form a new plant. | Strawberries and currants. |
Vegetative: Bulbs | Modified stems consisting of a bud covered by layers of modified fleshy leaves, that form a swollen structure usually found underground. The leaves serve as a food source for the developing bud and eventually dry up. A bulb can divide and form more bulbs that will form a new organism. | Onions, garlic, hyacinths, daffodils, lilies, and tulips. |
Vegetative: Tuber | They develop from roots (rhizomes) or stems (stolons) when a portion is swollen from storing high quantities of nutrients. Shoot and root systems develop from the tuber. | Potatoes, yams (stem tubers), sweet potatoes, dahlias, parsnip (root tubers). |
Vegetative: Corms | They are physically similar to bulbs. They are modified stems that store nutrients and grow underground. The difference is that corms consist of solid fleshy tissue usually surrounded by papery leaves, while a bulb has a central bud covered by layers of fleshy leaves. Shoots and roots develop from the corm. | Crocus, gladiolus, and taro. |
Vegetative: Plantlets | Vegetative structures that grow along the margin of leaves from the meristem (growth tissue in plants) and look like miniature plants. they develop roots and eventually detach from the leaf. | Kalanchoe (Bryophyllum daigremontianum) |
Apomixis | Production of unfertilized seeds. | Kentucky bluegrass, blackberries, dandelions. |
Advantages and disadvantages of asexual reproduction in plants
Asexual reproduction can have many advantages for plants, or other organisms, under the right conditions (which we will discuss shortly). Some of these advantages for plants are the following:
- It does not require the investment of resources for producing flowers, seeds, and fruits, which are high resource-consuming processes.
- Faster development. The new plant reaches maturity faster and has higher survival probabilities. The new plant avoids the seed germination stage, where seeds and developing seedlings are very vulnerable to predation, pathogens, wildfires, and other external conditions.
- Traits highly adapted to an environment are passed on without modifications (excluding mutations) to the clones.
- Increased number of progeny. Creating clones is less resource-consuming, thus, a plant can generate more clones and more quickly, than sexually produced progeny. This allows for rapid increases in population size in a relatively short time.
The first two advantages do not really apply to reproduction through apomixis, as seeds are still produced. However, the parent plant can save some resources by not waiting for male gametes and have the advantage of seed and fruit dispersals which enable plants to reach more distant locations.
On the other hand, the main disadvantage of asexual reproduction for any organism is the lack of genetic diversity among the new organisms. A population with low genetic diversity is more vulnerable to environmental changes as it is less likely that some of the individuals will have specific traits or alleles to overcome any new challenge (diseases, climate change, predators, etc.)
In summary, asexual reproduction is usually more advantageous than sexual reproduction in stable environments where individuals are not faced with new threats or environmental changes. Under stable conditions, a clone would encounter the same environment as the parent plant and the inherited traits would likely be highly adapted to that environment.
Many crops are vegetatively reproduced since new plants grow and produce progeny faster. Farmers take care of crops to some extent; thus, external conditions are usually stable for crops (in terms of water supply and pathogen control). However, extreme conditions like eventual droughts, flooding, and especially disease outbreaks can occur.
This is what happened with the Irish potato famine in the 1840s.
At that time, the main source of food for the Irish population was the “lumper” potato, a vegetatively propagated crop. When the plant pathogen Phytophthora infestans appeared, it swops out almost all of the crops, since all the plants were clones and vulnerable to the potato blight. It is estimated that one eighth of the Irish population died of starvation during a three-year period.
Artificial methods of asexual reproduction in plants
Methods used in agriculture and horticulture are considered artificial methods of plant reproduction because they involve some level of human manipulation. Some of these methods just take advantage of the natural vegetative propagation methods that plants use or accelerate them.
The use of rooting hormones is common to speed up the development of adventitious roots in stems or root fragments.
- Grafting: a stem section of a plant, the scion, is grafted onto the stem of another plant that is rooted, the stock. To do this, both stems are cut obliquely so the matching surfaces fit when they are tied up together. The result is that the vascular systems of the two plants fuse and can grow as one organism which is called a graft. With this method, desirable traits of both plants can be maintained (like the fruits of the scion and the root characteristics of the stock).
- It is commonly used for some varieties of roses, citric fruits, and grapes.
- Cutting: a stem section that contains some nods is cut and placed in soil. The fragment will develop roots and shoots. Some plants’ stems will also develop when placed in water.
- Examples of plants reproduced with cuttings are coleus and money plants.
- Layering: a part of a young stem or branch that can be easily bent while still attached to the plant. After some time, the buried part of the stem will develop roots and can be removed to be transplanted.
- Plants that can be reproduced this way are jasmines and bougainvillaea (paper flowers).
- Suckering: in many shrubs and trees, sprouts appear from the root system (usually rhizomes), called suckers. These suckers can be cut out and planted to get new plants, but they are also commonly pruned in crops when they appear in excess as they consume resources from the parent plant.
- Tissue culture: plant tissues are commonly cultured under laboratory conditions for agricultural or conservationist research. Several types of plant tissues or cells can be used which are placed in a nutritious medium. A mass of cells forms first and eventually a large number of plantlets develop that can be transplanted.
Figure 2: Examples of artificial methods of asexual reproduction in plants. Left: layering, right: grafting where A is the scion and B is the stock. Source: both images Pearson Scott Foresman, Public domain, via Wikimedia Commons.
Asexual Reproduction in Plants - Key takeaways
- Many plants can reproduce both sexually and asexually and can shift from one method to the other depending on the circumstances.
- Plants naturally present two types of asexual reproduction: fragmentation or vegetative propagation, through the detachment of sections of modified stems, roots, or leaves, and apomixis, the formation of unfertilized seeds.
- Fragmentation is the most common type of asexual reproduction in plants and is based on the presence of meristematic tissue, the ability of parenchymatic cells to differentiate into other types of cells, and the ability to develop adventitious roots.
- Some advantages of asexual reproduction in plants are the investment of fewer resources, a faster development of the plant, more clones are produced and more quickly, etc.
- Asexual reproduction can be more advantageous than sexual reproduction in stable environments where individuals are not faced with new threats or environmental changes.
References
- Lisa Urry et al., Biology, 12th edition, 2021.
- Mary Ann Clark et al., Biology 2e (section 32.3 Asexual Reproduction), 2022
- UC Museum of Paleontology Understanding Evolution team, Monoculture and the Irish Potato Famine: cases of missing genetic variation, 2022
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