r
62
CHAPTER 3
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
UN[T 11
EVOLUTION AND DIVERSITY OF PLANTS
gemmae propagules
2 rows of
dorsal leaves
sperm cells
(sterile “jacket” layer)
1 row of
ventral leaves
neck
sperm cells
“
gemmae cup
‘fl
I
/
pore
dorsal (upper)
view
B
A
leafy liverwort
thalloid liverwort
FIGURE 3.10
A. Antheridia. B. Archegonia. Both are apomorphies of land plants.
attractant, acting as a homing device for the swimming sperm.
Sperm cells enter the neck of the archegonium and fertilize
the egg cell to form a diploid (2n) zygote. In addition to
effecting fertilization, the archegonium serves as a site for
embryo/sporophyte development and the establishment of a
nutritional dependence of the sporophyte upon gametophytic
tissue.
The land plants share other possible apomorphies: the
presence of various ultrastructural modifications of the sperm
cells, fiavonoid chemical compounds, and a proliferation of
heat shock proteins. These are not discussed here.
DIVERSITY OF NONVASCULAR LAND PLANTS
During the early evolution of land plants, three major,
monophyletic lineages diverged before the vascular plants
(discussed in Chapter 4). These lineages may collectively be
called the nonvascular land plants or “bryophytes” and
include the liverworts, mosses, and hornworts. “Bryophytes”
are a paraphyletic group, defined by the absence of derived
features; the name, placed in quotation marks, is no longer
formally recognized.
Liverworts, mosses, and homworts differ from the vascular
plants in lacking true vascular tissue and in having the game—
tophyte as the dominant, photosynthetic, persistent, and freeliving phase of the life cycle. It is likely that the ancestral
gametophyte of the land plants was thalloid in nature, similar
to that of the hornworts and many liverworts. The sporophyte
4
(,,
of the liverworts, mosses, and hornworts is relatively small,
ephemeral, and attached to and nutritionally dependent upon
the gametophyte (see later discussion).
The relationships of the liverworts, mosses, and hornworts
to one another and to the vascular plants remain unclear.
Many different relationships among the three lineages have
been proposed, one recent of which is seen in Figure 3.6.
LIVERWORTS
Liverworts, also traditionally called the Hepaticae, are one of
the monophyletic groups that are descendents of some of the
first land plants. Today, liverworts are relatively minor com
ponents of the land plant flora, growing mostly in moist,
shaded areas (although some are adapted to periodically dry,
hot habitats). Among the apomorphies of liverworts are
(1) distinctive oil bodies and (2) specialized structures called
elaters, elongate, nonsporogenous cells with spiral wall thick
enings, found inside the sporangium. Elaters are hygroscopic,
meaning that they change shape and move in response to
changes in moisture content. Elaters function in spore
dispersal; as the sporangium dries out, the elaters twist out of
the capsule, carrying spores with them (Figures 3.11, 3.12K).
There are two basic morphological types of liverwort
gametophytes: thalloid and leafy (Figures 3.11—3.13).
Thalloid liverworts consist of a thallus, a flattened mass of
tissue; this is likely the ancestral form, based on cladistic
studies. As in hornworts and mosses, the gametophyte bears
rhizoids, uniseriate, filamentous processes that function in
anchorage and absorption. Pores in the upper surface of the
ventral (lower)
view
••1•
archegonium
(n)
archegoniophore (n)
(longitudinal-section)
archegoniophore (n)
(longitudinal-section)
fertilization
capsule
I
Ri
I)
sporophyte
(2n)
elater
antheridiophore (n)
(longitudinal section)
antheridium
(0)
spore
(n)
germinating spore
F IGURE 3.11
Liverwort morphology and life cycle.
63
r
62
CHAPTER 3
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
UN[T 11
EVOLUTION AND DIVERSITY OF PLANTS
gemmae propagules
2 rows of
dorsal leaves
sperm cells
(sterile “jacket” layer)
1 row of
ventral leaves
neck
sperm cells
“
gemmae cup
‘fl
I
/
pore
dorsal (upper)
view
B
A
leafy liverwort
thalloid liverwort
FIGURE 3.10
A. Antheridia. B. Archegonia. Both are apomorphies of land plants.
attractant, acting as a homing device for the swimming sperm.
Sperm cells enter the neck of the archegonium and fertilize
the egg cell to form a diploid (2n) zygote. In addition to
effecting fertilization, the archegonium serves as a site for
embryo/sporophyte development and the establishment of a
nutritional dependence of the sporophyte upon gametophytic
tissue.
