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Angiosperms

Angiosperms

Some plants produce flowers, while others don't. Flowers primarily function in sexual reproduction, and when the ovule of a female plant is fertilized, a seed-bearing fruit is formed. Flowering plants are collectively known as angiosperms. In the following, we will first define angiosperms and distinguish them from gymnosperms. We will then discuss the life cycle of angiosperms, including their sexual and asexual reproduction.

What is the definition of angiosperms?

Angiosperms are plants that bear flowers and fruits. This group of organisms are part of the same common ancestor.

The below shows a definition for the term angiosperm.

Angiosperms (or flowering plants) are vascular plants whose seeds are enclosed in ovaries. They produce flowers and seed-bearing fruits.

Vascular plants are those with vascular tissues–called xylem and phloem–that conduct water and nutrients to various parts of the plant.

What are some examples of angiosperms?

Angiosperms are the largest and most species-rich group of plants, with over 300,000 species. Examples of angiosperms range from dandelions and grasses to beans and fruits.

There are many examples, in nature, of angiosperms.

Angiosperms vs gymnosperms

Vascular plants reproduce through seeds or spores. Both angiosperms and gymnosperms reproduce by seeds and, as such, are referred to as seed plants.

The main difference between angiosperms and gymnosperms is how their seeds are developed. As mentioned earlier, the seeds of angiosperms are enclosed in the ovaries of flowers, which mature into fruits.

In contrast, the seeds of gymnosperms are exposed. Instead of being enclosed in fruits, their seeds are found in unisexual cones and are not visible until they reach maturity. Examples of gymnosperms include pines, ginkgo, and cycads.

Angio-” means “vessel,” referring to the ovary that contains the seeds. On the other hand, "gymno-" means naked or exposed.

What is the life cycle of angiosperms?

All plants have a life cycle with alternating generations, where haploid and diploid generations alternately produce each other.

Recall that diploid means having two sets of chromosomes (one from each parent), whereas haploid means having one set of chromosomes.

The diploid plant–called sporophyte–produces haploid spores through meiosis. These spores undergo mitosis to produce gametophytes, male and female haploid plants that produce gametes (sperm and eggs).

The fusion of these gametes–a process called fertilization–results in a diploid zygote. When the zygote divides through mitosis, it forms a new sporophyte.

The sporophyte generation of an angiosperm is more dominant than its gametophyte generation.

What are the key characteristics of angiosperms?

The key characteristics of angiosperms can be summarized as “3Fs”:

  1. Flowers;

  2. Double fertilization; and

  3. Fruits

Let’s go through each one.

Angiosperms have flowers that attract pollinators

Flowers are sporophytic structures that function in sexual reproduction. Flowers are made up of four main organs: carpels, stamens, petals, and sepals, all of which are attached to a part of the stem called the receptacle.

Carpels and stamens are modified leaves that function in reproduction, called sporophylls. On the other hand, sepals and petals are sterile modified leaves.

Carpel

The carpel (or megasporophyll) represents the female reproductive parts of the flower. It typically occupies the center of the flower. It consists of three main parts: the ovary, the style, and the stigma.

  • The ovary is found near the base of the carpel. It contains one or more ovules, which become seeds upon fertilization. The female gametophyte called the embryo sac develops inside each ovule.

  • The style is the long, stalk-like part of the carpel that raises the stigma above the ovary and other parts of the flower.

  • The stigma is found at the elevated end of the style. It is a sticky structure specialized in capturing pollen.

The carpels are joined in most species, creating a compound ovary with two or more chambers that each contain one or more ovules. Sometimes, a single carpel or two or more fused carpels are referred to as a pistil.

Stamen

The stamen (or microsporophyll) represents the male reproductive parts of the flower. Like the carpel, it typically occupies the center of the flower. It consists of two main parts: the anther and the filament.

  • The anther consists of sac-like structures called microsporangia that produce pollen. In angiosperms, the pollen grain is the male gametophyte that produces sperm.

  • The filament is the stalk-like structure that connects the anther to the flower.

