EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS

Transcription

3
EVOLUTION AND DIVERSITY OF
GREEN AND LAND PLANTS
Hornworts
Polysporangiophytes/Pan-Tracheophyta
69
70
THE GREEN PLANTS
55
EMBRYOPHYTA- LAND PLANTS
59
DIVERSITY OF NONVASCULAR LAND PLANTS
62
REVIEW QjESTIONS
71
62
65
EXERCISES
72
REFERENCES FOR FURTHER STUDY
72
Liverworts
Mosses
THE GREEN PLANTS
the cells, acting as a sort of cellular exoskeleton. The evolu
tion of a cellulosic cell wall was a preamble to the further
evolution of more complex types of growth, particularly of
self-supporting shoot systems. It is not clear if a cellulosic
cell wall constitutes an apomorphy for the Viridiplantae
alone, as it may have evolved much earlier, constituting an
apomorphy for the Viridiplantae plus one or more other
groups; in any case, its adaptive significance seems clear.
Perhaps the primary apomorphy for the Viridiplantae is a
specialized type of chloroplast (Figure 3.2). As discussed in
Chapter 1, chloroplasts are one of the major defining charac
teristics of traditionally defined “plants”; their adaptive sig
nificance as organelles functioning in photosynthesis, the
conversion of light energy to chemical energy, is unques
tioned. Chloroplasts in the Viridiplantae, the green plants, differ
from those of most other organisms, such as the red and brown
“algae,” in (1) containing chlorophyll b in addition to chloro
phyll a, the former of which acts as an accessory pigment in
light capture; (2) having thylakoids, the chlorophyll-containing
membranes, that are stacked into grana, which are pancakelike aggregations (see Figure 3.2B,C); and (3) manufacturing as
a storage product true starch, a polymer of glucose sugar
units (= polysaccharide) in which the glucose molecules are
chemically bonded in the alpha-1,4 position (ci-l,4glucopyranoside). Thus, all green plants, from filamentous
green “algae” in a pond or tide pooi to giant sequoia or
The green plants, formally called the Viridiplantae or
Chiorobionta, are a monophyletic group of eukaryotic organ
isms that includes what have traditionally been called “green
algae” plus the land plants or embryophytes (Figure 3.1).
Like all eukaryotes, the Viridiplantae have cells with
membrane-bound organelles, including a nucleus (containing
chromosomes composed of linear chains of DNA bound to
proteins, that are sorted during cell division by mitosis), micro
tubules, mitochondria, an endoplasmic reticulum, vesicles,
and golgi bodies. Although the interrelationships of the non—
land plant Viridiplantae will not be covered in detail here, it
is important to realize that some of the evolutionary innova
tions, or apomorphies, that we normally associate with land
plants actually arose before plants colonized the land.
Several apomorphies unite the Viridiplantae (Figure 3.1).
One possible novelty for this group is a cellulosic cell wall
(Figure 3.2A). Cellulose, like starch, is a polysaccharide,
but one in which the glucose sugar units are bonded in the
beta-l,4 position (=3-1,4-g1ucopyranoside). This slight change
in chemical bond position results in a very different molecule.
Cellulose is secreted outside the plasma membrane as micro
scopic fiber-like units called microfibrils that are further
intertwined into larger fibril units, forming a supportive
meshwork. The function of cellulose is to impart rigidity to
-
55
02010 Elsevier Inc. All rights reserved.
doi: 10. 1016/B978-0- 12-374380-0.00003-9
r
56
CHAPTER 3
UNIT II
EVOLUTION AND DIVERSITY OF PLANTS
57
Viridiplantae [Chiorobionta] Green Plants
Streptophytes
—
Chiorophytes
1i
“Green Algae” (a paraphyletic group)
IE
-
fi
Charophytes
=
Land Plants
Embryophytes
of
FIGURE 3.3 Examples of non—land plant Viridiplantae. A. Chlansydomonas reinhardtii, a unicellular form. (Photo courtesy
large,
with
form,
vegetative
Rick Bizzoco.) B. Ulva, a thalloid form. C. Volvox, a colonial form. D. Spirogyra, a filamentous form. Above:
spiral chloroplasts. Below: reproductive conjugation stage, showing + and mating strains and nonmotile zygotes.
archegonium
—
antheridium
parenchyma
Eucalyptus trees have this same type of chioroplast. Recent
data imply that chioroplasts found in the green plants today
were modified from those that evolved via endosymbiosis,
the intracellular cohabitation of an independently living, uni
cellular prokaryote inside a eukaryotic cell (see Chapter 1).