The land plants share other possible apomorphies: the
presence of various ultrastructural modifications of the sperm
cells, fiavonoid chemical compounds, and a proliferation of
heat shock proteins. These are not discussed here.
DIVERSITY OF NONVASCULAR LAND PLANTS
During the early evolution of land plants, three major,
monophyletic lineages diverged before the vascular plants
(discussed in Chapter 4). These lineages may collectively be
called the nonvascular land plants or “bryophytes” and
include the liverworts, mosses, and hornworts. “Bryophytes”
are a paraphyletic group, defined by the absence of derived
features; the name, placed in quotation marks, is no longer
formally recognized.
Liverworts, mosses, and homworts differ from the vascular
plants in lacking true vascular tissue and in having the game—
tophyte as the dominant, photosynthetic, persistent, and freeliving phase of the life cycle. It is likely that the ancestral
gametophyte of the land plants was thalloid in nature, similar
to that of the hornworts and many liverworts. The sporophyte
4
(,,
of the liverworts, mosses, and hornworts is relatively small,
ephemeral, and attached to and nutritionally dependent upon
the gametophyte (see later discussion).
The relationships of the liverworts, mosses, and hornworts
to one another and to the vascular plants remain unclear.
Many different relationships among the three lineages have
been proposed, one recent of which is seen in Figure 3.6.
LIVERWORTS
Liverworts, also traditionally called the Hepaticae, are one of
the monophyletic groups that are descendents of some of the
first land plants. Today, liverworts are relatively minor com
ponents of the land plant flora, growing mostly in moist,
shaded areas (although some are adapted to periodically dry,
hot habitats). Among the apomorphies of liverworts are
(1) distinctive oil bodies and (2) specialized structures called
elaters, elongate, nonsporogenous cells with spiral wall thick
enings, found inside the sporangium. Elaters are hygroscopic,
meaning that they change shape and move in response to
changes in moisture content. Elaters function in spore
dispersal; as the sporangium dries out, the elaters twist out of
the capsule, carrying spores with them (Figures 3.11, 3.12K).
There are two basic morphological types of liverwort
gametophytes: thalloid and leafy (Figures 3.11—3.13).
Thalloid liverworts consist of a thallus, a flattened mass of
tissue; this is likely the ancestral form, based on cladistic
studies. As in hornworts and mosses, the gametophyte bears
rhizoids, uniseriate, filamentous processes that function in
anchorage and absorption. Pores in the upper surface of the
ventral (lower)
view
••1•
archegonium
(n)
archegoniophore (n)
(longitudinal-section)
archegoniophore (n)
(longitudinal-section)
fertilization
capsule
I
Ri
I)
sporophyte
(2n)
elater
antheridiophore (n)
(longitudinal section)
antheridium
(0)
spore
(n)
germinating spore
F IGURE 3.11
Liverwort morphology and life cycle.
63
64
CHAPTER 3
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
UNIT II
EVOLUTION AND DIVERSITY OF PLANTS
65
Hepaticae Leafy liverworts. A. Bazania trilobata, a leafy liverwort. B. Porella, a leafy liverwort, showing third row of
reduced leaves at arrows (lower side facing).
FIGURE 3.13
FIGURE 3.12 Hepaticae Liverworts. A. Conocephaluin sp., a thalloid liverwort. B. Marchantia, thallus with gemma cups and gemmae.
Note whitish pores. C. Asterella, a thalloid liverwort with archegoniophores. D—L. Marchantia. D. Thallus with antheridiophores and arche
goniophores. E. Antheridiophore, close-up. F. Archegoniophore, showing capsules beneath lobes. G. Antheridiophore, longitudinal-section.
H. Archegoniophore, longitudinal-section. 1. Archegonium. J. Capsule, longitudinal-section, showing sporogenous tissue. K. Close-up,
sporogenous tissue, showing spores and elaters. L. Cross-section of thallus, showing rhizoids and upper pores.
—
I
—
thallus of some species function in gas exchange (Figure
3.12B,L). These pores are not true stomata (discussed later),
as they have no regulating guard cells. Some liverworts, like
the hornworts (discussed later), have a symbiotic relationship
with cyanobacteria. On the upper surface of the gametophytes
of some thalloid liverworts, such as Marchantia, are special
ized structures called gemma cups, which contain propagules
called gemmae (Figure 3.11, 3.12B). These structures func
tion in vegetative (asexual) reproduction; when a droplet of
water falls into the gemma cup, the gemmae themselves may
be dispersed some distance away, growing into a haploid
genetic clone of the parent.
