Plant Life Cycle - Alternation of Generations

Creeping feather-moss
This image shows alternation of generations in creeping feather-moss. The sporophyte generation (spore capsules and stalks) extend upward from the gametophyte generation (green vegetation) below.

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Alternation of generations describes a plant's life cycle as it alternates between a sexual phase, or generation and an asexual phase. The sexual generation in plants produces gametes, or sex cells and is called the gametophyte generation. The asexual phase produces spores and is called the sporophyte generation. Each generation develops from the other continuing the cyclical process of development. Alternation of generations is also observed in other organisms. Fungi and protists, including algae, exhibit this type of life cycle.

Plant vs Animal Life Cycles

Tiger Butterfly
This image shows a common tiger butterfly on a flower.

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Plants and some animals are capable of reproducing both asexually and sexually. In asexual reproduction, the offspring are an exact duplicate of the parent. Types of asexual reproduction commonly seen in both plants and animals include parthenogenesis (offspring develops from an unfertilized egg), budding (offspring develops as a growth on the parent's body), and fragmentation (offspring develops from a part or fragment of the parent). Sexual reproduction involves the uniting of haploid cells (cells containing only one set of chromosomes) to form a diploid (containing two chromosome sets) organism.

In multicellular animals, the life cycle consists of a single generation. The diploid organism produces haploid sex cells by meiosis. All other cells of the body are diploid and produced by mitosis. A new diploid organism is created by the fusion of male and female sex cells during fertilization. The organism is diploid and there is no alternation of generations between haploid and diploid phases.

In plant multicellular organisms, life cycles vacillate between diploid and haploid generations. In the cycle, the diploid sporophyte phase produces haploid spores via meiosis. As haploid spores grow by mitosis, the multiplied cells form a haploid gametophyte structure. The gametophyte represents the haploid phase of the cycle. Once mature, the gametophyte produces male and female gametes. When haploid gametes unite, they form a diploid zygote. The zygote grows via mitosis to form a new diploid sporophyte. Thus unlike in animals, plant organisms can alternate between diploid sporophyte and haploid gametophyte phases.

Non-Vascular Plants

Hairy Cap adn Bog Moss
Two species of primitive non-vascular plants, hairy cap moss and bog moss, are seen in this image. The green vegetation is the plant gametophyte.

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Alternation of generations is seen in both vascular and non-vascular plants. Vascular plants contain a vascular tissue system that transports water and nutrients throughout the plant. Non-vascular plants do not have this type of system and require moist habitats for survival. Non-vascular plants include mosses, liverworts, and hornworts. These plants appear as green mats of vegetation with stalks protruding from them.

The primary phase of the plant life cycle for non-vascular plants is the gametophyte generation. The gametophyte phase consists of green mossy vegetation, while the sporophtye phase consists of elongated stalks with a sporangium tip that encloses the spores.

Seedless Vascular Plants

Fern Leaf With Spores
This is a close-up of the underside of a fern leaf showing sporangia (orange). Sporangia produce the haploid spores which germinate to form fern gametophytes.

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The primary phase of the plant life cycle for vascular plants is the sporophtye generation. In vascular plants that do not produce seeds, such as ferns and horsetails, the sporophtye and gametophyte generations are independent. In ferns for example, the leafy fronds represent the mature diploid sporophyte generation.

The sporangia on the undersides of the fronds produce the haploid spores, which germinate to form the haploid fern gametophytes (prothallia). These plants thrive in damp environments as water is required for the male sperm to swim toward and fertilize the female egg.

Seed-Bearing Vascular Plants

Apple Core With Seed
This apple seed is the result of pollination and fertilization within the plant ovary. The seed is the beginning of a new sporophyte generation.

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Vascular plants that produce seeds are not necessarily dependent upon moist environments to reproduce. The seeds protect the developing embryos. In both flowering plants and nonflowering plants (gymnosperms), the gametophyte generation is totally dependent upon the dominant sporophtye generation for survival.

In flowering plants, the reproductive structure is the flower. The flower produces both male microspores and female megaspores. The male microspores are contained within pollen and are produced in the plant stamen. They develop into the male gametes or sperm. The female megaspores are produced in the plant ovary. They develop into female gametes or eggs.

During pollination, pollen is transfered via wind, insects or other animals to the female part of a flower. Male and female gametes unite in the ovary and develop into a seed, while the ovary forms the fruit. In gymnosperms such as conifers, pollen is produced in male cones and eggs are produced in female cones.

Key Points

  • Alternation of generations is the life cycle of plants and some other organisms that alternates between a sexual phase and an asexual phase.
  • The sexual phase of this life cycle is called the gametophyte generation. Gametes are produced in this phase.
  • The asexual phase is called the sporophyte generation and spores are produced in this phase.
  • The primary phase of the plant life cycle for non-vascular plants is the gametophyte generation. Non-vascular plants include mosses, liverworts, and hornworts.
  • The primary phase of the plant life cycle for vascular plants is the sporophtye generation.
  • In seedless vascular plants, such as ferns and horsetails, the sporophtye and gametophyte generations are independent.
  • In seed producing vascular plants, such as angiosperms and gymnosperms, the gametophyte generation is totally dependent upon the sporophtye generation.

Sources

  • Britannica, The Editors of Encyclopaedia. "Alternation of Generations." Encyclopædia Britannica, Encyclopædia Britannica, Inc., 13 Oct. 2017, www.britannica.com/science/alternation-of-generations.
  • Gilbert, SF. "Plant Life Cycles." Developmental Biology, 6th ed., Sinauer Associates, 2000, www.ncbi.nlm.nih.gov/books/NBK9980/.