recap

27.2 recap

The transition of plants to land required numerous adaptations, including the cuticle, stomata, gametangia, protected embryos, pigments, thick spore walls, and mutually beneficial associations with fungi. Nonvascular land plants rely on liquid water for reproduction.

learning outcomes

You should be able to:

  • Describe one or more key innovations in land plants that were most important for their transition to a terrestrial environment, and justify your choice(s).

  • Distinguish alternation of generations as seen in plants from the life cycles of animals.

Question 1

Explain what is meant by “alternation of generations.” Why do we not use this phrase for humans?

Alternation of generations is a life cycle in which a multicellular diploid sporophyte generation produces spores by meiosis, alternating with a multicellular haploid gametophyte generation that produces gametes by mitosis. Humans (and other animals) do not have alternating sexual and asexual multicellular stages. They undergo meiosis to produce sex cells and use mitosis for growth and maintenance, not for reproduction.

Question 2

Describe key adaptations of plants to the terrestrial environment, and describe the distribution of those adaptations among the liverworts, mosses, and hornworts.

Terrestrial adaptations found in all major lineages of land plants (including liverworts, mosses, and hornworts) include the cuticle, which provides a coating of waxy lipids that retards water loss; gametangia (multicellular organs that enclose plant gametes), which prevent gametes from drying out; protected embryos; pigments that provide protection against ultraviolet radiation; thick-walled spores; and a mutually beneficial association with fungi that promotes nutrient uptake from the soil. Stomata, which are small closable openings in leaves and stems that are used to regulate gas exchange, are a terrestrial adaptation that is found in all land plants except liverworts.

New features appeared in plants as they continued to adapt to the terrestrial environment. One of the most important of these was vascular tissues, the characteristic that defines the vascular plants.