Leafy liverworts have gametophytes consisting of a stem
axis bearing three rows of thin leaves. In most leafy liver
worts, the stem is prostrate and the leaves are modified such
that the upper two rows of leaves are larger and the lower
most row (on the stem underside) are reduced (Figures 3.11,
3.13). Other leafy liverworts are more erect, with the three
rows of leaves similar. The leaves of leafy liverworts evolved
independently from those of mosses (discussed later) or
vascular plants (Chapter 4).
As in all of the early diverging land plant lineages, liver
worts have antheridia and archegonia that develop on the
gametophyte. In some liverwort taxa (e.g., Marchantia), the
gametangia form as part of stalked, peltate structures: anthe
ridiophores bearing antheridia and archegoniophores bear
ing archegonia (Figures 3.11, 3.12). Sperm released from an
antheridium of the antheridiophore swims in a film of water to
the archegonia of the archegoniophore, effecting fertilization.
After fertilization the zygote divides mitotically and even
tually differentiates into a diploid (2n) embryo, which matures
into the diploid (2n) sporophyte. This sporophyte is relatively
small, nonphotosynthetic, and short lived. It consists almost
entirely of a sporangium or capsule (Figure 3.11, 3. 12F,J).
At a certain stage, the internal cells of the capsule divide mei
otically, forming haploid (n) spores (see Figure 3.7). In liver
worts the spores are released by a splitting of the capsule into
four valves. The spores may land on a substrate, germinate
(under the right conditions), and grow into a new gameto
phyte, completing the life cycle.
MOSS
The mosses, or Musci, are by far the most speciose and
diverse of the three major groups of nonvascular land plants
and inhabit a number of ecological niches. Mosses may share
one apomorphy with the hornworts (discussed later) and vas
cular plants: possession of stomates (Figure 3.6). Stomates
(also termed stomata) are specialized epidermal cells gener
ally found on leaves, but sometimes on stems. Stomata con
sist of two chloroplast-containing cells, the guard cells,
which, by changes in turgor pressure, can increase or decrease
the size of the opening between them, the stoma (Figure
3.14). Each guard cell has one or more ridge-like deposits on
the side facing the stoma (Figure 3.14). This material, which is
rich in suberin, a waxy, water-resistant substance, functions to
better seal the stoma. Stomata function in regulation of gas
exchange, in terms of both photosynthesis and water uptake.
Carbon dioxide passing through the stoma diffuses to the
chloroplasts of photosynthetic cells within and is used in the
dark reactions of photosynthesis. Oxygen, a by-product of
photosynthesis, exits via the stoma. Stomata also allow water
vapor to escape from the leaf. In most plants stomata open
during the day when photosynthesis takes place; thus, heat
from the sun may cause considerable water loss through sto
mata. In some plants, loss of water via stomata is simply a
by-product, a price to be paid for entry of carbon dioxide,
64
CHAPTER 3
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
UNIT II
EVOLUTION AND DIVERSITY OF PLANTS
65
Hepaticae Leafy liverworts. A. Bazania trilobata, a leafy liverwort. B. Porella, a leafy liverwort, showing third row of
reduced leaves at arrows (lower side facing).
FIGURE 3.13
FIGURE 3.12 Hepaticae Liverworts. A. Conocephaluin sp., a thalloid liverwort. B. Marchantia, thallus with gemma cups and gemmae.
Note whitish pores. C. Asterella, a thalloid liverwort with archegoniophores. D—L. Marchantia. D. Thallus with antheridiophores and arche
goniophores. E. Antheridiophore, close-up. F. Archegoniophore, showing capsules beneath lobes. G. Antheridiophore, longitudinal-section.
H. Archegoniophore, longitudinal-section. 1. Archegonium. J. Capsule, longitudinal-section, showing sporogenous tissue. K. Close-up,
sporogenous tissue, showing spores and elaters. L. Cross-section of thallus, showing rhizoids and upper pores.
—
I
—
thallus of some species function in gas exchange (Figure
3.12B,L). These pores are not true stomata (discussed later),
as they have no regulating guard cells. Some liverworts, like
the hornworts (discussed later), have a symbiotic relationship
with cyanobacteria. On the upper surface of the gametophytes
of some thalloid liverworts, such as Marchantia, are special
ized structures called gemma cups, which contain propagules
called gemmae (Figure 3.11, 3.12B). These structures func
tion in vegetative (asexual) reproduction; when a droplet of
water falls into the gemma cup, the gemmae themselves may
be dispersed some distance away, growing into a haploid
genetic clone of the parent.
Leafy liverworts have gametophytes consisting of a stem
axis bearing three rows of thin leaves. In most leafy liver
worts, the stem is prostrate and the leaves are modified such
that the upper two rows of leaves are larger and the lower
most row (on the stem underside) are reduced (Figures 3.11,
3.13). Other leafy liverworts are more erect, with the three
rows of leaves similar. The leaves of leafy liverworts evolved
independently from those of mosses (discussed later) or
vascular plants (Chapter 4).
