Angiosperms Definition and Examples

Introduction:

• The word "angiosperm" has been derived from two Greek words 'Angeion' which means container and 'Sperma' which means seed. It refers to plants that form seeds within the fruit.
• Angiosperms are the most advanced plants that have flowers, covered ovules/seeds.
• They are the most dominant and highest evolved plants on this earth. The tertiary period of the Cenozoic era is called the Age of angiosperms.
• Angiosperms have been reported in every habitat.
• All types of plants are found in angiosperms annuals/biennials/perennials; autotrophs/parasites/epiphytes/insectivorous/saprophytes.

Detailed Explanation:

1. Salient features:

• Angiosperms, or flowering plants, have pollen grains and ovules developed in specialized structures called flowers. The seeds of angiosperms are enclosed within the fruits.
• Angiosperms create the largest and most diverse group of flowering plants in the kingdom Plantae, with approximately 300,000 species. Angiosperms account for roughly 82% of all currently known green plants.
• The diversity of forms found in angiosperms is wider than in any other plant phylum. The size range is quite remarkable, ranging from the smallest flowering plant, the Wolffia which is less than 2 millimeters, to the tallest angiosperms which are Eucalyptus regnans (Australia's mountain ash tree), at approximately 100 meters (330 feet). Angiosperms of nearly every single size and shape exist between these two extremes.
• The wide range of habitats in which angiosperms grow, as well as their nearly complete worldwide distribution, reflects the wide variation in angiosperm form.
• Angiosperms are classified into two groups based on the type of cotyledon present, these are monocotyledons and dicotyledons, respectively. The seeds of dicotyledonous plants as the name suggests have two cotyledons, while monocotyledonous plants have one cotyledon.

• The characteristic features are presence of vessels in xylem and presence of companion cells in phloem.
• Secondary growth is found in dicots.
• Antheridia and archegonia are absent.
• Presence of flowers is the most important feature. Sex organs enclosed in the flower.

2. Life Cycle of Angiosperms:

• The life cycle of an angiosperm is of diplontic type and consists of two stages: sporophyte and gametophyte.
• The sporophyte (2n) is the main plant body seen in an angiosperm which is differentiated into roots, stem and leaves.
• When sporophyte cells undergo meiotic division in order to prepare for reproduction, they produce reproductive cells with only half of the entire chromosomes (haploid) which develop to form gametophytes (pollen grains or embryo sac). Meiosis occurs in the floral parts of angiosperms.
• A vast array of floral structures are found in angiosperms. The flower is the sexual reproductive structure of angiosperms and it is the modified shoot of an angiosperm plant.
• A flower has four whorls which are calyx (contains sepals), corolla (contains petals), androecium (contains stamens), and gynoecium (contains pistil or carpel).
• The stamen/microsporophyll is a male sex organ of a flower. Each stamen is made up of a thin filament with an anther attached at its tip.
• The pistil or carpel or megasporophyll is the female sex organ in a flower. The pistil is made up of an ovary that houses one to numerous ovules.
• Angiosperms are heterosporous as they produce two types of spores which are microspores (male) and megaspores (female).
• Microspores are produced within the anthers of the stamen. Anthers have microsporangium (pollen sacs) that contain microspore mother cells.
• Microspore mother cells divide by meiosis inside the anthers' microsporangia to produce haploid microspores, which then undergo mitosis and form pollen grains. Each pollen grain usually has two cells, one generative cell that divides into two male gametes and one vegetative or tube cell that develops into the pollen tube cell.
  
  
  
  
   
• However, megaspores are produced from the megaspore mother cells of megasporangium (present within the ovules) by meiosis. The megasporangium is shielded by two layers of integuments and an ovary wall.
• A megaspore mother cell undergoes meiosis within each megasporangium, producing four megaspores. Only one megaspore towards the chalazal end survives, and it gives rise to the female gametophyte known as the embryo sac (7-celled, 8- nucleated). The megaspore undergoes three mitotic divisions for the development of female gametophyte.
• Each embryo-sac contains three antipodal cells towards the chalazal end, two polar nuclei of the central cell, and a three-celled egg apparatus consisting of one egg cell and two synergids towards the micropylar end.
• By pollination, pollen grains are transferred to the stigma of a flower. If the pollen is compatible with the stigma, it starts germinating on that stigma.
• While the pollen grains germinate on the stigma, the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule. The pollen tubes enter the embryo-sac where two male gametes are discharged.
• One of the male gametes fuses with the egg cell to form a zygote (syngamy/true fertilization). The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN). Because of the involvement of two fusions, this event is termed double fertilization, an event unique to angiosperms.
• The zygote develops into an embryo (with one or two cotyledons) and the PEN develops into an endosperm which provides nourishment to the developing embryo. The synergids and antipodals degenerate after fertilization.
• During these events, the ovules develop into seeds and the ovaries develop into a fruit.
• These seeds are dispersed by various means and when fall on the appropriate substratum, they germinate to form a new plant. Thus, the cycle repeats again.

