Alternating Life Cycles

All plants have a life cycle that involves two phases, which are named for the type of reproductive cells they produce. Recall that cells having two sets of chromosomes are referred to as diploid, and cells having only one set of chromosomes are referred to as haploid. The first phase of a plant's life cycle consists of a diploid (2n) sporophyte (SPOH-ruh-FlET) plant that produces spores. The second phase consists of a haploid (1n) gametophyte (guh-MEET-uh-FlET) plant that produces eggs and sperm. A life cycle that alternates between the gametophyte phase and sporophyte phase is called alternation of generations.

Such deforestation is occurring at a rate of several hundred thousand square miles per year. Although this rate seems overwhelming, people can help reforestation efforts in once naturally forested areas by planting seeds or seedlings. It's best to choose trees that are well adapted to your area. For more information, call your local or state parks department, the U.S. Forest Service, a county extension agent, or a nearby college forestry department.

Gametophyte (1n) Gametophyte (1n)

(a) NONVASCULAR PLANT LIFE CYCLE (b) VASCULAR PLANT LIFE CYCLE

figure 28-3

In the life cycle of a plant, there is an alternation of the haploid gametophyte generation and the diploid sporophyte generation. (a) The life cycle of a nonvascular plant, such as a moss, is characterized by a long, thin sporophyte growing up from the top of the more prominent gametophyte. (b) The life cycle of a vascular plant, such as a fern, is characterized by a large sporophyte and a very small gametophyte.

Gametophyte (1n) Gametophyte (1n)

(a) NONVASCULAR PLANT LIFE CYCLE (b) VASCULAR PLANT LIFE CYCLE

Figure 28-3 shows the life cycles of a nonvascular plant and a vascular plant. In alternation of generations, the gametophyte (1n) undergoes mitosis to form gametes—eggs and sperm. Once an egg is fertilized by a sperm and produces a zygote, the plant begins the diploid phase of its life cycle. The zygote divides by mitosis to form a sporophyte plant. The sporophyte (2n) produces cells that undergo meiosis to form haploid spores. These spores are released by most seedless plants but are retained by seed plants. The life cycle begins again when spores divide by mitosis to form new gametophytes.

In nonvascular plants, the gametophyte is the dominant phase. In contrast, the sporophyte is the dominant phase of vascular plants. Oak trees are large sporophytes that dominate some landscapes. In seedless vascular plants, the gametophyte is usually a separate small organism quite different from the sporophyte. In seed plants, the gametophyte is a very small parasite of the sporophyte. For example, gametophytes of flowering plants are microscopic parts of their flowers that are not photosynthetic.

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