As in all of the early diverging land plant lineages, liver
worts have antheridia and archegonia that develop on the
gametophyte. In some liverwort taxa (e.g., Marchantia), the
gametangia form as part of stalked, peltate structures: anthe
ridiophores bearing antheridia and archegoniophores bear
ing archegonia (Figures 3.11, 3.12). Sperm released from an
antheridium of the antheridiophore swims in a film of water to
the archegonia of the archegoniophore, effecting fertilization.
After fertilization the zygote divides mitotically and even
tually differentiates into a diploid (2n) embryo, which matures
into the diploid (2n) sporophyte. This sporophyte is relatively
small, nonphotosynthetic, and short lived. It consists almost
entirely of a sporangium or capsule (Figure 3.11, 3. 12F,J).
At a certain stage, the internal cells of the capsule divide mei
otically, forming haploid (n) spores (see Figure 3.7). In liver
worts the spores are released by a splitting of the capsule into
four valves. The spores may land on a substrate, germinate
(under the right conditions), and grow into a new gameto
phyte, completing the life cycle.
MOSS
The mosses, or Musci, are by far the most speciose and
diverse of the three major groups of nonvascular land plants
and inhabit a number of ecological niches. Mosses may share
one apomorphy with the hornworts (discussed later) and vas
cular plants: possession of stomates (Figure 3.6). Stomates
(also termed stomata) are specialized epidermal cells gener
ally found on leaves, but sometimes on stems. Stomata con
sist of two chloroplast-containing cells, the guard cells,
which, by changes in turgor pressure, can increase or decrease
the size of the opening between them, the stoma (Figure
3.14). Each guard cell has one or more ridge-like deposits on
the side facing the stoma (Figure 3.14). This material, which is
rich in suberin, a waxy, water-resistant substance, functions to
better seal the stoma. Stomata function in regulation of gas
exchange, in terms of both photosynthesis and water uptake.
Carbon dioxide passing through the stoma diffuses to the
chloroplasts of photosynthetic cells within and is used in the
dark reactions of photosynthesis. Oxygen, a by-product of
photosynthesis, exits via the stoma. Stomata also allow water
vapor to escape from the leaf. In most plants stomata open
during the day when photosynthesis takes place; thus, heat
from the sun may cause considerable water loss through sto
mata. In some plants, loss of water via stomata is simply a
by-product, a price to be paid for entry of carbon dioxide,
F
66
CHAPTER 3
UNIT II
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
guard cells
of stomate
EVOLUTION AND DIVERSITY OF PLANTS
chloroplast
peristome teeth
operculum
/
0
calyptra
/
/
0
closed
open
operculum
guard cells
of stomate
mature capsule
(sporangium)
FIGURE 3.14 The stomate, an innovation for mosses, hornworts, and vascular plants. A. Face view, slightly open. B. Diagram, face view,
open and closed. C. Diagram, cross-section.
which is essential for photosynthesis. However, in other plants,
such as tall trees, stomatal water loss may actually be adap
tive and functional, as a large quantity of water must flow
through the leaves in order to supply sufficient quantities of
mineral nutrients absorbed via the roots.
A second apomorphy, possibly shared among mosses,
hornworts, and vascular plants, is an elongate, aerial sporo
phyte axis (Figure 3.6). The elongate, aerial sporophyte seen
in mosses and hornworts may be a possible precursor to the
evolution of the dominant, aerial sporophytic stem in vascu
lar plants (see Chapter 4).
Mosses have a number of autapomorphies. First, some
mosses have specialized conductive cells called hydroids,
which function in water conduction, and leptoids, which
function in sugar conduction. These cells resemble typical
xylem tracheary elements and phloem sieve elements (Chapter
4), but lack the specializations of the latter cell types. They
likely evolved independently of vascular tissue (Figure 3.6),
although alternative hypotheses of “bryophyte” relationships
argue that hydroids and leptoids may represent intermediate
structures in the evolution of true vascular tissue. Second, the
spores of mosses have a thick outer layer called a perine
layer (Figure 3.15), which maybe apomorphic for the mosses
alone (Figure 3.6) or possibly for the mosses and vascular
plants combined. The perine layer may function in prevent
ing excess desiccation and provide additional mechanical
protection of the spore cytoplasm. As with liverworts and
hornworts, a three-lined structure, called a trilete mark,
develops on the spore wall; the trilete mark is the scar of
attachment of the adjacent three spores of the four spores pro
duced at meiosis (Figure 3.15; see also Chapter 4). Thus,
moss gametophytes are always leafy, with a variable number
of leaf ranks or rows (Figures 3.16, 3.17B,C). The leaves of
mosses are thought to have evolved independently from those
in liverworts and, thus, constitute an apomorphy for the
mosses alone. Moss leaves are mostly quite small and thin,
but may have a central costa (Figure 3.17C), composed of
conductive cells, which resembles a true vein.
