Chapter 22
Bryophytes
THE LEAP ONTO LAND
Bryophytes Faced Many Problems When
They Moved onto the Land
Key Innovations in Land Plants First Appear
in the Bryophytes
BRYOPHYTES ARE NOT A NATURAL GROUP
Bryophyte Relationships Remain Uncertain
HORNWORTS
LIVERWORTS
Some Liverworts Have Thalloid Bodies
Most Liverworts Have Leafy Bodies
MOSSES
Gametophytes Have Protonemal, Bud, and
Leafy Phases
Mosses Have Several Forms of Asexual
Reproduction
Sexual Reproduction Typically Occurs at the
Ends of Stems
Moss Sporophytes Have Complex Capsules
Mosses Have Significant Economic and
Ecological Value
SUMMARY
PLANTS, PEOPLE, AND THE
ENVIRONMENT: The Ultimate Shade Plants
ECONOMIC BOTANY: Mining Moss: Peat
for Profit
1
, KEY CONCEPTS
1. Bryophytes are land plants that differ from all other plants lacking lignified
vascular tissue by having the gametophyte generation dominant and having
unbranched sporophytes that produce a single sporangium.
2. Modern bryophytes almost certainly evolved from a single common ancestor, and
they likely represent several lineages along the evolutionary path to vascular plants.
Recent fossil discoveries push back the earliest appearance of bryophytes to 475
million years ago. Existing bryophytes preserve a suite of ancestral characteristics
that give us insight into the origin of land plants. Bryophyte relationships remain
uncertain; however, existing bryophytes fall into three lineages: liverworts
(Marchantiophyta), hornworts (Anthocerotophyta), and mosses (Bryophyta).
Traditionally, liverworts have been considered the earliest evolving lineage of
bryophytes, but recent evidence suggests that hornworts may be the earliest.
Mosses are likely the closest sister group to vascular plants.
3. Key innovations of the bryophyte radiation, not present in their algal ancestors,
include multicellular gametangia (antheridia and archegonia) that protect and
insulate gametes from the environment; a multicellular sporophyte that develops
from an embryo embedded within and nutritionally dependent on the gametophyte;
and the presence of a waxy coating on the shoots (cuticle) and the spores
(sporopollenin). The most complex bryophyte sporophytes also contain novel
structures such as stomata and water- and sugar-conducting tissue (unlignified
vascular tissue). Bryophyte gametophytes are not able to control their water
balance, and they dry out rapidly in the absence of free water. The desiccated plants
are still alive and can become active within minutes of being rewetted.
4. Mosses are important in many ecosystems. They provide most of the biomass in
boreal vegetation such as tundra, they dominate the understory of cool-temperate
forests, and they are common in damp microenvironments. Some species are
aquatic and most require humid conditions, but some can colonize dry, exposed
habitats such as rock outcrops and desert soil surfaces.
22.1 THE LEAP ONTO LAND
Despite being common almost everywhere, bryophytes often are ignored because of
their small stature, lack of familiar features, and the fact that in many environments
they are dormant for much of the year. Yet bryophytes are exceptionally diverse,
with nearly 25,000 named species. Among land plants, only flowering plants and
ferns have more species. Bryophytes also are extremely widespread, being present
on all continents, including Antarctica. They have a long evolutionary history, and
some bryophytes are a sister group to vascular plants (Fig. 22.1).
2
Bryophytes
THE LEAP ONTO LAND
Bryophytes Faced Many Problems When
They Moved onto the Land
Key Innovations in Land Plants First Appear
in the Bryophytes
BRYOPHYTES ARE NOT A NATURAL GROUP
Bryophyte Relationships Remain Uncertain
HORNWORTS
LIVERWORTS
Some Liverworts Have Thalloid Bodies
Most Liverworts Have Leafy Bodies
MOSSES
Gametophytes Have Protonemal, Bud, and
Leafy Phases
Mosses Have Several Forms of Asexual
Reproduction
Sexual Reproduction Typically Occurs at the
Ends of Stems
Moss Sporophytes Have Complex Capsules
Mosses Have Significant Economic and
Ecological Value
SUMMARY
PLANTS, PEOPLE, AND THE
ENVIRONMENT: The Ultimate Shade Plants
ECONOMIC BOTANY: Mining Moss: Peat
for Profit
1
, KEY CONCEPTS
1. Bryophytes are land plants that differ from all other plants lacking lignified
vascular tissue by having the gametophyte generation dominant and having
unbranched sporophytes that produce a single sporangium.
2. Modern bryophytes almost certainly evolved from a single common ancestor, and
they likely represent several lineages along the evolutionary path to vascular plants.
Recent fossil discoveries push back the earliest appearance of bryophytes to 475
million years ago. Existing bryophytes preserve a suite of ancestral characteristics
that give us insight into the origin of land plants. Bryophyte relationships remain
uncertain; however, existing bryophytes fall into three lineages: liverworts
(Marchantiophyta), hornworts (Anthocerotophyta), and mosses (Bryophyta).
Traditionally, liverworts have been considered the earliest evolving lineage of
bryophytes, but recent evidence suggests that hornworts may be the earliest.
Mosses are likely the closest sister group to vascular plants.
3. Key innovations of the bryophyte radiation, not present in their algal ancestors,
include multicellular gametangia (antheridia and archegonia) that protect and
insulate gametes from the environment; a multicellular sporophyte that develops
from an embryo embedded within and nutritionally dependent on the gametophyte;
and the presence of a waxy coating on the shoots (cuticle) and the spores
(sporopollenin). The most complex bryophyte sporophytes also contain novel
structures such as stomata and water- and sugar-conducting tissue (unlignified
vascular tissue). Bryophyte gametophytes are not able to control their water
balance, and they dry out rapidly in the absence of free water. The desiccated plants
are still alive and can become active within minutes of being rewetted.
4. Mosses are important in many ecosystems. They provide most of the biomass in
boreal vegetation such as tundra, they dominate the understory of cool-temperate
forests, and they are common in damp microenvironments. Some species are
aquatic and most require humid conditions, but some can colonize dry, exposed
habitats such as rock outcrops and desert soil surfaces.
22.1 THE LEAP ONTO LAND
Despite being common almost everywhere, bryophytes often are ignored because of
their small stature, lack of familiar features, and the fact that in many environments
they are dormant for much of the year. Yet bryophytes are exceptionally diverse,
with nearly 25,000 named species. Among land plants, only flowering plants and
ferns have more species. Bryophytes also are extremely widespread, being present
on all continents, including Antarctica. They have a long evolutionary history, and
some bryophytes are a sister group to vascular plants (Fig. 22.1).
2