The Viridiplantae as a whole are classified as two sister
groups: chiorophytes, or Chlorophyceae, and streptophytes,
or Streptophyceae (Figure 3.1). The traditional “green algae”
are a paraphyletic group (which is why the name is placed in
quotation marks) and are defined as the primarily aquatic
Viridiplantae, consisting of all chiorophytes and the non—land
plant streptophytes. “Green algae” occur in a tremendous
variety of morphological forms. These include single cells
(Figure 3.3A) with or without flagella, thalloid forms (Figure
3.3B), motile and nonmotile colonies (Figure 3.3C), and nonmotile filaments (Figure 3.3D). Many have flagellated motile
cuticle
sporophyte/embryo (alternation of generations)
true starch storage compound
thylakoids stacked in grana
chlorophyll b (chlorophyll a is ancestral)
Unique green plant chloroplast features
cellulose in cell wall (may have evolved earlier & thus not a synapomorphy for Chlorobionta alone)
FIGURE 3.1 Cladogram of the green plants (Viridiplantae or Chlorobionta), modified from Bremer (1985), Mishler and Churchill (1985),
and Mishler et al. (1994). Important apomorphies discussed in the text are listed beside thick hash marks.
cells in at least one phase of their life history. “Green
algae” inhabit fresh and marine waters and some live in or on
soil (or even on snow!) or in other terrestrial but moist
habitats.
The primitive type of green plant sexual reproduction
seems to have been the production of flagellate, haploid (n)
gametes that are “isomorphic,” that is, that look identical.
Fertilization occurs by union of two of these gametes, result
ing in a diploid (2n) zygote (Figure 3.4A). The zygote, which
is free-living, then divides by meiosis to form four haploid
spores, each of which may germinate and develop into a new
haploid individual, which produces more gametes, complet
ing what is termed a haplontic (or “haplobiontic”) life cycle
(Figure 3.4A).
Within the streptophyte lineage that gave rise to the land
plants, a few innovations evolved that may have been
HAPLOJD (n)
Multicelled Stage
HAPLOID (n)
//Z Multicelled Stage
granum
.thylakoid
—
-7:-’
..-
-
mitosis
:
/
I
HAPLONTIC
Isogamy
Spores
(n)
-i’
FIGURE 3.2 A. Elodea, whole leaf in face view, showing apomorphies of the Viridiplantae: a cellulosic cell wall and green plant
chloroplasts. B. Diagram of chloroplast structure of green plants, showing thylakoids and grana. C. Electron micrograph of Chlamydomonas
reinhardtii, a unicellular “green alga,” showing granum of chioroplast. (Photo courtesy of Rick Bizzoco.)
A
•;
Gamete Gamete
(n)
(n)
1/
fertilization
Jneiosis
‘i•:
•;
Z3ote
(2n)
FIGURE 3.4
HAPLONTIC
Oogamy
C
Spores i
(n)
\
.
/
Egg Sperm
(n)
(n)
//
fertilization
nze,oszs
Zygote
(2n)
Haplontic life cycles in some of the green plants. A. Jsogamy. B. Oogamy.
ii
56
CHAPTER 3
UNIT II
57
EVOLUTIONANDDIVERSITYOFPLANT5
Viridiplantae [Chiorobionta] Green Plants
Chiorophytes —j
Streptophytes
—
r
“Green Algae” (a paraphyletic group)
J::
-
Charophytes
-
=
.
Land Plants
Embryophytes
I
archegonium
Examples of non—land plant Viridiplantae. A. Chiarnydornonas reinhardtii, a unicellular form. (Photo courtesy of
with large,
Rick Bizzoco.) B. Ulva, a thalloid form. C. Volvox, a colonial form. D. Spirogyra, a filamentous form. Above: vegetative form,
zygotes.
nonmotile
and
spiral chloroplasts. Below: reproductive conjugation stage, showing + and mating strains
FIGURE 3.3
—
antheridjum
parenchyma
Eucalyptus trees have this same type of chloroplast. Recent
data imply that chioroplasts found in the green plants today
cuticle
true starch storage compound
thylakoids stacked in grana
were modified from those that evolved via endosymbiosis,
the intracellular cohabitation of an independently living, uni
cellular prokaryote inside a eukaryotic cell (see Chapter 1).
