29.2: Bryophytes Have a Dominant Gametophyte Generation
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Learning Objectives
- Describe the characteristics of bryophytes
Bryophytes
Bryophytes are the group of seedles plants that are the closest-extant relative of early terrestrial plants. The first bryophytes (liverworts) probably appeared in the Ordovician period, about 450 million years ago. However, because they lack of lignin and other resistant structures, bryophyte fossil formation is improbable and the fossil record is poor. Some spores protected by sporopollenin have survived and are attributed to early bryophytes. By the Silurian period, however, vascular plants had spread through the continents. This compelling fact is used as evidence that non-vascular plants must have preceded the Silurian period.
More than 25,000 species of bryophytes thrive in mostly-damp habitats, although some live in deserts. They constitute the major flora of inhospitable environments like the tundra where their small size and tolerance to desiccation offer distinct advantages. They generally lack lignin and do not have actual tracheids (xylem cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells. Although the term non-tracheophyte is more accurate, bryophytes are commonly called non-vascular plants.
In a bryophyte, all the conspicuous vegetative organs, including the photosynthetic leaf-like structures, the thallus, stem, and the rhizoid that anchors the plant to its substrate, belong to the haploid organism, or gametophyte. The sporophyte is barely noticeable. Thus, the gametophyte is the dominant and most familiar form; the sporophyte appears for only a short period. The gametes formed by bryophytes swim with a flagellum. The sporangium, the multicellular sexual reproductive structure, is present in bryophytes and absent in the majority of algae. The sporophyte embryo also remains attached to the parent plant, which protects and nourishes it. This is a characteristic of land plants. The bryophytes are divided into three phyla: the liverworts (Hepaticophyta), the hornworts (Anthocerotophyta), and the mosses (true Bryophyta).
Key Points
- Bryophytes are the closest-living relative of early terrestrial plants; liverworts were the first Bryophytes, probably appearing during the Ordovician period.
- Bryophytes fossil formation is improbable since they do not possess lignin.
- Bryophytes thrive in mostly-damp habitats; however, some species can live in deserts while others can inhabit hostile environments such as the tundra.
- Bryophytes are nonvascular because they do not have tracheids; instead, water and nutrients circulate inside specialized conducting cells.
- In a bryophyte, all the vegetative organs belong to the gametophyte, which is the dominant and most familiar form; the sporophyte appears for only a short period.
- The sporophyte is dependent on the gametophyte and remains permanently attached to it in order to gain nutrition and protection.
Key Terms
- bryophyte : seedless, nonvascular plants that are the closest extant relative of early terrestrial plants
- tracheid : elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts
- sporangium : a case, capsule, or container in which spores are produced by an organism