Antheridia and archegonia in mosses are usually produced
at the apex of gametophytic stems (Figures 3.16, 3.1 7D—F).
After fertilization, the sporophyte grows upward (Figures
3.16, 3.17G) and often carries the apical portion of the origi
nal archegonium, which continues to grow. This apical arche
gonial tissue, known as a calyptra (Figures 3.16, 3.17H),
may function in protecting the young sporophyte apex. The
sporophyte generally develops a long stalk, known as a stipe,
at the apex of which is born the sporangium or capsule (Figures
3.16, 3.17G,H). The capsule of most mosses has a specialized
mechanism of dehiscence. At the time of spore release, a lid
known as an operculum falls off the capsule apex, revealing
a whorl of peristome teeth. The peristome teeth, like the
elaters of liverworts, are hygroscopic. As the capsule dries
up, the peristome teeth retract, effecting release of the spores
(Figures 3.16, 3.17H,I).
Under the right environmental conditions, moss spores
will germinate and begin to grow into a new gametophyte.
The initial development of the gametophyte results in the
formation of filamentous structure, known as a protonema
(Figures 3.16, 3. 17A). The protonema probably represents an
ancestral vestige, resembling a filamentous green “alga.”
After a period of growth, the protonema grows into a paren
chymatous gametophyte.
II
germinating
spore
protonema
.1/
antheridia
sporophyte
(2n)
stipe
costa
archegonial
neck
archegonia
penile layer
trilete mark
1/
gametophyte
(n)
FIGURE 3.15
trilete mark.
Moss spore. Note protective perine layer and
FIGURE 3.16
Moss morphology and life cycle.
67
F
66
CHAPTER 3
UNIT II
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
guard cells
of stomate
EVOLUTION AND DIVERSITY OF PLANTS
chloroplast
peristome teeth
operculum
/
0
calyptra
/
/
0
closed
open
operculum
guard cells
of stomate
mature capsule
(sporangium)
FIGURE 3.14 The stomate, an innovation for mosses, hornworts, and vascular plants. A. Face view, slightly open. B. Diagram, face view,
open and closed. C. Diagram, cross-section.
which is essential for photosynthesis. However, in other plants,
such as tall trees, stomatal water loss may actually be adap
tive and functional, as a large quantity of water must flow
through the leaves in order to supply sufficient quantities of
mineral nutrients absorbed via the roots.
A second apomorphy, possibly shared among mosses,
hornworts, and vascular plants, is an elongate, aerial sporo
phyte axis (Figure 3.6). The elongate, aerial sporophyte seen
in mosses and hornworts may be a possible precursor to the
evolution of the dominant, aerial sporophytic stem in vascu
lar plants (see Chapter 4).
Mosses have a number of autapomorphies. First, some
mosses have specialized conductive cells called hydroids,
which function in water conduction, and leptoids, which
function in sugar conduction. These cells resemble typical
xylem tracheary elements and phloem sieve elements (Chapter
4), but lack the specializations of the latter cell types. They
likely evolved independently of vascular tissue (Figure 3.6),
although alternative hypotheses of “bryophyte” relationships
argue that hydroids and leptoids may represent intermediate
structures in the evolution of true vascular tissue. Second, the
spores of mosses have a thick outer layer called a perine
layer (Figure 3.15), which maybe apomorphic for the mosses
alone (Figure 3.6) or possibly for the mosses and vascular
plants combined. The perine layer may function in prevent
ing excess desiccation and provide additional mechanical
protection of the spore cytoplasm. As with liverworts and
hornworts, a three-lined structure, called a trilete mark,
develops on the spore wall; the trilete mark is the scar of
attachment of the adjacent three spores of the four spores pro
duced at meiosis (Figure 3.15; see also Chapter 4). Thus,
moss gametophytes are always leafy, with a variable number
of leaf ranks or rows (Figures 3.16, 3.17B,C). The leaves of
mosses are thought to have evolved independently from those
in liverworts and, thus, constitute an apomorphy for the
mosses alone. Moss leaves are mostly quite small and thin,
but may have a central costa (Figure 3.17C), composed of
conductive cells, which resembles a true vein.
