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Angiosperm (Flowering Plants) : It's Definition, Types and Importance
- Kingdom Plantae: General Characteristic and Basis of Classification of Plants
Kingdom Plantae is one of the major groups of organism that comprises about 266,000 known species of ferns, mosses, herbaceous and woody plants, liverworts, vines, bushes, trees and other forms of plants.
- Parts of Angiosperm; Its Importance and General Characteristics
Angiosperm also known as flowering plants belongs to Phylum Angiospermophyta. The name is derived from the Latin word “Angi” which means “enclosed” & Greek word “Sperma” which means “seed”.
Angiosperm which means “enclosed seed” commonly known as flowering plants have various roles of sexual reproduction, seed development and fruit production; numerous plants produce very visible flowers that have typical size, fragrance and color. Some of the most common beautiful flowers are roses, orchids and tulips though some plants have green and gray flowers which remain unnoticed such as oaks, beeches, maples and grasses.
All flowers, whether eye-catching or unremarkable produces male or female sex cells that are necessary for sexual reproduction. Botanist approximately believed that there are more than 240,000 species of flowering plants/angiosperm. Flowering plants are more dominant than another group of plants. They flourish on every continent from the swamps and marshland of the Arctic tundra to the arid soil of Antarctica. Every bone such as grassland, rainforest, deserts shows unique flower species. Even lakes, streams, rivers and bogs are refuge of many flowering plants.
The diversity of the environment where flowers thrive made them evolve into irreplaceable living things in the intricate, interdependent communities of organisms that build up an ecosystem. As producers, they are considered one of the main sources of food for animals and man. In addition, an enormous number of insects, birds, bats and other small mammals feed on its nectar which is a sweet liquid that is produced by flowers. Flowers, seeds and fruits are food of many preys such as lizard, salamander, frogs and fish which is as good food for larger animals or predators such as snakes and owl.
Importance of Flowers
Flowering plants play a vital role in the lives of human. Some of the important roles of angiosperms are:
- Flowers hue gives color to the environment. It brightens landscape and adds attractiveness that beautify the surroundings (homes, parks and roadsides etc)
- Flowers produce fleshy fruits such as apples, grapes, strawberries and oranges. They produce food for all organisms in the world. Some fruits have hard-shelled coverings like pecans, chestnuts and hazelnuts. Flowers also yield grains that are dietary mainstay of human such as wheat, rice, oats and corns.
- Some flowers are popular vegetable such as broccoli and cauliflower.
- Flowers are also a good source of natural dye and perfumes. Some flowers such as jasmine and damask rose produce oils that are made into fragrant.
- Flowers have also medicinal properties such as red clover blossoms
- Edible flowers enhance color and flavor to some dishes.
- In ancient times, flowers are used to symbolize emotions in some important rituals like funerals and weddings.
Structure of the Flower
Pistil and Stamen are the part of flower that is involved in sexual reproduction. These are essential parts that play a specific function for reproduction.
1. Pistil – the female organ of flower. It has three parts:
- Stigma – the expanded tip of the pistil where pollen grains land and germinate
- Style – the tube connecting the stigma to the base of the pistil
- Ovary – the base of the pistil where the egg cell is formed.
2. Stamen – the male organ of flower. It has two parts:
- Anther – the place where pollen grains are produced.
- Filament – the slender stalk which holds the anther.
What is your favorite flower?
Formation of the Egg Cell
The egg cell is formed inside the ovule which is a structured that is located inside the ovary. When the egg cell is fertilized, the ovule turns into a seed. An ovule contains a central mass of cell called a nucleus and an integument which is two outer layers of cells. As the ovule becomes bigger, one of the cells of the nucleus distinguishes into a reproductive cell known as a megaspore mother cell.
The ovule has a minuscule opening called micropyle which proceeds to the nucleus. The megaspore mother cell is considered a diploid. It goes through meiosis and eventually form three times that will give rise to nuclei, then four nuclei and finally to eight nuclei. Temporarily, the functional megaspore flourishes larger which eventually becomes a whole structure embryo sac.
In the embryo sac, only six out of eight nuclei develop walls about them and turn into complete cells. On the other hand, three of these cells move to the end of the embryo sac closest the micropyle, while the other three move to the other end. The two remaining naked cells, which do not have walls move to the middle are known as polar nuclei. The embryo sac which yields the egg is the female gametophyte in angiosperms.