The Viridiplantae as a whole are classified as two sister
groups: chiorophytes, or Chlorophyceae, and streptophytes,
or Streptophyceae (Figure 3.1). The traditional “green algae”
are a paraphyletic group (which is why the name is placed in
quotation marks) and are defined as the primarily aquatic
Viridiplantae, consisting of all chiorophytes and the non—land
plant streptophytes. “Green algae” occur in a tremendous
variety of morphological forms. These include single cells
(Figure 3.3A) with or without flagella, thalloid forms (Figure
3.3B), motile and nonmotile colonies (Figure 3.3C), and nonmotile filaments (Figure 3.3D). Many have flagellated motile
Unique green plant chioroplast features
chlorophyll b (chlorophyll a is ancestral)
cellulose in cell wall (may have evolved earlier & thus not a synapomorphy for Chiorobionta alone)
FIGURE 3.1 Cladogram of the green plants (Viridiplantae or Chlorobionta), modified from Bremer (1985), Mishler
and Churchill (1985),
and Mishler et al. (1994). Important apomorphies discussed in the text are listed beside thick hash marks.
//
granum
hyIakoid_),
_-
.,.“
cells in at least one phase of their life history. “Green
algae” inhabit fresh and marine waters and some live in or on
soil (or even on snow!) or in other terrestrial but moist
habitats.
The primitive type of green plant sexual reproduction
seems to have been the production of flagellate, haploid (n)
gametes that are “isomorphic,” that is, that look identical.
Fertilization occurs by union of two of these gametes, result
ing in a diploid (2n) zygote (Figure 3.4A). The zygote, which
is free-living, then divides by meiosis to form four haploid
spores, each of which may germinate and develop into a new
haploid individual, which produces more gametes, complet
ing what is termed a haplontic (or “haplobiontic”) life cycle
(Figure 3.4A).
Within the streptophyte lineage that gave rise to the land
plants, a few innovations evolved that may have been
HAPLOID (n)
HAPLOH) (n)
Multicelled Stage
Multicelled Stage
(Adult)
..,
mitosis
4
rd
VI
HAPLONTIC
Isogamy
©©
Spores
I
-
-.
.:
FIGURE 3.2 A. Elodea, whole leaf in face view, showing apomorphies of the Viridiplantae:
a cellulosic cell wall and green plant
chloroplasts. B. Diagram of chloroplast structure of green plants, showing thylakoids and grana. C. Electron
micrograph of Chlamydornonas
reinhardtjj, a unicellular “green alga,” showing granum of chioroplast. (Photo courtesy of Rick
Bizzoco.)
I
A
Spores
(n)
Zygote
(2n)
FIGURE 3.4
B
Egg Sperm
(n)
(n)
//
fertilization
meiosis
fertilization
meiosis
4
Gamete Gamete
(n)
(n)
1/
-
‘b
HAPLONTIC
Oogamy
Zygote
(2n)
Haplontic life cycles in some of the green plants. A. Isogamy. B. Oogamy.
58
CHAPTER 3
UNIT II
EVOLUTION AND DIVERSiTY OF GREEN AND LAND PLANTS
“preadaptations” to survival on land. First of these was the
evolution of oogamy, a type of sexual reproduction in which
one gamete, the egg, becomes larger and nonflagellate; the
other gamete is, by default, called a sperm cell (Figure 3.4B).
Oogamy is found in all land plants but independently evolved
in many other groups, including many other “algae” and in
the animals.
Several other apomorphies of and within the Viridiplantae
include ultrastructural specializations of flagella and some
features of biochemistry. Although these have been valuable
in elucidating phylogenetic relationships, their adaptive
significance is unclear, and they will not be considered
further here.
An apomorphy for the charophytes, a dade within the
streptophytes that includes Coleochaete (Figure 3.5B),
Charales (Figure 3.5C—E), and the land plants (Figure 3.1),
are plasmodesmata. Plasmodesmata are essentially pores in
the primary (10) cell wall through which membranes traverse
between cells, allowing for transfer of compounds between
cells (Figure 3.5A). Plasmodesmata may function in more
efficient or rapid transport of solutes, including regulatory
and growth-mediating compounds, such as hormones.