Antheridia and archegonia in mosses are usually produced
at the apex of gametophytic stems (Figures 3.16, 3.1 7D—F).
After fertilization, the sporophyte grows upward (Figures
3.16, 3.17G) and often carries the apical portion of the origi
nal archegonium, which continues to grow. This apical arche
gonial tissue, known as a calyptra (Figures 3.16, 3.17H),
may function in protecting the young sporophyte apex. The
sporophyte generally develops a long stalk, known as a stipe,
at the apex of which is born the sporangium or capsule (Figures
3.16, 3.17G,H). The capsule of most mosses has a specialized
mechanism of dehiscence. At the time of spore release, a lid
known as an operculum falls off the capsule apex, revealing
a whorl of peristome teeth. The peristome teeth, like the
elaters of liverworts, are hygroscopic. As the capsule dries
up, the peristome teeth retract, effecting release of the spores
(Figures 3.16, 3.17H,I).
Under the right environmental conditions, moss spores
will germinate and begin to grow into a new gametophyte.
The initial development of the gametophyte results in the
formation of filamentous structure, known as a protonema
(Figures 3.16, 3. 17A). The protonema probably represents an
ancestral vestige, resembling a filamentous green “alga.”
After a period of growth, the protonema grows into a paren
chymatous gametophyte.
II
germinating
spore
protonema
.1/
antheridia
sporophyte
(2n)
stipe
costa
archegonial
neck
archegonia
penile layer
trilete mark
1/
gametophyte
(n)
FIGURE 3.15
trilete mark.
Moss spore. Note protective perine layer and
FIGURE 3.16
Moss morphology and life cycle.
67
r
68
CHAPTER 3
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
UNIT II
EVOLUTION AND DIVERSITY OF PLANTS
69
I
FIGURE 3.18 Sphagnum, or peat moss. A. Clonal population. B. Individual leaf at center, showing the specialized chlorophyllous and
hyaline cells. C. Leaf close-up, showing chiorophyllous cells, hyaline cells, pores, and spiral wall thickenings of hyaline cells.
I
One economically important moss worth mentioning is the
genus Sphagnum, or peat moss, containing numerous species.
Sphagnum grows in wet bogs and chemically modifies its
environment by making the surrounding water acidic. The
leaves of Sphagnum are unusual in having two cell types:
chiorophyllous cells, which form a network, and large,
clear hyaline cells, having characteristic pores and helical
thickenings (Figure 3.18). The pores of the hyaline cells give
Sphagnum remarkable properties of water absorption and
retention, making it quite valuable horticulturally in potting
mixtures. Peat is fossilized and partially decomposed
Sphagnum and is mined for use in potting mixtures and as an
important fuel source in parts of the world.
HORNWORTS
The homworts, formally known as Anthocerotae, are a mono
phyletic group comprising a third extant lineage of nonvascular
land plants. Hornworts are similar to the thalloid liverworts in
gametophyte morphology and are found in similar habitats.
Homworts differ from liverworts, however, in lacking pores,
--I
I‘
FIGURE 3.17 Mosses. A. Protonema of Sphagnum. B. Atrichum sp. garnetophyte. C. Mnium leaf, showing median costa. D. Polytrichum
sp. gametophyte, face view, showing antheridia at tips of branches. E,F. Mnium sp. E. Antheridia, longitudinal-section, showing external wall
(sterile layer) and internal sporogenous tissue. F. Archegonia, showing stalk, egg cell, neck, and neck canal cells. C. Sporophytes of moss, show
ing capsules. H. Moss sporophyte close-up, showing developmental series (left to right). I. Mnium, capsule (sporangium) longitudinal-section,
showing operculum, one of several peristome teeth, and spores within sporangium.
with some species having stomates, a presumed apomorphy of
all land plants except liverworts (Figure 3.6). All homworts have
a symbiotic relationship with cyanobacteria (blue-greens), which
llve inside cavities of the thallus. This relationship is found in a
few thafloid liverworts as well (probably evolving indepen
dently), but not in mosses. Interestingly, hornworts and liver
worts may also have a symbiotic association between the
gametophytes and a fungus, similar to the mycorrhizal associa
tion with the roots of vascular plants.
The basic life cycle of hornworts is similar to that of
liverworts and mosses. The sporophyte of hornworts is
similar to that of mosses in being aerial and elongate, but
unique in being cylindrical, and photosynthetic (Figure
3.19A,B). This cylindrical sporophyte has indeterminate
(potentially continuous) growth, via a basal, intercalary
meristem (Figure 3.19E). The intercalary meristern is a
region of actively dividing cells near the base of the
sporophyte (just above the point of attachment to the game
tophyte), constituting an apomorphy for the hornworts.
