por He plant life cycle It describes the different stages through which these living beings go from the beginning of their lives to the end of their lives. This cycle begins with a seed that germinates and continues with a small plant that develops roots.
Unlike humans, who can reproduce sexually in only one way, plants are capable of reproducing by various methods, both sexual and asexual.
The asexual reproduction of plants requires a single parent, that is, a plant gives rise to another genetically identical plant, so, in this case, we are not talking about «males» or «females».
On the other hand, the sexual reproduction of plants always requires two different parents, usually a “male” plant and a “female” plant, which mix their genes to produce offspring that are genetically different from both.
In the plant kingdom, the same plant that reproduces asexually at one time may do so sexually at another time, but this depends on several factors that we are not going to mention in this text.
However, there are also plants that reproduce exclusively sexually or exclusively asexually.
The sexual reproduction of many plants is usually related to special structures with which we are very familiar: flowers and seeds. The plants in whose sexual reproduction we see these structures belong to a large group known as angiosperms or flowering plants.
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Life cycle of a flowering plant (sexual reproduction)
1- A seed that germinates
The life cycle of almost all flowering plants begins with a seed, but what is a seed? A seed is the structure where the embryo of a plant is enclosed, which we can identify as a «baby plant».
This embryo is the result of the fusion of two very special sexual cells: a pollen grain (microspore) and an ovule (megaspore), which are equivalent to the spermatozoon and the ovule of animals.
The seeds generally contain enough food to support the life of the embryo inside until the external conditions are suitable for germination. In addition, they also have a resistant cover, which we call seed cover, which protects everything inside.
It is important that we mention that there are other plants that do not have flowers and whose sexual reproduction does not begin with the germination of a seed, but with a very small spore.
– The dispersion
Seeds can be dispersed over long distances in various ways. Some are transported inside fruits, which can be torn from the plants by different animals, which can eat them and disperse them with their waste or spread them wherever they go.
Others are spread by wind or water, and still others by birds, insects, and mammals. Human beings also participate in the dispersal of seeds and normally use them to grow the food that sustains us daily.
– The germination
Once the seeds of a plant reach their final destination, they can germinate, that is, the embryo inside receives certain signals from the outside and begins to grow.
Among these signs we can mention the presence of water, sunlight, oxygen and the right temperature, although these vary depending on the type of plant.
When the embryo begins to grow, it begins to «push» the seminal coat until it manages to break it and get out of it.
Normally, the first thing we see when a seed germinates is a very small root. Shortly after we can see one or two simple leaves, which we call cotyledons and which will help the growing seedling to carry out photosynthesis to feed itself.
2- A seedling that takes root
The growth of the seedling is made possible thanks to the fact that its roots go deeper into the soil and branch out in it, increasing its capacity to find and absorb water and other mineral nutrients.
It is very common for growing seedlings to «seek» to orient themselves in the direction of the sun’s rays, since it is thanks to the energy contained in these that they can feed by photosynthesis through a pigment known as chlorophyll.
3- An adult who grows
As the seedling grows, it develops into an adult plant. Adult plants generally develop deeper roots, branches and new «true» leaves, increasing in size and area of coverage.
Through their roots, adult plants can “suck” water and nutrients from the soil, driven by forces that arise in the stems and leaves. These nutrients are transported to the other structures of the plant body, in order to nourish and hydrate them.
4- An adult that blooms
When an adult plant begins to flower, we say that it has «entered» its reproductive stage, since the flowers (which grow on the apices or tips of the stems) are the reproductive organs of plants, just as the genitalia are in humans.
There are different types of flowers: some are male and others are female, while there are others that are hermaphrodites, that is, they are both male and female. Hermaphrodite flowers are very common and are generally made up of the same basic elements:
– a “foot” or stem that supports the entire structure,
– some petals that have different colors, with which they “seek” to attract the little animals that help pollination (usually insects and birds),
– some stamens, formed by filaments and anthers, which are the sites where pollen is produced by meiosis, so we could say that they are the “male” part of the flower and
– a pistil, made up of a stigma, a style and an ovary, which are the sites where the pollen grains are received, the channel through which they germinate and the container that contains the ovules (produced by meiosis), respectively. We can say that this corresponds to the «feminine part of the flower».
Some flowers also have a kind of «containers» in which they produce sugary substances, which attract the attention of the insects that pollinate them and can be seen as a «reward» for them.
5- A flower that is pollinated
The process of transferring pollen from the stamen of one flower to the stigma of another is called pollination. This depends, to a large extent, on those insects, birds or other animals that visit the flowers and take the pollen with them, leaving it “accidentally” on other flowers that they visit.
This can also happen without the participation of another living organism, but can be given through the wind or water, for example.
Pollination usually leads to the germination of one or more pollen grains on the stigma, which produce a tube that «grows» to reach the ovary and ovules within.
Through this structure, known as the pollen tube, the pollen grains discharge their internal contents into the ovules. Remember that both the pollen grains and the ovules have half the genetic load of the plant that gave rise to them.
When the nucleus of a pollen grain fuses with the nucleus of an ovule through fertilization, the genetic makeup is restored into a cell known as a zygote, from which an embryo is formed.
6- The cycle that begins again
The embryo produced by sexual reproduction is «kidnapped» inside a seed and, sometimes, inside a fruit.
The cycle begins again when this seed is dispersed in some way, reaches the soil and the right conditions, and germinates, giving birth to a new seedling, with characteristics shared between two different plants.
The plant that gave rise to said seed can die after reproducing, but it can also continue to live and carry out many other cycles of flowering and fruiting, as is the case with perennial fruit trees, for example.
Life cycle by asexual or vegetative reproduction
Unlike what we have just studied, the asexual reproduction of plants, also known as vegetative reproduction, does not involve the production and germination of a seed.
Instead, many plants develop special structures that help them multiply in a short time and without the need for two different parents; the result of this multiplication is a group of genetically identical individuals, often called clones.
Thanks to sexual reproduction, a plant that is adapted to a relatively stable environment can multiply rapidly, very «sure» that its «offspring» will also succeed in the same place.
Consider, for example, a plant that arose from a seed and now reproduces by asexual reproduction.
– This can develop horizontal “stems” known as stolons, for example, which, moving away from the plant, can develop their own roots and establish themselves as a new individual.
– It may also be the case that one of its leaves touches the ground and roots develop at the contact site, which can later make a new individual independent.
– Suppose, furthermore, that a horticulturist cuts or extracts from the plant a portion, say a fragment of the stem, and plants it in a different pot. This fragment can develop roots and become a new plant.
References
Bales, K. (2020). ThoughtCo. Retrieved April 26, 2020, from thoughtco.com
Nabors, MW (2004). Introduction to botany (No. 580 N117i). Pearson,.
Raven, PH, Evert, RF, & Eichhon, S. (2014). Vegetable biology.
Solomon, E.P., Berg, L.R., & Martin, D.W. (2011). Biology (9th edn). Brooks/Cole, Cengage Learning: USA.
Walbot, V., & Evans, MM (2003). Unique features of the plant life cycle and their consequences. Nature Reviews Genetics, 4(5), 369-379.