Members of the Charales, such as the genera Chara and
Nitella, are perhaps the closest living relatives to the land
plants. These fresh water, aquatic organisms have a haplontic
life cycle, and consist of a central axis bearing whorls of lat
eral branches (Figure 3.5D) or (if small) “leaves” on the hap
bid body. Some Charales are capable of precipitating calcium
carbonate as an outer layer of the plant body (accounting for
the common names “brittleworts” or “stoneworts”). Members
of the Charales grow by means of a single apical cell, similar
to that of some land plants and representing a possible syna
pomorphy with them. However, the Charales differ from
land plants in lacking true parenchyma (see later discussion).
The Charales have specialized male and female gametangia,
termed antheridia and oogonia (Figure 3.5C,D). The oogonia
are distinctive in having a spirally arranged group of outer
“tube” cells (Figure 3.5D); fossilized casts of oogonia retain
the outline of these tube cells (Figure 3.5E). Oogonia and
antheridia of the Charales resemble the archegonia and
Embryophyta
1
—
land plants
Pan-Tracheophytal
Polysporangiophytes
0
gametophyte
leafy (in some)
-
Tracheophytes
vascular plants
t t
gametophytic
leaves
59
—
pseudo-elaters
in sporangium
columella in
sporangium
sporophyte branched
with multiple sporangia
elaters in
sporangium
oil bodies
sporophyte photosynthetic, nutritionally independent
aerial sporophyte axis
stomates
archegonium
antheridium
parenchyma
cuticle
t
=
extinct
FIGURE 3.6 One hypothesis of relationships of the land plants (Embryophyta), with major apomorphies indicated. After Qiu et al. (2007),
some apomorphies after Bremer (1985); Mishler and Churchill (1985); Mishler et al. (1994).
antheridia of land plants (see later discussion) in having an
outer layer of sterile cells, but the gametangia of the two
groups are generally thought not to be directly homologous
because of major differences in structure and development.
However, members of the Charales retain the egg and zygote
(although the latter only briefly) on the plant body. This
retention of egg and zygote on the haploid body may repre
sent a transition to their permanent retention on the gameto
phyte of land plants (see later discussion).
FIGURE 3.5 A. Diagram of plasmodesmata in cellulosic cell wall, an apomorphy of some green plants, including the land plants.
B. Coleochaete sp., a close relative to the embryophytes. (Photo courtesy of Linda Graham.) C—E. Charales. C. Nitella sp., oogonia and
antheridia. D. Chara sp., oogonium and antheridium. Note spiral tube cells of oogonia. E. Tectochara helicteres, a fossil oogonium from the
Eocene, showing remnants of spiral tube cells.
The Embryophyta, or embryophytes (commonly known as
land plants), are a monophyletic assemblage within the green
plants (Figures 3.1, 3.6). The first colonization of plants on
land during the Silurian period, ca. 400 million years ago,
was concomitant with the evolution of several important fea
tures. These shared, evolutionary novelties (Figure 3.6) con
stituted major adaptations that enabled formerly aquatic
green plants to survive and reproduce in the absence of a sur
rounding water medium.
One major innovation of land plants was the evolution of
the embryo and sporophyte (Figure 3.6). The sporophyte is
a separate diploid (2n) phase in the life cycle of all land
plants. The corresponding haploid, gamete-producing part of
the life cycle is the gametophyte. The life cycle of land
plants, having both a haploid gametophyte and a diploid
sporophyte, is an example of a haplodiplontic (also called
“diplobiontic”) life cycle, commonly called alternation of
generations (Figure 3.7). Note that alternation of generations
does not necessarily mean that the two phases occur at differ
ent points in time; at any given time, both phases may occur
in a population.
The sporophyte can be viewed as forming from the zygote
by the delay of meiosis and spore production. Instead of mei
osis, the zygote undergoes numerous mitotic divisions, which
result in the development of a separate entity. The embryo is
defined as an immature sporophyte that is attached to or sur
rounded by the gametophyte. In many land plants, such as the
58
CHAPTER 3
UNIT II
EVOLUTION AND DIVERSiTY OF GREEN AND LAND PLANTS
“preadaptations” to survival on land. First of these was the
evolution of oogamy, a type of sexual reproduction in which
one gamete, the egg, becomes larger and nonflagellate; the
other gamete is, by default, called a sperm cell (Figure 3.4B).
Oogamy is found in all land plants but independently evolved
in many other groups, including many other “algae” and in
the animals.
Several other apomorphies of and within the Viridiplantae
include ultrastructural specializations of flagella and some
features of biochemistry. Although these have been valuable
in elucidating phylogenetic relationships, their adaptive
significance is unclear, and they will not be considered
further here.
