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Flowering Plants: Life Cycle & Examples

Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

You likely know that a flower is the reproductive part of a flowering plant (or angiosperm). Did you know that a flower can be male, or female, or both? Or that an angiosperm can reproduce sexually or asexually? This lesson covers the angiosperm life cycle.

What is a Flowering Plant?

Many plants can reproduce either sexually or asexually. Flowers allow for genetic diversity. Just like us humans, flowering plants have sperm and eggs. Unlike us, flowering plants can have sperm and eggs produced in the same organ. But flowering plants do not have to reproduce sexually. For example, a strawberry plant can make flowers and fruit (via sexual reproduction) or send out a runner (via asexual reproduction). The table below lists the groups of angiosperms. The matching images are in the summary of this lesson.

Table of angiosperms
Table of angiosperms

The Plant Life Cycle

Angiosperms (flowering plants) have two alternative life cycles because they undergo sexual and asexual reproduction. Asexual reproduction creates a clone of the original plant. The advantage to asexual reproduction is that an individual plant can propagate the species. Sexual reproduction creates genetic diversity. The seeds produced by sexual reproduction can also help distribute plants to far away areas. Thus, there are two advantages to sexual reproduction. Figure 1 shows the angiosperm life cycle.

Angiosperm Life Cycle
Image of angiosperm life cycle

Meristems

The growing tip of a plant is called the meristem. You can see this at the tip of a branch, and it is where new leaves form. Angiosperms shift between a vegetative growth phase (no flowers) and a reproductive growth phase (flowers). The tip of the growing plant is called an apical meristem when the plant is in a vegetative phase. It becomes a floral meristem when the plant transitions into the reproductive phase.

Other than the obvious flower versus leaves visible difference, there is another difference between these meristems. Apical meristems have indeterminate growth. That is, they can keep growing and growing. Flowers do not keep growing. Thus, floral meristems form a flower and stop growing. This is determinant growth.

Diploid vs. Haploid

Like most other higher organisms, plants are diploid. This means that they have two copies of each gene in their nucleus. Once copy comes from each parent, which is how we get genetic diversity. Each parent is genetically different, thus the combination of their genetic material (DNA) creates a new variation. For example, someone might have said to you, 'You have your father's eyes, but your mother's nose.'

Sperm and egg cells (gametes or gametophytes) are haploid. This means that they have half of the genetic material of a diploid cell. Sexual reproduction requires that the diploid organism halves its DNA. If it did not, then the offspring would inherit all of each parent's genetic material. They would have twice as much DNA as necessary. So, developing gametes undergo two stages of cell division, mitosis (cell division with DNA replication) and meiosis (cell division without DNA replication). The resulting mature gamete is haploid.

The Angiosperm Sexual Reproduction Organ: The Flower

When you look at a flowering plant, the part you can see is called a sporophyte. The flower contains the male and/or female gametophytes. Flowers are actually modified leaves. Each flower contains four different types of modified leaves: carpels, petals, stamens, and sepals. Figure 2 shows the different parts of a flower.

Anatomy of a Flower
Image of Flower anatomy

The Female Gametophyte

The carpel is the female sex organ. Female gametophytes develop within the carpel. The carpel (megametophyte) is comprised of a stigma, style, ovary, and ovule. The megaspores (egg cells or female gametophytes) reside within the ovule (macrospore).

An individual macrospore will undergo meiosis twice to produce four haploid macrospores. Three of these haploid macrospores will die. The remaining macrospores undergo mitosis (cell division with DNA replication) three times. This process is called megagametogenesis, and it results in an embryo sac with one egg. Only the egg is fertilized - the rest of the cells aid with fertilization.

The Male Gametophyte

Male gametophytes are called microspores. They develop within the male sex organ (anther). They also undergo two rounds of meiosis to produce four haploid cells. However, all four haploid cells will develop into pollen grains (microgametophytes). To form a pollen grain, an individual microspore forms a spore wall. Inside the spore wall the haploid cell divides via mitosis. The resulting two cells are called a generative cell and a tube cell.

Fertilization

Pollen grains are released from the anther and travel to the stigma. Once bound to the stigma, the tube cell forms a pollen tube which delivers the sperm to the egg. Only one sperm nucleus fuses with the egg. The other fuses with the polar nuclei. An embryo then begins to develop within the ovary. The ovary becomes a seed. If the angiosperm develops fruit, then the ovary wall thickens and creates the edible portion of the fruit.

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