Petals

Petals are typically larger and more brightly colored than sepals. Their color and fragrance serve to attract insects and other animal pollinators. Some flowers, including those pollinated by wind or water, have no petals at all. The entire whorl of petals is referred to as the corolla.

Sepals

Sepals typically resemble leaves more than other parts of the flower. They are usually green and leaf-like, although some flowers may have colored sepals. Sepals enclose and protect the flower bud before it opens. The entire whorl of sepals is referred to as the calyx.

Angiosperms can have complete or incomplete flowers. Complete flowers have all four floral organs, while incomplete flowers lack one or more of the organs. Examples of complete flowers include hibiscus, magnolias, and roses. Examples of incomplete flowers include corn (which have neither petals nor sepals) and papaya (which have only either male or female reproductive parts).

Did you know that a single sunflower is actually made up of hundreds of tiny flowers? The central disk is made up of incomplete flowers, while the “yellow petals” are actually individual, sterile incomplete flowers!

Angiosperms sexually reproduce through double fertilization

Double fertilization is a phenomenon in which two fertilization events occur: one sperm cell fertilizing the egg and another fertilizing two polar nuclei.

Double fertilization is unique to angiosperms; it does not occur in other plants.

When the pollen reaches the stigma of a carpel, pollination takes place. This can occur by wind, water, or animals. Once the pollen makes contact with the stigma, it germinates.

The stigma contains two cells: a generative cell and a tube cell. When pollen germinates, the pollen tube cell grows and extends into the style, and the generative cell enters the tube, where it divides via mitosis to form two sperm cells. The sperm cells remain inside the tube cell as the pollen tube goes through an opening in the ovule called a micropyle.

One sperm cell fertilizes the egg, forming a diploid zygote. The other sperm cell fertilizes two polar nuclei, forming a triploid cell in the center of the large central cell of the embryo sac. This triploid cell develops into the endosperm, which will become a food source for the growing embryo. These two fertilization events are collectively referred to as double fertilization.

Fruits facilitate seed dispersal

The fertilized ovule becomes the seed, and the ovary forms the fruit which, in turn, encloses the seed and aids in its dispersal. By spreading to areas at a distance from the parent plant, seeds can potentially germinate and grow in more favorable and less competitive environments.

Seeds consist of a dormant embryo surrounded by a food supply stored and protective tissues. Embryonic leaves called cotyledons absorb nutrients stored within the seed until the plant produces true leaves and begin undergoing photosynthesis.

Angiosperms can be classified according to the number of cotyledons they have:

  • Monocots have a single cotyledon.

  • Dicots have two cotyledons.

Seeds germinate when optimal environmental conditions are met. These conditions include temperature, light, and water availability.

Fruits can be classified based on their developmental origin:

  • Simple fruits are those derived from a single carpel or several fused carpels from a single flower.

    • Examples of simple fruits include bananas, oranges, and apples.

  • Aggregate fruits are those derived from a single flower that has two or more separate carpels, each forming a tiny “fruitlet.” These fruitlets are grouped together in one receptacle.

    • Examples of aggregate fruits include blackberries and raspberries.

  • Multiple fruits are those derived from a group of flowers crowded together in the same inflorescence. As the walls of ovaries begin to thicken, they combine to form a single fruit.

    • Examples of multiple fruits include pineapples and jackfruits.

Other parts of the flowering plant can contribute to the formation of the fruit. These are called accessory fruits. The strawberry is a type of accessory fruit: its red fleshy part is actually the receptacle (which, if you recall, is actually the thickened part of a stem), while the structures embedded on its surface are actually tiny fruits, each bearing a single seed!

A common misconception is that all fruits are sweet. In biology, any seed-bearing structure that develops from the ovary of a flowering plant is a fruit. That means tomatoes, squash, and chilli peppers are all fruits!

Some angiosperms also reproduce asexually

We have discussed the three Fs that characterize angiosperms, but these only tackle sexual reproduction. Sexual reproduction is important in angiosperms as it produces most of the genetic variations that allow plants to evolve with better adaptations to their environment.

However, some angiosperms can also undergo asexual reproduction.