An apomorphy for the charophytes, a dade within the
streptophytes that includes Coleochaete (Figure 3.5B),
Charales (Figure 3.5C—E), and the land plants (Figure 3.1),
are plasmodesmata. Plasmodesmata are essentially pores in
the primary (10) cell wall through which membranes traverse
between cells, allowing for transfer of compounds between
cells (Figure 3.5A). Plasmodesmata may function in more
efficient or rapid transport of solutes, including regulatory
and growth-mediating compounds, such as hormones.
Members of the Charales, such as the genera Chara and
Nitella, are perhaps the closest living relatives to the land
plants. These fresh water, aquatic organisms have a haplontic
life cycle, and consist of a central axis bearing whorls of lat
eral branches (Figure 3.5D) or (if small) “leaves” on the hap
bid body. Some Charales are capable of precipitating calcium
carbonate as an outer layer of the plant body (accounting for
the common names “brittleworts” or “stoneworts”). Members
of the Charales grow by means of a single apical cell, similar
to that of some land plants and representing a possible syna
pomorphy with them. However, the Charales differ from
land plants in lacking true parenchyma (see later discussion).
The Charales have specialized male and female gametangia,
termed antheridia and oogonia (Figure 3.5C,D). The oogonia
are distinctive in having a spirally arranged group of outer
“tube” cells (Figure 3.5D); fossilized casts of oogonia retain
the outline of these tube cells (Figure 3.5E). Oogonia and
antheridia of the Charales resemble the archegonia and
Embryophyta
1
—
land plants
Pan-Tracheophytal
Polysporangiophytes
0
gametophyte
leafy (in some)
-
Tracheophytes
vascular plants
t t
gametophytic
leaves
59
—
pseudo-elaters
in sporangium
columella in
sporangium
sporophyte branched
with multiple sporangia
elaters in
sporangium
oil bodies
sporophyte photosynthetic, nutritionally independent
aerial sporophyte axis
stomates
archegonium
antheridium
parenchyma
cuticle
t
=
extinct
FIGURE 3.6 One hypothesis of relationships of the land plants (Embryophyta), with major apomorphies indicated. After Qiu et al. (2007),
some apomorphies after Bremer (1985); Mishler and Churchill (1985); Mishler et al. (1994).
antheridia of land plants (see later discussion) in having an
outer layer of sterile cells, but the gametangia of the two
groups are generally thought not to be directly homologous
because of major differences in structure and development.
However, members of the Charales retain the egg and zygote
(although the latter only briefly) on the plant body. This
retention of egg and zygote on the haploid body may repre
sent a transition to their permanent retention on the gameto
phyte of land plants (see later discussion).
FIGURE 3.5 A. Diagram of plasmodesmata in cellulosic cell wall, an apomorphy of some green plants, including the land plants.
B. Coleochaete sp., a close relative to the embryophytes. (Photo courtesy of Linda Graham.) C—E. Charales. C. Nitella sp., oogonia and
antheridia. D. Chara sp., oogonium and antheridium. Note spiral tube cells of oogonia. E. Tectochara helicteres, a fossil oogonium from the
Eocene, showing remnants of spiral tube cells.
The Embryophyta, or embryophytes (commonly known as
land plants), are a monophyletic assemblage within the green
plants (Figures 3.1, 3.6). The first colonization of plants on
land during the Silurian period, ca. 400 million years ago,
was concomitant with the evolution of several important fea
tures. These shared, evolutionary novelties (Figure 3.6) con
stituted major adaptations that enabled formerly aquatic
green plants to survive and reproduce in the absence of a sur
rounding water medium.
One major innovation of land plants was the evolution of
the embryo and sporophyte (Figure 3.6). The sporophyte is
a separate diploid (2n) phase in the life cycle of all land
plants. The corresponding haploid, gamete-producing part of
the life cycle is the gametophyte. The life cycle of land
plants, having both a haploid gametophyte and a diploid
sporophyte, is an example of a haplodiplontic (also called
“diplobiontic”) life cycle, commonly called alternation of
generations (Figure 3.7). Note that alternation of generations
does not necessarily mean that the two phases occur at differ
ent points in time; at any given time, both phases may occur
in a population.