Formation of the Sperm
The anther contains four lobes, each lobes consist numerous cells which distinguish into reproductive cells called microspore mother cells. This microspore mother cell undergoes meiosis that forms four haploid microspores. The haploid microspore nucleus is eventually separated by mitosis into two. One more nucleus stays as the nucleus of the microspore, while the other one is engulfed by its own cytoplasm within the microspore. The cell produced inside a cell is known as generative cell which is a two-celled structure commonly called pollen grains. The pollen grain is considered the male gametophyte in angiosperms. After some time, the pollen grain matures and finally the anther schism open and release the pollen grains into the environment.
Pollination is the transfer of pollen grains from the anther (male organ of plant) to the pistil (female organ) of a plant. Each pollen grain consists of cells that will become male sex cells or sperm. The plant is prepared for fertilization (union of male and female sex cells) by pollination thus it plays an essential role in order for grains, vegetables, fruits, flowers and trees to be pollinated and fertilized to yield seed or fruit and for the production of significant agricultural crops such as corn, wheat, rice, oranges, apples, tomatoes and squash.
There are two types of pollination:
- Self-pollination – the transfer of the pollen grain from the male structure to the stigma of the same flower or another.
- Cross pollination – the transfer of pollen grains from the male structure of one flower to the stigma of another flower or another plant of the similar kind.
Pollination of flowers
Pollen grains are scattered and reach the pistils of flowers through the help of some significant agents which transfer it in the environment. There are four agents:
1. Insects (butterflies, bees, moths, flies and beetles)
- Bees are attracted to flower because of scent (sweet smell), attractive hue of petals, typically blue, yellow or mixture of the two and nectar
- Butterflies and moths have the same structure that sips nectar through their proboscis which is a long tube-like structure. For butterflies they are much attracted to luminous colors usually flowers with red and orange colored petals. On the other hand, moths are attracted to strong scents and for nectar.
- Flies are also an agent for pollination which you would not think of. Certain fly species like Amorphophallus campanulatus are attracted to heavy smells, some species of flowers have very strong smell like the Syzgium cuminii which are found in high mountains are pollinated by flies.
- Birds like hummingbird feeds on flowers to suck nectar, some eat pollen grains and other feed on insects that lives in the flower. Accidentally, they transfer pollen grains from one flower to another.
- Long-nosed bat is a type of bat that feeds on pollen grains of nocturnal flowers. They are typically draws on flowers with strong fragrance and feeds at night. Most of the flowers that are pollinated by bats are areca palm, durian and candle tree.
- Wind pollinates flowers by helping flowers diffuse its pollen grains into the air. The characteristic of flowers that is pollinated by wind are flowers which no petals with stamens and stigma that are open to the wind, flowers with feathery or sticky stigma and flowers which produces numerous amounts of very tiny pollen grains.
Germination of the Pollen Grains and Fertilization
After the pollen grain reached the stigma, it absorbs the liquid (water with dissolved sugar) on the male structure and germinates and soon grows an extension. This extension is called pollen tube. The tube nucleus of the pollen grains arranges and guides the pollen tube while it grows which is similar to generative cells within the pollen grains that are separated by mitosis into two. These are the male sex cells or sperms.
After the pollen tube reaches the ovule, the wall at the edge of the pollen tube cracks and the male sex cells are discharged into the embryo sac. One of it will have union to one egg, which eventually forms a 2N zygote. On the other hand, the other male sex cells combine with the two polar nuclei which will form a single 3n nucleus. Finally, the tube nucleus: synergids and antipods cells break.
Changes in the Flower after Fertilization
After fertilization there will be two major changes that will happen:
- Ovary turn into fruit
- Ovule turns into a seed
Each fertilized ovary propagate in size which stores food into simple organic molecules such as sugar that is the cause why fruits have sweet taste. Within the fruit, fertilized ovule turns into a seed. The 2N zygote that is developed by the union of the male sex cell and the female sex cell will start a new generation which will undergo different mitotic divisions that will form an embryo (young plant inside the seed). The embryo distinguishes to develop one or two cotyledons (hypocotyls and epicotyls). The 3N nucleus developed by the union of one sperm nucleus and the two polar nuclei will also go through different mitosis that will form an embryo. The endosperm sustains food for the developing embryo. When the fruit get ripens and the seed matures, an additional part of the flower gets desiccated.
Plant seeds will eventually fall to the ground because of some agents and if the condition favors the seed, it will germinate. The seed coat break as the seed gets water from the ground. The seed hypocotyls of the embryo will develop in the soil and will sprout the main root of the plant. The epicotyls will flourish towards the light and become stem and leaves. The seedlings will develops and will become a mature plant and will bloom flowers and the cycle will be repeated!
References; Science and Technology by Lilia M. Rabago Ph. D , Crescensia C. Joaquin Ph.D, Catherine B. Lagunzad , PH. D, Encarta