Like so many other living organisms, plants reproduce sexually. However, there is a step along the way that mystifies many beginning gardeners: pollination. Oftentimes, pollination is confused for fertilization, mistakenly interpreted to mean the same thing. In this brief article about plant life reproduction, I will explain the tremendous difference between the pollination and fertilization processes.
Pollination occurs before fertilization when the pollen (sex cells) from a male plant is transferred to the sexual organ of a female plant. Fertilization only occurs when the pollen successfully bonds with the egg cells of the female. When a female plant is fertilized, the result is a zygote: a seed.
You, like many others, might be surprised to see the same terminology used as when we discuss mammalian reproduction. It can be difficult to wrap one’s head around at first, so let’s dive in and discuss everything a beginner needs to know about pollination and fertilization.
What is Pollination?
The reproduction cycle of flowering plants begins with pollination. This is the introduction of pollen, the male plant gamete, from a male plant’s anthers to the stigma of a female plant. The female plant may be of the same or a different species. When one plant pollinates another, this is referred to as cross-pollination. This is not the only form of pollination possible. Self-pollination occurs in many plants.
Self-pollination occurs in many plant species but leaves these species at a distinct disadvantage due to the lack of genetic diversity. Plants resulting from self-pollination will essentially be clones of the same plant, thus leaving them vulnerable to possessing the same flaws and weaknesses.
Cross-pollinated plants have the genetic variety of two parent plants. Because of the natural advantages of cross-pollination over self-pollination, most plant types cannot pollinate themselves by nature’s design. Some plants even have male and female reproductive components on the same plant but on different flowers of that plant.
What is Fertilization?
When the pollen introduced during pollination reaches the ovules, the female gamete located inside the ovary, the reproductive process continues. The male gamete and the female gamete’s chromosomes combine during this time. This results in a normal, complementary formation of a seed. Providing the flower is able to grow a seed successfully as a result of pollination and then fertilization, that seed will contain an embryo and the nourishment needed for it to thrive.
How many seeds a plant has speaks to how many ovules the female has in its ovary. An avocado, for instance, only has one ovule. Therefore, it only has one seed. Kiwi and pomegranate, on the other hand, have many ovules to produce many seeds.
Fertilization is necessary for the continuing life cycle of not only plants but almost every living organism on Earth.
What Happens Between Pollination and Fertilization?
After pollen from a male plant’s anthers has come into contact with the stigma of the female plant, the female readies itself for fertilization. This does not, in itself, ensure that fertilization will eventually occur. It is between pollination and fertilization that the pollen grain on the stigma must form a pollen tube. Without the formation of this tube, the pollen cannot reach the ovules within the female.
How Does Pollen Transfer from Male to Female Plants?
Pollination is not autonomous, requiring the assistance of external influences to get the pollen from the male anthers to the stigma of the female plant.
Most often, flying insects are responsible for carrying the pollen. Because flowering plants require this help in reproducing, they have been crafted by nature to be incredibly appealing to pollinators who are in search of food. Pollen is packed with protein. Nectar is a great source of energy for many of these insects.
Some of the most common insects responsible for plant cross-pollination include:
Winged insects are not the only living creatures that can (and do) transfer pollen from point A to point B. Flying animals like bats and birds are often responsible for this stage of the reproductive process. Even a grounded animal simply prancing in a bed of flowering plants could transfer pollen with their body could indirectly introduce pollen to female plants’ anatomies.
It doesn’t take more than a gentle springtime breeze for the wind to carry pollen grains from one place to another – and a lot of them, too. In a matter of moments, the wind can assist in pollinating hundreds of female plants.
Within a controlled setting, humans can, have and will continue to manually transfer pollen from male plants to female ones. On a large scale this is fairly inefficient, so orchards and similar enterprises will make use of bees and other insects for the purpose of pollination. Humans perform hand-pollination for a few different reasons:
- The creation of specific hybrid plants.
- Helping a species of plant thrive when there aren’t enough natural pollinators in the environment.
- Controlling cross-pollination of plants grown together.
- Saving space that would be required for male and female plants to co-exist.
- Saving energy and time that would be used for the growth of additional male plants.
Hand-pollination is popular among gardeners in urban areas, where planting on a small scale is typically a must. This type of pollination management can be performed fairly easily by using a swab to transfer pollen from a male or even removing the petals of the male plant and brushing the pollen onto the female plant’s anatomy directly.
Pollination and Fertilization are Different but Equally Important in Reproduction
Fertilization cannot occur if not for pollen and the external forces that are responsible for the exchange of pollen from male to female plants. All humans and many other living organisms rely heavily on the reproduction of plants to sustain life all over the globe, so the importance of these processes cannot be overstated. And it all begins with a little bit of pollen making its way to a female plant on the back of a honeybee.
Once pollen has reached a female plant’s stigma, fertilization can begin. After the development of the pollen tube, which brings the pollen to the female gamete, a seed will form if all goes well and nothing disturbs the process. Within this seed will be an embryo, as well as the nourishment that the seed will need to continue to the next step in its development.
Humans have long had their hands in the pollination (and therefore fertilization) process, but it is really remarkable how well nature performs this incredibly important task. Hand-pollination, however, allows urban gardeners and greenhouse operators to have full control and save space in their cross-pollination pursuits.