Learning objectives
By the end of this section, you will be able to do the following:
- 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 (liverworts) most likely appeared in the Ordovician period, about 450 million years ago. Because they lack lignin and other resistant structures, the likelihood of bryophytes forming fossils is rather small. Some spores protected by sporopollenin have survived and are attributed to early bryophytes. By the Silurian period (435 MYA), 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 (“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 (fig-ch25_03_01). Lobate liverworts form a flat thallus, with lobes that have a vague resemblance to the lobes of the liver (fig-ch25_03_02), 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 fig-ch25_03_01.
View original figure (external; image not reproduced in this text edition)
Original figure description: The illustration shows a variety of liverworts, which all share a branched, leafy structure.
View original figure (external; image not reproduced in this text edition)
Original figure description: Photo shows a liverwort with lettuce-like leaves. The gemma cup, described in the paragraph and subsequent life-cycle diagram, is highlighted.
Openings in the thallus that allow the movement of gases may be observed in liverworts (fig-ch25_03_02). 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. fig-ch25_03_03 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 will give rise, by meiosis, to the next generation of haploid spores, which can be disseminated by wind or water. In many liverworts, spore dispersal is facilitated by elaters—long single cells that suddenly change shape as they dry out and throw adjacent spores out of the spore capsule. 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 fig-ch25_03_03 and fig-ch25_03_04)—are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes.
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Original figure description: The liverwort has a flat, leaf-like structure called a thallus, which is haploid (1n). Root-like rhizoids grow from the bottom of the thallus. Gemma cups used for asexual reproduction grow on top of the thallus. A slender stalk extends from the thallus, and a haploid (1n) archegonial head sits at its top. The archegonial head is the female gametophyte. The archegonial head has fronds, like a palm tree. The underside of the archegonial head contains protrusions called archegonia, which house the eggs. Another stalk extends from another thallus, and a haploid (1n) antheridial head sits at its top. The antheridial head is the male gametophyte. The outside of the antheridial head has antheridia which release sperm. The haploid (1n) sperm swim through a hole in the bottom of the archegonium and fertilize the haploid (1n) egg to produce a diploid (2n) embryo. The embryo grows into a stalk called a seta. Meiosis produces haploid (1n) spores in a sac at the tip of the stalk. The sac bursts open, releasing the spores. The spores sprout, producing a new thallus and rhizoids.
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 (fig-ch25_03_04).
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Original figure description: The base of the hornwort plant, called the thallus, has a wrinkled, leaf-like appearance. The sporophytes, a cluster of slender green stalks with brown tips, grow from this wrinkled mass.
Stomata (air pores that can be opened and closed) appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus each contain a single chloroplast. Meristem cells at the base of the plant keep dividing and adding to the height of the sporophyte. This growth pattern is unique to the hornworts. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.
The lifecycle of hornworts (fig-ch25_03_05) 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.
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Original figure description: In hornworts, the gametophyte is a haploid 1 n leaf-like structure with slender stalks called rhizoids underneath. Male sex organs called antheridia produce sperm, and female sex organs called archegonia produce eggs. Both male and female sex organs form just beneath the surface of the gametophyte, and are exposed to the surface as the organs mature. The sperm swims to the egg or is propelled by water. When the egg is fertilized, the embryo grows into a hollow tube-like structure called a sporophyte. Meiosis inside the sporophyte produces haploid 1 n spores. The spores are ejected from the top of the tube. They grow into new gametophytes, completing the cycle.
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. They are also sensitive to copper salts, so these salts are a common ingredient of compounds marketed to eliminate mosses from lawns.
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. Mosses have stomata only on the sporophyte. 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 lack of a true root system explains why it is so easy to rip moss mats from a tree trunk. 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 fig-ch25_03_06. 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 fig-ch25_03_07, contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule.
View original figure (external; image not reproduced in this text edition)
Original figure description: In the photo, setae appear as long, slender, bent stems with oval-shaped capsules at the tips.
Spore mother 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.
Section Summary
Seedless non-vascular plants are small, having the gametophyte as the dominant stage of the lifecycle. Without a vascular system and roots, they absorb water and nutrients on all their exposed surfaces. Collectively known as bryophytes, the three main groups include the liverworts, the hornworts, and the mosses. Liverworts are the most primitive plants and are closely related to the first land plants. Hornworts developed stomata and possess a single chloroplast per cell. Mosses have simple conductive cells and are attached to the substrate by rhizoids. They colonize harsh habitats and can regain moisture after drying out. The moss sporangium is a complex structure that allows release of spores away from the parent plant.
Visual Connection Questions
fig-ch25_03_06 Which of the following statements about the moss life cycle is false?
- The mature gametophyte is haploid.
- The sporophyte produces haploid spores.
- The calyptra buds to form a mature gametophyte.
- The zygote is housed in the venter.
Review Questions
Which of the following structures is not found in bryophytes?
- a cellulose cell wall
- chloroplast
- sporangium
- root
D
Stomata appear in which group of plants?
- Charales
- liverworts
- hornworts
- mosses
C
The chromosome complement in a moss protonema is:
- 1n
- 2n
- 3n
- varies with the size of the protonema
A
Why do mosses grow well in the Arctic tundra?
- They grow better at cold temperatures.
- They do not require moisture.
- They do not have true roots and can grow on hard surfaces.
- There are no herbivores in the tundra.
C
A botanist travels to an area that has experienced a long, severe drought. While examining the bryophytes in the area, they notice that many are in the same life-cycle stage. Which life-cycle stage should be the most common?
- zygote
- gametophyte
- sporophyte
- archegonium
B
Critical Thinking Questions
In areas where it rains often, mosses grow on roofs. How do mosses survive on roofs without soil?
Mosses absorb water and nutrients carried by the rain and do not need soil because they do not derive much nutrition from the soil.
What are the three classes of bryophytes?
The bryophytes are divided into three phyla: the liverworts or Marchantiophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.
Describe two adaptations that are present in mosses, but not hornworts or liverworts, which reflect steps of evolution toward land plants.
Potential answers include:
- Mosses exhibit a primitive conductive system in their stalks that transports nutrients and water.
- Mosses exhibit a more complex, multicellular rhizoid system.
Bryophytes form a monophyletic group that transitions between green algae and vascular plants. Describe at least one similarity and one difference between bryophyte reproduction and green algae reproduction.
Similarities include:
- Sexual reproduction is dependent upon water in which the male gamete swims.
- The haploid organism is the dominant part of the life cycle.
Differences include:
- Bryophyte gametotangia protect the gametes and the growing embryo.
- Bryophytes make sporangium to produce spores.
Glossary
- capsule
- case of the sporangium in mosses
- gemma
- (plural, gemmae) leaf fragment that spreads for asexual reproduction
- hornworts
- group of non-vascular plants in which stomata appear
- liverworts
- most primitive group of the non-vascular plants
- mosses
- group of bryophytes in which a primitive conductive system appears
- peristome
- tissue that surrounds the opening of the capsule and allows periodic release of spores
- protonema
- tangle of single-celled filaments that forms from the haploid spore
- rhizoids
- thin filaments that anchor the plant to the substrate
- seta
- stalk that supports the capsule in mosses