This region is surrounded by a protective collar of
gametophytic tissue (Figure 3.19C). The proximal end of
the sporophyte, known as the foot, is embedded within
gametophytic tissue, its surface somewhat lobed (Figure
3.19D,E). Other apomorphies include a unique central
column of sterile (nonspore-producing) tissue called a col
amelia and the production of specialized structures in the
sporangium called pseudo-elaters (Figure 3.19F), groups
of cohering, nonsporogenous, elongate, generally hygro
scopic cells, which are nonhomologous with but have a
similar function to the elaters of liverworts.
Some recent molecular analyses place the hornworts as
sister to the vascular plants (Figure 3.6). One possible apo
morphy shared between them is the sporophyte. The sporo
phyte of hornworts is photosynthetic and relatively long-lived.
In fact, the sporophyte of some hornworts is capable of per
sisting independent of the gametophyte for long periods. In
addition, the foot of hornworts is somewhat lobed and the
surface compared to incipient rhizoids. Thus, hornwort
r
68
CHAPTER 3
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
UNIT II
EVOLUTION AND DIVERSITY OF PLANTS
69
I
FIGURE 3.18 Sphagnum, or peat moss. A. Clonal population. B. Individual leaf at center, showing the specialized chlorophyllous and
hyaline cells. C. Leaf close-up, showing chiorophyllous cells, hyaline cells, pores, and spiral wall thickenings of hyaline cells.
I
One economically important moss worth mentioning is the
genus Sphagnum, or peat moss, containing numerous species.
Sphagnum grows in wet bogs and chemically modifies its
environment by making the surrounding water acidic. The
leaves of Sphagnum are unusual in having two cell types:
chiorophyllous cells, which form a network, and large,
clear hyaline cells, having characteristic pores and helical
thickenings (Figure 3.18). The pores of the hyaline cells give
Sphagnum remarkable properties of water absorption and
retention, making it quite valuable horticulturally in potting
mixtures. Peat is fossilized and partially decomposed
Sphagnum and is mined for use in potting mixtures and as an
important fuel source in parts of the world.
HORNWORTS
The homworts, formally known as Anthocerotae, are a mono
phyletic group comprising a third extant lineage of nonvascular
land plants. Hornworts are similar to the thalloid liverworts in
gametophyte morphology and are found in similar habitats.
Homworts differ from liverworts, however, in lacking pores,
--I
I‘
FIGURE 3.17 Mosses. A. Protonema of Sphagnum. B. Atrichum sp. garnetophyte. C. Mnium leaf, showing median costa. D. Polytrichum
sp. gametophyte, face view, showing antheridia at tips of branches. E,F. Mnium sp. E. Antheridia, longitudinal-section, showing external wall
(sterile layer) and internal sporogenous tissue. F. Archegonia, showing stalk, egg cell, neck, and neck canal cells. C. Sporophytes of moss, show
ing capsules. H. Moss sporophyte close-up, showing developmental series (left to right). I. Mnium, capsule (sporangium) longitudinal-section,
showing operculum, one of several peristome teeth, and spores within sporangium.
with some species having stomates, a presumed apomorphy of
all land plants except liverworts (Figure 3.6). All homworts have
a symbiotic relationship with cyanobacteria (blue-greens), which
llve inside cavities of the thallus. This relationship is found in a
few thafloid liverworts as well (probably evolving indepen
dently), but not in mosses. Interestingly, hornworts and liver
worts may also have a symbiotic association between the
gametophytes and a fungus, similar to the mycorrhizal associa
tion with the roots of vascular plants.
The basic life cycle of hornworts is similar to that of
liverworts and mosses. The sporophyte of hornworts is
similar to that of mosses in being aerial and elongate, but
unique in being cylindrical, and photosynthetic (Figure
3.19A,B). This cylindrical sporophyte has indeterminate
(potentially continuous) growth, via a basal, intercalary
meristem (Figure 3.19E). The intercalary meristern is a
region of actively dividing cells near the base of the
sporophyte (just above the point of attachment to the game
tophyte), constituting an apomorphy for the hornworts.
This region is surrounded by a protective collar of
gametophytic tissue (Figure 3.19C). The proximal end of
the sporophyte, known as the foot, is embedded within
gametophytic tissue, its surface somewhat lobed (Figure
3.19D,E). Other apomorphies include a unique central
column of sterile (nonspore-producing) tissue called a col
amelia and the production of specialized structures in the
sporangium called pseudo-elaters (Figure 3.19F), groups
of cohering, nonsporogenous, elongate, generally hygro
scopic cells, which are nonhomologous with but have a
similar function to the elaters of liverworts.