Differences between Gymnosperms and Angiosperms are summarized in the table below-
 

3. Economic Importance of Angiosperms:

• Angiosperms supply us with food, feed, fuel, medicines, and a variety of other commercially important products.
• Agriculture relies almost entirely on angiosperms, either directly or indirectly via livestock feed.
• The Poaceae, or grass family, is by far the most important of all flowering plant families, providing the majority of cereals like wheat, rice, corn (maize), barley, millet, rye, oats, etc.
• The plants of Fabaceae, or legume family, provide products rich in proteins.
• The Solanaceae family includes potatoes, tomatoes, and peppers whereas the Cucurbitaceae, or gourd family includes pumpkins and melons.
• The Brassicaceae, or mustard plant family includes rapeseed, cauliflower, radish, broccoli and cabbage.
• Many of the fruits including apples, pears, cherries, apricots, plums, etc, and flowers like rose, tulip, etc are provided by angiosperms.
• Another economic benefit of angiosperms is that they provide a variety of pharmaceuticals.
• Apart from some antibiotics, the compositions of almost all medicines are derived and extracted directly from angiosperms, or if synthesized, their major elements are found in angiosperms.
• Vitamins, narcotics, aspirin, and quinine are among the medications on the list.
• Some such angiosperms that are incredibly toxic to living things have been shown to be very useful in the reduction of cancer, leukemia, and a variety of heart problems.
• Quinine is used to cure malaria, vincristine to treat leukemia, curare to relax muscles during open-heart surgery, and diosgenin as a starting material in oral contraceptives.
• Flowering plants are the sources of wood, paper, spices, fibers like cotton, hemp, flax, etc.
• The contribution of angiosperms in maintaining our habitat is of great significance.
• The diversity of sources of food and supply of oxygen in our environment is heavily reliant on the variety of angiosperms found.
• A significant decrease in the angiosperms will have a significant impact on survival of our habitat.

Frequently asked questions (FAQ)

Q1. Name the smallest and the largest angiosperm.
Ans.

• Angiosperms are flower-producing vascular seed plants in which the ovule gets fertilized to develop into a seed within the ovary.
• The size range is quite remarkable in the angiosperms.
• The smallest flowering plant is Wolffia which is less than 2 millimeters and the tallest angiosperm is Eucalyptus regnans (Australia's mountain ash tree), at approximately 100 meters (330 feet).

Q2. What is double fertilization?
Ans.

• Flowering plants are known for their double fertilization. It is named so as fertilization occurs twice. One of the male gametes fertilizes the egg, leading to the production of a diploid zygote referred as true fertilization or syngamy, while the other male gamete combines with two polar nuclei (which fuse to form a secondary nuclei just prior to fertilization) to form the triploid endosperm referred as triple fusion.

Q3. Write two economic significance of angiosperms as a source of food.
Ans.

• The Poaceae, or grass family, is by far the most important of all flowering plant families, providing the majority of cereals like wheat, rice, corn (maize), barley, millet, rye, oats, etc.
• The plants of Fabaceae, or legume family, provide products rich in proteins.
• Many of the fruits including apples, pears, cherries, apricots, plums, etc are provided by angiosperms.

Q4. Explain the 7-celled, 8-nucleated embryo sac/ female gametophyte of angiosperms.
Ans.

• An embryo-sac consists of three antipodal cells towards the chalazal end, two polar nuclei of the central cell, and a three-celled egg apparatus consisting of one egg cell and two synergids towards the micropylar end.

Q5. Mention the features that show the angiosperms are evolutionarily advanced.
Ans.
The following features show that angiosperms are evolutionarily advanced-

• Presence of flower
• Presence of seed enclosed within the fruit
• Double fertilization
• Well developed sporophyte and reduced gametophyte
• Presence of vessels in xylem and companion cells in phloem.