25.3 Bryophytes
Learning Outcomes
- Identify the main characteristics of bryophytes
- Describe the distinguishing traits of liverworts, hornworts, and mosses
- Chart the development of land adaptations in the bryophytes
- Describe the events in the bryophyte lifecycle
Bryophytes are the closest extant relatives of early terrestrial plants. The first bryophytes most likely appeared about 450 million years ago. More than 25,000 species of bryophytes thrive in mostly damp habitats. 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 tracheids (xylem cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells. bryophytes are non-vascular plants.
In a bryophyte, all the conspicuous vegetative organs belong to the haploid gametophyte, which is the dominant phase of the bryophyte life cycle. Bryophytes structure includes photosynthetic leaf-like structure, called a thallus (pl. thalli), stem, and rhizoids that anchors the plant to its substrate. Although it is tempting to correlate these structures with the familiar plant structures of leaves, stems, and roots, it is important to remember than bryophytes are non-vascular. In bryophytes so these structures do not transport organic molecules or water. In bryophytes, the male gametes swim with a flagellum, so fertilization is dependent on the presence of water. The bryophyte embryo remains attached to the parent plant, which protects and nourishes it. The diploid sporophyte that develops from the embryo is barely noticeable. Bryophytes have sporangium, sexual reproductive structures, which produces haploid spores via meiosis. Sporanguim are also present in land plants but not in the majority of algae.
The bryophytes are divided into three phyla: the liverworts, the hornworts, and the mosses.
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.
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 ensues. The zygote grows into a small sporophyte still contained in the archegonium. The diploid zygote, the sporophyte, will produce haploid spores via meiosis. The haploid spores germinate into thalli, creating a new generation of liverwort. Liverwort plants can also reproduce asexually, by the breaking of “branches” or the spreading of leaf fragments called gemmae. In this latter type of reproduction, the gemmae—small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus (shown in Figure 25.11. and Figure 25.12)—are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes.
Hornworts
The defining characteristic of the hornworts (Anthocerotophyta) is the narrow, pipe-like sporophyte. Hornworts have colonized a variety of habitats on land, although they are never far from a source of moisture. The short, blue-green gametophyte is the dominant phase of the life cycle of a hornwort. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant (Figure 25.12).
The lifecycle of hornworts (Figure 25.13) follows the general pattern of alternation of generations. The gametophytes grow as flat thalli on the soil with embedded male and female gametangia. Flagellated sperm swim to the archegonia and fertilize eggs. The zygote develops into a long and slender sporophyte that eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the spores and help propel them farther in the environment. The haploid spores germinate and give rise to the next generation of gametophytes.
Mosses
The mosses are the most numerous of the non-vascular plants. More than 10,000 species of mosses have been catalogued. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox. Mosses are very sensitive to air pollution and are used to monitor air quality.
Mosses form diminutive gametophytes, which are the dominant phase of the lifecycle. Green, flat structures with a simple midrib—resembling true leaves, but lacking stomata and vascular tissue—are attached in a spiral to a central stalk. Water and nutrients are absorbed directly through the leaflike structures of the gametophyte. Some mosses have small branches. A primitive conductive system that carries water and nutrients runs up the gametophyte’s stalk, but does not extend into the leaves. Additionally, mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids, precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals. The mosses therefore occupy a threshold position between other bryophytes and the vascular plants.
The moss lifecycle follows the pattern of alternation of generations as shown in Figure 25.14. The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema—usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Male and female gametangia develop at the tip of separate gametophores. The antheridia (male organs) produce many sperm, whereas the archegonia (the female organs) each form a single egg at the base (venter) of a flask-shaped structure. The archegonium produces attractant substances and at fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.
The moss sporophyte is dependent on the gametophyte for nutrients. The slender seta (plural, setae), as seen in Figure 25.15, contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule.
Spore cells in the sporangium undergo meiosis to produce haploid spores. The sporophyte has several features that protect the developing spores and aid in their dispersal. The calyptra, derived from the walls of the archegonium, covers the sporangium. A structure called the operculum is at the tip of the spore capsule. The calyptra and operculum fall off when the spores are ready for dispersal. The peristome, tissue around the mouth of the capsule, is made of triangular, close-fitting units like little “teeth.” The peristome opens and closes, depending on moisture levels, and periodically releases spores.
Alternation of Generations
The lifecycle of bryophytes follows the general pattern of alternation of generations. The haploid gametophyte spores germinate into flattened thalli attached to the substrate by rhizoids. Stalk-like structures 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 (haploid sperm) develop within antheridia (male gametangia). The female gametes (haploid eggs) develop within archegonia (female gametangia). Once released, the male gametes use their flagellum to swim to an archegonium, where fertilization occurs. The zygote grows into a small diploid sporophyte, still contained in the archegonium. The sporophyte will produce haploid spores by meiosis. The spores are dispersed by wind or water.