Some recent molecular analyses place the hornworts as
sister to the vascular plants (Figure 3.6). One possible apo
morphy shared between them is the sporophyte. The sporo
phyte of hornworts is photosynthetic and relatively long-lived.
In fact, the sporophyte of some hornworts is capable of per
sisting independent of the gametophyte for long periods. In
addition, the foot of hornworts is somewhat lobed and the
surface compared to incipient rhizoids. Thus, hornwort
70
CHAPTER 3
UNIT II
EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS
EVOLUTION AND DIVERSITY OF PLANTS
REVIEW Q..VESTIONS
GREEN PLANTS
1. What are two formal names for the green plants?
2. What are apomorphies for the green plants?
3. The bulk of the primary cell wall of green plants is composed of what substance? (Give the common name and chemical name.)
4. Is the cell wall synthesized inside or outside the plasma membrane?
5. What are the unique features of green plant chloroplasts?
6. How are chloroplasts thought to have originated (i.e., by what evolutionary process)?
7. What is a haplontic life cycle? Draw and label.
8. What is oogamy?
9. Describe and give the function of plasmodesmata.
I
FIGURE 3.19 Homworts, Anthoceros sp. A. Population of gametophytes with attached sporophytes. B. Gametophyte with attached, cylin
drical sporophyte. C. Close-up of sporophyte base, showing ensheathing collar of gametophytic tissue surrounding intercalary meristem of
sporophyte. D. Whole mount of sporophyte base, showing foot embeded within gametophyte. E. Longitudinal-section of sporophyte base.
Note basal foot and actively dividing cells of intercalary meristem at sporophyte base. F. Sporophyte longitudinal-section, showing columella,
spores, and pseudo-elaters.
sporophytes may represent a transition to the very dominant,
long-lived sporophytes of vascular plants (Chapter 4).
POLYSPOBANGIOPHYTES/PAN-TRACHEOPHYTA
This group is inclusive of a few, basal fossil taxa plus all of the
true vascular plants, or tracheophytes (Chapter 4). The
first-evolving polysporangiophytes (formally the Polysporan-
giomorpha or Pan-Tracheophyta; see Chapter 4), such as the genus
Horneophyton (not illustrated), were similar to hornworts, liver
worts, and mosses in lacking vascular tissue. However, they are
different from “bryophytes,” and linked to the vascular plants, in
having branched stems with multiple sporangia (Figure 3.6). Thus,
the polysporangiophytes include taxa that were transitional to the
tracheophytes.
LAND PLANTS
10. What is the formal name for the land plants?
11. Name the major apomorphies of the land plants.
12. Draw and label the basic haplodiplontic life cycle (alternation of generations) of all land plants, illustrating all structures,
processes, and ploidy levels.
13. What is an embryo?
14. What is a sporangium?
15. Name the possible adaptive features of the sporophyte.
16. What are cutin and cuticle and what are their adaptive significance?
17. Define apical growth and parenchyma.
18. In land plants what is the name of the pectic-rich layer between adjacent cell walls that functions to bind them together?
19. What is an antheridium? Draw and label the parts.
20. What is an archegonium? Draw and label the parts.
LIVERWORTS, MOSSES, AND HORNWORTS
21. Draw a phylogenetic tree denoting relationships of the liverworts, mosses, hornworts, and vascular plants.
22. What is the formal name of the liverworts?
23. Name two apomorphies of the liverworts.
24. What is the function of elaters?
25. What are the two major morphological forms of liverworts? Which is likely ancestral?
26. What are gemmae and gemma cups?
27. Describe the morphology of the leaves of leafy liverworts.
28. What is an antheridiophore? An archegoniophore?
29. Describe the structural makeup and function of a stomate.
30. What land plant groups possess stomates?
31. What possible apomorphies may be shared by the mosses, hornworts, and vascular plants?
32. What is the formal name of the mosses?
33. Name major apomorphies shared by the mosses alone.
34. What is a calyptra, stipe, operculum, peristome tooth?
35. What is the scientific name of peat moss?
36. What feature of the leaf anatomy of peat moss enables the leaves to absorb and retain water?
37. How is peat moss of economic importance?
38. What is the formal name of hornworts?
39. Describe the major features of hornworts, citing how they differ from the liverworts and mosses.
40. What is the function of pseudo-elaters, and how do they differ structurally from the elaters of liverworts?
41. What feature of the sporophyte might unite the hornworts with the vascular plants?
42. What apomorphy links the Pan-Tracheophyta/polysporangiophytes with the vascular plants?
71