More than 25,000 species of bryophytes thrive in mostly damp habitats, although
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 (“plant body”), stem, and the rhizoid that anchors the plant to its substrate—belong to the haploid organism or gametophyte. The male gametes formed by bryophytes swim with a flagellum, so fertilization is dependent on the presence of water. The bryophyte embryo also remains attached to the parent plant, which protects and nourishes it. The sporophyte that develops from the embryo is barely noticeable. The sporangium—the multicellular sexual reproductive structure in which meiosis produces haploid spores—is present in bryophytes and absent in the majority of algae. This is also a characteristic of land plants.
The bryophytes are divided into three phyla: the liverworts or Marchantiophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.
Liverworts
Liverworts (Marchantiophyta) are currently classified as the plants most closely related to the ancestor of vascular plants that adapted to terrestrial environments. In fact, liverworts have colonized every terrestrial habitat on Earth and diversified to more than 7000 existing species (Figure 25.9). Lobate liverworts form a flat thallus, with lobes that have a vague resemblance to the lobes of the liver (Figure 25.10), which accounts for the name given to the phylum. Leafy liverworts have tiny leaflike structures attached to a stalk. Several leafy liverworts are shown in Figure 25.9.
Openings in the thallus that allow the movement of gases may be observed in liverworts (Figure 25.10). However, these are not stomata, because they do not actively open and close by the action of guard cells. Instead, the thallus takes up water over its entire surface and has no cuticle to prevent desiccation, which explains their preferred wet habitats. Figure 25.11 represents the lifecycle of a lobate liverwort. Haploid spores germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Stalk-like structures (gametophores) grow from the thallus and carry male and female gametangia, which may develop on separate, individual plants, or on the same plant, depending on the species. Flagellated male gametes develop within antheridia (male gametangia). The female gametes develop within archegonia (female gametangia). Once released, the male gametes swim with the aid of their flagella to an archegonium, and fertilization ensue