7.3: Plant Reproduction and Life Cycle

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Do all plants have flowers?

No, plants do not all have flowers. For example, the mosses and ferns pictured here are both types of plants. However, they never produce flowers. They don't produce seeds, either. They do, however, make tiny spores to reproduce.

Plant Reproduction and Life Cycle

The life cycle of a plant is very different from the life cycle of an animal. Humans are made entirely of diploid cells (cells with two sets of chromosomes, referred to as ''2n''). Our only cells that are haploid cells (cells with one set of chromosomes, ''n'') are sperm and egg cells. Plants, however, can live when they are at the stage of having haploid cells or diploid cells. If a plant has a haploid chromosome number of 20, what is the diploid chromosome number? If the diploid chromosome number is 20, what is the haploid number?

Plants alternate between diploid-cell plants and haploid-cell plants. This is called alternation of generations, because the plant type alternates from generation to generation. In alternation of generations, the plant alternates between a sporophyte that has diploid cells and a gametophyte that has haploid cells.

Alternation of generations can be summarized in the following four steps: follow along in the Figure below as you read through the steps.

The haploid gametophyte produces the gametes, or sperm and egg, by mitosis. Remember, gametes are haploid, having one set of chromosomes.
Then, the sperm fertilizes the egg, producing a diploid zygote that develops into the sporophyte, which of course, is diploid.
The diploid sporophyte produces haploid spores by meiosis.
The haploid spores go through mitosis, developing into the haploid gametophyte.

As we will see in additional Plants concepts, the generation in which the plant spends most of its life cycle is different between various plants. In the plants that first evolved, the gametophyte takes up the majority of the life cycle of the plant. During the course of evolution, the sporophyte became the major stage of the life cycle of the plant.

Summary

Plants undergo alternation of generations, meaning they alternate between diploid-cell plants and haploid-cell plants.
During the course of evolution, the sporophyte (diploid) became the major stage of the life cycle of the plant.

Explore More

Use the resource below to answer the questions that follow.

Plant Reproduction at http://www.youtube.com/watch?v=ZvBMQIrlhYw (2:23)

When does meiosis occur during a plant's life cycle?
When does mitosis occur during a plant's life cycle?
How do gametophytes with both male and female parts avoid self-fertilization?
Describe the fertilization process for an Angiosperm.

Review

What is the difference between a haploid and diploid cell?
What does "alternation of generations" mean?
What part of the plant life cycle is made of haploid cells, the sporophyte or the gametophyte?
What process is used by the sporophyte to produce spores?