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In the asexual reproduction, the offspring are genetically identical to the
parent. It only requires –and only allows for- one parent.
Not all plants make attractive flowers in order to reproduce. Some plants will
not make seeds and pollen either. These plants have other ways to create
new offspring. We will look at a few key methods of vegetative propagation.
You may know that plants produce seeds and pollen in order to create new
plants. However, this form of reproduction is not the only way to create new
plants.
For plants that do not make seeds, there are many options for making new
offspring. We will look at a few of these forms of reproduction. But first, let's
understand some general aspects of asexual plant reproduction.
Asexual reproduction occurs when offspring are genetically identical to the
parent. Asexual reproduction only requires - and only allows for - one parent.
Most of the time we think of two parents -a male and a female- making an
offspring, however, asexual reproduction only uses one parent. In plants,
asexual reproduction can occur by spores or by vegetative propagation.
Many plants form spores during their biological life cycle in a process
called sporogenesis which takes place in the sporophyte, as you remember in
ferns and mosses life cycle.
Vegetative propagation, like many things, has its advantages and
disadvantages. It is beneficial for plants that are well suited for their
environment and when the environment is stable. Remember that asexual
reproduction results in genetically identical organisms, so these organisms
must be well adapted to their environment in order to survive. Because
asexual reproduction doesn't allow for evolution and adaptations to occur as
frequently as sexual reproduction, vegetative propagation is not advantageous
for plants that live in changing environments. In unstable environments,
plants that are identical to each other may all die out at once. However, if
plants are genetically different, which is a result of sexual reproduction, some
plants may survive in an unstable environment.
For example, if we have a specific type of plant that grows very well in hot,
humid environments, all of the plants may die out if there is a change in the
climate. If the temperature or humidity change, the plants that produce
asexually are all the same and therefore, they will all die out because they
are not adapted to the new climate. However, a plant that produces sexually
will have some diversity in the individual plants and some may survive this
change in temperature or humidity. We also see this in plants that are
affected by diseases - those plants that are genetically identical are more
likely to die out than those plants that are genetically diverse.
Bulbs
Now that we have explored what vegetative propagation is, as well as some
benefits and downsides, let's now look at a few forms of asexual reproduction
in plants. We will first look at how bulbs result in vegetative propagation.
Generally, you may think of bulbs that you plant in the fall in order to get
flowers, such as daffodils or tulips, in the spring. Bulbs are underground
buds that have fleshy leaves extending from them. Bulbs are food storage
units for future developing plants. Bulbs contain several buds near the node,
which is where leaves are produced. These new buds can eventually develop
into new plants. If you plant one bulb, you may find that years later you have
several plants coming from that one bulb. If you want, you can separate these
new bulbs and plant more flowers. Because this one plant produces new
offspring that are genetically identical, bulbs are a form of vegetative
propagation.
Tubers
Tubers are somewhat similar to bulbs. Tubers, such as potatoes,
are enlarged, fleshy underground stems. While potatoes do not always
reproduce by using tubers, if you've ever left a potato for too long, you know
that new potato plants can develop from the tuber. These new stems start on
the eyes of the potato and, if you plant a potato with new growth from the
eyes, a potato plant will develop. We can see the stems starting to develop on
the eyes of the potato here. You can even see that leaves have started to
develop.
Rhizomes and Stolons
Let's next look at two ways stems can be involved in vegetative reproduction.
First, Rhizomes are horizontal underground stems. These horizontal stems
will grow and then develop a new vertical stem at certain critical points.
Rhizomes are responsible for the growth of many grasses, sedges and weeds.
The rhizomes grow out from the original plant and invade the nearby soil.
They then make new flowering stalks. Even though we think of grasses and
weeds most often with this type of vegetative propagation, this method of
asexual reproduction can even be seen in irises. Irises can reproduce sexually
through pollen and seeds but sometimes will use rhizomes to reproduce as
well. This is why you may see a patch of irises growing close together - they
are all sharing the same root system.
The second type of stem growth we will look at uses stolons, which
are horizontal aboveground stems. These horizontal stems grow along the
ground and may develop adventitious roots along with new flowering plant
stems. The most common example of stolons can be found in strawberries.
Strawberries have runners, which are really stolons that grow along the
ground and continue to develop new flowering stems and roots. Again, this
form of vegetative propagation can explain why you will see a large number of
strawberry plants in the same area - they are all connected by the stolons.