Asexual reproduction is when offspring are produced from a parent plant without the fusion of egg and sperm. The resulting offspring is thus genetically identical to its parent.

Asexual reproduction helps angiosperms reproduce even in the absence of a pollinator. There are various mechanisms for asexual reproduction in angiosperms.

One of these is fragmentation, where a parent plant is split into two or more parts that each develop into a whole individual. For example, if you cut off the part of a potato with an “eye” (which is actually a bud), it can grow into a whole plant.

Another mechanism is apomixis. This is a form of asexual reproduction where seeds are produced without pollination or fertilization, such as those in dandelions. The embryo is instead created by a diploid cell within the ovule, and the ovules develop into seeds.

Angiosperms - Key takeaways

  • Angiosperms are flowering plants that produce seed-bearing fruits.
  • In angiosperms, the seed is enclosed in an ovary, whereas in gymnosperms the seed is exposed or found in cones.
  • Double fertilization is where two fertilization events occur. It is an adaptation that occurs only in angiosperms.
  • The fertilized ovule becomes the seed, and the ovary forms the fruit which encloses the seed.
  • Angiosperms reproduce sexually and/or asexually.

References

  1. Jane B. Reece, et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
  2. Georgia Tech Biological Sciences, Plant Reproduction | Organismal Biology, 10 Nov. 2016.
  3. W.P. Armstrong, Fruit Identification Outline, Wayne's Word, 12 Jun. 2001.
  4. UC Museum of Paleontology, Monocots vs. Dicots, 2022.

Frequently Asked Questions about Angiosperms

Angiosperms are flowering plants that produce seed-bearing fruits.

Angiosperms can be classified into two main types, based on the number of cotyledons they have: monocots have one cotyledon while dicots have two.

In angiosperms, meristematic tissue is responsible for growth. It is typically found in the tips of roots and shoots.

Angiosperms produce seeds, and as such are considered seed plants alongside gymnosperms.

The main difference between angiosperms and gymnosperms is whether or not the seeds are contained in an ovary. In angiosperms, the seed is enclosed in an ovary, whereas in gymnosperms the seed is exposed or found in cones.

Final Angiosperms Quiz

Question

____ are flowering plants.

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Answer

Angiosperms

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Question

The ____ generation is more dominant in angiosperms.

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Answer

Diploid sporophyte

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Question

Which of the following main organs contain the female reproductive parts of the flower?

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Answer

Carpel

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Question

The ____ is found at the elevated end of the style. It is a sticky structure specialized in capturing pollen. 

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Answer

stigma

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Question

The _____ represents the male reproductive parts of the flower. 

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Answer

stamen

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Question

In angiosperms, the ____ is the male gametophyte that produces sperm.

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Answer

pollen

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Question

The ____ is the stalk-like structure that connects the anther to the flower.

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Answer

filament

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Question

Flowers are made up of four main organscarpelsstamenspetals, and sepals, all of which are attached to a part of the stem called the _____. 

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Answer

receptacle

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Question

What is the function of sepals?

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Answer

Sepals enclose and protect the flower bud before it opens. 

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Question

What is double fertilization?

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Answer

Double fertilization is a phenomenon in which two fertilization events occur: one sperm cell fertilizing the egg and another fertilizing two polar nuclei

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Question

What is the function of cotyledons?

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Answer

Cotyledons absorb nutrients stored within the seed until the plant produces true leaves and begin undergoing photosynthesis

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Question

What part of the flower turns into the seed and the fruit?

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Answer

The fertilized ovule becomes the seed, and the ovary forms the fruit.

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Question

____ is a mechanism of asexual reproduction where a parent plant is split into two or more parts that each develop into a whole individual.

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Answer

Fragmentation

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Question

______ is a form of asexual reproduction where seeds are produced without pollination or fertilization.

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Answer

Apomixis

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Question

What are the key characteristics of angiosperms?

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Answer

  1. Angiosperms produce flowers that attract pollinators
  2. Angiosperms sexually reproduce through double fertilization
  3. Angiosperms produce fruits that aid in seed dispersal

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