Ray was a member of Science Olympiad, participates in science and health writing competitions, and studied at a sci-tech school.
What Is Pollination?
Pollination is the process of reproduction in plants. It is the transfer of pollen from a male part of a plant to a female part of a plant. This transfer of pollen enables fertilization and the production of seeds. The process pollination requires at least one pollination agent. How do pollen grains reach the pistils? Among the most important agents which transfer pollen grains in nature are the following:
- Insects such as bees, butterflies, moths, beetles, and flies
Difference Between Self-Pollination and Cross-Pollination
Self-pollination and cross-pollination are the two types of pollination. The transfer of pollen grains from the anther to the stigma of the same flower or another flower in the same plant is known as self-pollination. On the other hand, the transfer of pollen grains from the anther of one flower to the stigma of another flower in another plant of the same kind is known as cross-pollination.
Gregor Johann Mendel, an Australian monk, performed many experiments on heredity. After carefully studying the structural characteristics and growing habits of several plants, Mendel finally selected the garden pea to be the subject of his experiment.
One reason he chose garden pea is that it has several contrasting characters. Another reason why Mendel chose the garden pea is a hardy plant and does not need much caring and cultivating.
Mendel experimented on one pair of contrasting characters at a time, say, round and wrinkled seeds. First, he allowed plants to self-pollinate for several generations. This way, he was sure that the round seeds were pure. It means that all the pea plants which he cultivated for this purpose produced only round seeds.
We describe such plants that produce the same character from generation to generation as pure-breeding. Mendel did the same with the plants bearing wrinkled seeds.
Mendel cross-pollinated pure-breeding plants bearing round seeds with pure-breeding plants bearing wrinkled seeds. When the stigma of the flower of one plant was not yet ripe or mature, Mendel removed all the anthers. Then, using a brush, he got the pollen grains from the ripe anther of another plant and transferred them to the first plant.
What makes bees good agents of pollination is their instinct to gather nectar from flowers of only one species at a time. Bees recognize the flower of one species by their smell, general form, and color. Examples of such flowers are those of squash, melon, pea, violets, some orchids, verbena, and many fruit trees. Observations show that bees are attracted to flowers which have the following:
- Sweet smell (fragrance)
- Brightly colored petals, usually blue, yellow, or a mixture of these two colors
2. Butterflies and Moths
Butterflies and moths have similar structures. Both of them suck nectar through a long tubelike structure called a proboscis. But they differ in the time they fly in search of food. Butterflies feed during the day, while moths feed during dusk and at night.
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Because of this difference in feeding time, the flowers visited by these two kinds of insects differ markedly in color. Those visited by butterflies are brightly colored, while those visited by moths are often white.
There are certain characteristics possessed by flowers pollinated by these insects. Butterflies pollinate nectars, and brightly colored petals, usually red and orange. Moths pollinate nectar and petals with heavy fragrance.
Examples of flowers pollinated by butterflies are lilies, carnations, and many of our garden plants. Examples of flowers pollinated by moths are tobacco, yucca, morning-glory, and many orchards.
Did you ever think of flies as agents of pollination? A good example of a fly-pollinated flower in tropical countries like the Philippines is the queer-looking plant called "pongapong." The flower exudes a very strong smell of rotting flesh. It attracts swarms of big blue bottle flies that crowd on top of the large flower.
A more familiar plant that attracts blue bottle flies is duhat (Syzygium cuminii). Many flowers pollinated by flies are found in high mountains where not too many insects can live.
Why do some birds visit flowers? They do so because they feed on the following:
- Insects which live in flowers
Incidentally, they transfer pollen grains as they travel from flower to flower. Examples of such birds are the hummingbirds of North and Central America. Birds have good vision but a poor sense of smell. Hence, the flowers they pollinate are often large and brightly colored, usually red and yellow and odorless.
Examples of flowers pollinated by birds are members of the pea, pineapple, banana, orchid and cactus families.
Do you know that bats help in pollination? The long-nosed bat feeds on pollen of night-blooming flowers. They feed at night. They are probably attracted to flowers by the smell. Examples of flowers pollinated by bats are those of areca palm, candle tree, and durian.
Wind is another good agent of pollination. Examples of wind-pollinated flowers are corn, rice, and grasses. Wind-pollinated flowers have the following characteristics:
- Most of them do not have petals. Their stamens and stigmas are exposed to the wind.
- The stigma is feathery or sticky. Pollen grains stick to it easily.
- They produce great numbers of very tiny pollen grains which are scattered far and wide by the wind.
Germination Process of Pollen Grains and Fertilization
What happens after pollen grain land on the stigma of a flower of the same species? Upon reaching the stigma, the pollen grain absorbs the liquid with dissolved sugar on the stigma and germinates, that is, it grows an extension. This outgrowth is referred to as the pollen tube.
Although it is named as a tube, it is not really a tube but an extension of the pollen grain. The tube nucleus of the pollen grain guides the growth of the pollen tube. Then something happens to the generative cell inside the pollen grain. It divides by mitosis into two. These are the sperms.
The diagram below shows a pollen grain germinated on the stigma of the flower. The pollen tube has grown through the style and reached the micropyle of the ovule. Then, the wall at the tip of the pollen tube breaks, and the sperms are discharged into the embryo sac. One sperm unites with the two polar nuclei, forming a single 3N nucleus. Eventually, the tube nucleus, synergids, and antipodal cells disintegrate.
Changes in the Flower After Fertilization
What major changes take place in the flower after fertilization? There are two major changes taking place after fertilization. These are:
a. The ovary develops into a fruit.
b. The ovule becomes a seed.
The fertilized ovary grows in size. Its stored food is digested into simple organic molecules such as sugars. This is why fruits become as sweet as they ripen. Inside the fruit, the fertilized ovule develops into a seed. The 2N zygote formed by the union of a sperm and an egg is the beginning of a new generation. It undergoes several mitotic divisions, forming an embryo or the young plant inside the seed.
The embryo differentiates to form one or two cotyledons, a hypocotyl, and an epicotyl. The 3N nucleus formed by the union of one sperm nucleus and the two polar nuclei also undergoes several mitoses, forming the embryo. The endosperm supplies the developing embryo with food. As the fruit ripens and the seed matures, the other parts of the flower dry up.
Why Don't Bananas Have Seeds?
Most cultivated bananas have triploid chromosome number (3N). Because of this uneven number of chromosome sets, they cannot divide by meiosis to produce eggs and sperms. As a result, fruits develop without fertilization. Hence, the fruits produce no seeds.
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© 2020 Ray
Ray (author) from Philippines on March 07, 2020:
You're very welcome Ms. Chitrangada. I love sharing things about science! I love science even before taking engineering in college. Thank you for taking some time reading my article.
Chitrangada Sharan from New Delhi, India on March 03, 2020:
A very well written and informative article about the agents of pollination.
We see them around regularly—in gardens, in parks, on our terrace; but we don’t even even realise, how much work these little creatures are doing.
You reminded me of my biology lessons in school. Loved the pictures.
Thanks for sharing this wonderful information.