Multiplication Of Plants

 

Vegetative Propagation

Fig. Bulbs of Vegetables

Bulbs are a form 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.

When plants make seeds and pollen, they often use a significant amount of time and energy. Additionally, remember that not all plants make seeds. 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 is 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 is called vegetative propagation.

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, 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.

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.

Pollination

In plants, pollination is a very important process through which pollen is transferred to a female reproduction of a plant to enable fertilization. The spermatophytes or the seed plant passes their genetic information to its next generation, just like all other living organisms. In plants, seed is known as reproductive unit and the pollination is an important step in seed production.

There are two types of pollination - cross-pollination and self-pollination.

Cross Pollination

The cross pollination is defined as deposition of pollen grains from a flower to the stigma of another flower. Commonly, the process is done pollinators that are insects and wind. By insects the process take place in several plants like tulips, grapes, plums, apples, pears, strawberries, daffodils, raspberries and others. While, by wind for different grasses, maples trees, dandelions, catkins, and others.

Self Pollination

In this process, the pollen grains transfer from stigma of the same or genetically similar flower. The self pollination can be seen in legumes like sunflowers, orchids, peanuts, oats, peas, potatoes, wheat, peaches, and more.

Difference Between Cross Pollination and Self Pollination
Cross Pollination Self Pollination
Transfer pollen grains from a flower to stigma of another flower of different or same species Transfer pollen grains from another to the stigma of same flower
The process take place in several plants like tulips, grapes, plums, apples, pears, strawberries, daffodils, raspberries and others. While, by wind for different grasses, maples trees, dandelions, catkins, and others. The self pollination can be seen in legumes like sunflowers, orchids, peanuts, oats, peas, potatoes, wheat, peaches, and more.
Transferred through insects, wind, water, animals, etc. Transfer directly onto stigma.
Transfers large number of pollen grains. Transfers small number of pollen grains.
Allogamy is a type of reproduction. Autogamy and Geitonogamy are types of reproduction.

Birds, bats, bees, butterflies, beetles, and other small mammals that pollinate plants are responsible for bringing us one out of every three bites of food. They also sustain our ecosystems and produce our natural resources by helping plants reproduce.

Development of the Seed

Monocot and dicot seeds develop in differing ways, but both contain seeds with a seed coat, cotyledons, endosperm, and a single embryo.

Parts of a Seed

The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat, known as the testa, and inner coat, known as the tegmen. The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl. The embryonic axis terminates in a radicle, which is the region from which the root will develop.

Fig. Parts of a Seed

Seed Growth

In angiosperms, the process of seed development begins with double fertilization and involves the fusion of the egg and sperm nuclei into a zygote. The second part of this process is the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary endosperm. Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after germination. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant: the inner integument forms the tegmen and the outer forms the testa. When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next.

In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce. The ovules after fertilization develop into the seeds.

Angiosperm Fruit Development

Development of the fertilized seed is normally accompanied by development of the fruit. In the simplest case the ovary wall becomes thickened to form the pericarp. This may be:

a. Dehiscent, splitting open when ripe to release the enclosed seeds; examples are the capsule (e.g. Eucalyptus), a multilocular fruit derived from a syncarpous ovary, and the leguminous pod (e.g. Cassia), which is derived from a single carpel and spilts along two sutures. The pericarp may be dry, semi-fleshy or fleshy at the time of dehiscence. Semi-fleshy to fleshy capsules are common in the humid tropics (e.g. Baccaurea, Durio, Dysoxylum, Myristica) and are often associated with the development of variously coloured, tasty or smelly pulp (aril or sarcotesta) around the seed.

b. Indehiscent or dry, closely fused with the seed; examples are the achene, a small hard one-seeded fruit with membranous pericarp, the samara, similar to the achene but with pericarp extended to form a wing (e.g. Triplochiton) and the nut, a rather large one-seeded fruit with woody or leathery pericarp (e.g. Shorea, Quercus).

c. Indehiscent and fleshy, often distinguished by colour, smell and taste to attract fruit-eating birds and animals. Two types are distinguished. The berry has an outer skin and inner fleshy mass, containing seeds that have a hardened seedcoat (e.g. Diospyros, Pouteria). The drupe has the inner layer of the pericarp hardened to protect the seeds (e.g. Prunus, Gmelina, Azadirachta, MangiferaIn some species other parts of the flower, as well as the ovary wall, take part in fruit formation.

The interval between flowering and maturation of seeds and fruits varies greatly with species, even within the same genus. In most species fertilization of one or more ovules must precede fruit formation. In a few species, however, fruits are set and mature without seed development and without fertilization of an egg.