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9.18: Plant Life Cycles

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Fertilization or pollination. How does this occur in the plant?

Pollination. A significant step in the life cycle of flowering plants. But fertilization must occur in the life cycles of all plants, not just those with flowers. Does it always use the birds or the bees?

General Plant Life Cycle

The life cycle of all plants is complex because it is characterized by alternation of generations. Plants alternate between diploid sporophyte and haploid gametophyte generations, and between sexual and asexual reproduction. The ability to reproduce both sexually and asexually gives plants the flexibility to adapt to changing environments. Their complex life cycle allows for great variation. A general plant life cycle is represented by the diagram in Figure below . From the figure, you can see that the diploid sporophyte has a structure called a sporangium (plural, sporangia) that undergoes meiosis to form haploid spores . A spore develops into a haploid gametophyte . The gametophyte has male or female reproductive organs that undergo mitosis to form haploid gametes (sperm or eggs). Fertilization of gametes produces a diploid zygote . The zygote grows and develops into a mature sporophyte, and the cycle repeats.

This diagram represents the life cycle that generally characterizes plants.

One of the two generations of a plant’s life cycle is typically dominant to the other generation. Whether it’s the sporophyte or gametophyte generation, individuals in the dominant generation live longer and grow larger. They are the green, photosynthetic structures that you would recognize as a fern, tree, or other plant (see Figure below ). Individuals in the nondominant generation, in contrast, may be very small and rarely seen. They may live in or on the dominant plant.

The dominant generation in nonvascular plants is the gametophyte; in vascular plants, it’s the sporophyte. Why is a dominant sporophyte generation an advantage on land?

All of these photos show plants of the dominant generation in their life cycle.

Summary

  • All plants have a life cycle with alternation of generations.
  • Plants alternate between diploid sporophyte and haploid gametophyte generations, and between sexual reproduction with gametes and asexual reproduction with spores.

Practice

Use this resource to answer the questions that follow.

  1. Distinguish between the sporophyte and gametophyte.
  2. Which phase, diploid or haploid, is the dominant phase in vascular plants?
  3. How does the plant life cycle compare to that of an animal?

Review

1. Outline the general life cycle of plants.

2. What are sporangia? What do they do?

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Feb 24, 2012

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Jul 30, 2014
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