The Angiosperms, or flowering plants, are the most successful group of plants. They are the most abundant of all plant groups on Earth, and they occupy all habitats. We will spend some time in this lesson learning about their unique and specialized reproduction.
Use the Angiosperm Reproduction Notes Guide to help you organize your learning about these concepts.
Flowers: Angiosperm Sexual Structures
Flowers are more than just ornamental parts of a plant. They are the reproductive structures for Angiosperms (the flowering plants). A flower consists of four whorls of modified leaves. From outside to inside, those whorls are: sepals, petals, stamens and carpels.
The stamens are the male reproductive parts that include the sporangia that produce pollen. The carpels are the female reproductive parts and include the sporangia that produce the egg.
Study the whorls of flower parts using the interactive.
Stop and Think: Can you name each of the flower parts of the Easter lily (Lilium longiflorum) pictured here?
Plants have different strategies for producing flowers and therefore seeds.
- Annual plants germinate, grow, produce seeds and die in a single growing season.
- Bi-annual plants germinate, and grow one season but mature and produce seeds the next, then die at the end of the second year.
- Perennial plants germinate, grow and produce seeds every growing seeds for an indeterminate period of time; this may repeat for several years or hundreds of years.
Pollen is powdery substance that serves to move plant sperm to an egg. Pollen grains have species-specific shapes. The shapes usually have a function in the way the pollen is distributed.
A pollen grain has two cells: a tube cell and a generative cell. The entire pollen grain is the male gametophyte.
Pollination is the placement of pollen on the stigma of the carpel. This pollen transfer can be accomplished by wind, insects, built in mechanical discharge, and other animals, including man.
- Wind pollination is a good strategy when many plants are close together. Wind pollination is not very efficient. Therefore, wind-pollinated plants must produce a great deal more pollen than animal-pollinated plants to be successful. (Hence the yellow ‘dust’ on everything outdoors in the spring!) Grasses and most temperate zone deciduous trees are pollinated by wind.
- Animal pollination depends on animals being attracted to the flower, usually for some reward. Nectar provides a carbohydrate energy source and pollen is rich in protein. Flowers are usually large and showy, and produce fragrant nectar to attract animal pollinators.
Once the pollen lands on the stigma, a series of chemical reactions takes place allowing the pollen grain’s tube nucleus to begin producing a structure called the pollen tube, which digests its way down the style toward the egg. The generative nucleus will divide and produce 2 sperm nuclei. The pollen tube will work its way through the style of the carpel and touch the micropyle of the ovule.
Watch the movie Floral Arrangements from WGBH Educational Foundation to learn more about how flowering plants are pollinated.
Fertilization is the union of sperm (from the pollen) and egg (in the ovule). In angiosperms, the sperm nuclei will enter the embryo sac. One will fertilize the egg and the other will fertilize the two polar nuclei; this is called double fertilization and is only found in Angiosperms.
The fertilized egg (2n) will develop into the immature seed plant, while the (3n) central cell will develop into the endosperm or food storage area of the seed. The result is a seed composed of the seed coat, the nutritive endosperm (cotyledon(s), and the embryo.
Self-fertilization (autogamy) occurs when pollen occurs between the male and female parts of the same flower. Cross-fertilization (allogamy) occurs when the sperm from one plant fertilizes the egg of another.
An angiosperm seed consists of the embryo (2N), the endosperm (3N) and the seed coat. The seed is protected by the seed coat. The endosperm serves as stored nutrients. The micropyle is the only opening into the seed. It is through here that the water will enter to start germination. The seed contains stored food in the form of seed leaves (cotyledons). Some seeds contain one (monocots) while others contain two seed leaves (dicots). Examine the difference below:
The embryo plant contains several areas: the area above the attachment of the cotyledons is the epicotyl. This will develop into the shoots and leaves of the developing plant. The area below the attachment is called the hypocotyl. This will develop into the roots of the plant.
A fruit typically develops from a ripened ovary containing seeds. Fruits protect the seeds and allow for their dispersal away from the parent plant (to avoid competition with the parent).
- Fruits dispersed by animals tend to be sweet, fleshy and attractive for eating. Their seed coats are often tough enough to survive an animal’s digestive tract.
- Fruits dispersed by wind and water are typically lightweight, allowing them to be carried in the air or to float on the surface of the water.
Learn more about the variety of fruits and how they can be classified.
Dormancy is a period during which the embryo of a seed is alive but not growing. Germination is the process of a new plant growing from its seed into a seedling. Sometimes the term germination is used in a more general sense for anything that begins growth (such as a spore or a pollen grain). A number of dormant seeds will not germinate until they are in the proper environmental conditions, such as those indicated below. Roll over the following images to learn more about the factors affecting germination: water, temperature, oxygen, light, scarification.
Once a seed begins to germinate, enzymes are activated that begin to break down stored food reserves to resume growth. The seed uses this energy to grow above the soil (toward light) and begin photosynthesis to generate energy.
Use the interactive to review the life cycle of Angiosperms and key features in their reproduction.