The sporophyte can be viewed as forming from the zygote
by the delay of meiosis and spore production. Instead of mei
osis, the zygote undergoes numerous mitotic divisions, which
result in the development of a separate entity. The embryo is
defined as an immature sporophyte that is attached to or sur
rounded by the gametophyte. In many land plants, such as the
60
CHAPTER 3
UNIT II
HAPLODIPLONTIC
LIFE CYCLE
(“Alternation of Generations”)
mechanical protection of inner tissue and to inhibit water
loss. The cuticle consists of a thin, homogeneous, transparent
layer of cutin, a polymer of fatty acids, and functions as a
sealant, preventing excess water loss. Cutin also impregnates
the outer cellulosic cell walls of epidermal cells; these are
known as a “cutinized” cell wall. The adaptive advantage of
cutin and the cuticle is obvious: prevention of desiccation
outside the ancestral water medium. In fact, plants that are
adapted to very dry environments will often have a particu
larly thick cuticle (as in Figure 3.8) to inhibit water loss.
A third apomorphy for the land plants was the evolution of
parenchyma tissue (Figure 3.9). All land plants grow by
means of rapid cell divisions at the apex of the stem, shoot,
and thallus or (in most vascular plants) of the root. This region
of actively dividing cells is the apical meristem. The apical
meristem of liverworts, hornworts, and mosses (discussed
later), and of the monilophytes (see Chapter 4) have a single
apical cell (Figure 3.9), probably the ancestral condition for
the land plants. In all land plants the cells derived from the
apical meristem region form a solid mass of tissue known as
parenchyma (Gr. para, “beside” + enchyma, “an infusion”;
in reference to a concept that parenchyma infuses or fills up
space beside and between the other cells). Parenchyma tissue
consists of cells that most resemble the unspecialized, undif
ferentiated cells of actively dividing meristematic tissue.
Structurally, parenchyma cells (1) are elongate to isodiamethc;
(2) have a primary (1°) cell wall only (rarely a secondary wall);
and (3) are living at maturity and potentially capable of
continued cell divisions. Parenchyma cells function in
Sporophyte Body
mitosis, growth, & differentiation
z
Embryo
Sporangium
/%
mitosis, growth, & dfferenuation
initosis, growth, & differentiation
SPOROPHYTE GENERATION
(2N)
Zygote
Sporocyte
——fertilization
rneiosis——
)
GAMETOPHYTE GENERATION
(N)
(Sperm nonflagellate in Conifers,
Gnetales, and Angiosperms)
Egg
Sperm
Spores©
/
lost by reduction and modificationf Archegonium Antheridium
in the Angiosperms
and some Gnetales
Gametophyte Body
FIGURE 3.7
Haplodiplontic “alternation of generations” in the land plants (embryophytes).
seed plants, the embryo will remain dormant for a period of
time and will begin growth only after the proper environmen
tal conditions are met. As the embryo grows into a mature
sporophyte, a portion of the sporophyte differentiates as the
spore-producing region. This spore-producing region of the
sporophyte is called the sporangium. The sporangium is
enveloped by a sporangial wall, which consists of one or
more layers of sterile, non-spore-producing cells. A sporan
gium contains sporogenous tissue, which matures into sporo
cytes, the cells that undergo meiosis. Each sporocyte produces,
by meiosis, four haploid spores (Figure 3.7).
One adaptive advantage of a sporophyte generation as a
separate phase of the life cycle is the large increase in spore
production. In the absence of a sporophyte, a single zygote
(the result of fertilization of egg and sperm) will produce four
spores. The elaboration of the zygote into a sporophyte and
sporangium can result in the production of literally millions
of spores, a potentially tremendous advantage in reproductive
output and increased genetic variation.
Another possible adaptive value of the sporophyte is
associated with its diploid ploidy level. The fact that a sporo
phyte has two copies of each gene may give this diploid phase
an increased fitness in either of two ways: (1) by potentially pre
ventiiig the expression of recessive, deleterious alleles (which,
in the sporophyte, may be “shielded” by dominant alleles,
but which, in the gametophyte, would always be expressed);
and (2) by permitting increased genetic variability in the
sporophyte generation (via genetic recombination from two
“parents”) upon which natural selection acts, thus increasing
the potential for evolutionary change.
A second innovation in land plants was the evolution of
cutin and the cuticle (Figure 3.8). A cuticle is a protective
layer that is secreted to the outside of the cells of the epider
mis (Gr. epi, “upon” + derma, “skin”), the outermost layer
of land plant organs. The epidermis functions to provide
cuticle
cell wall
—
single apical cell
epidernial cell
I
FIGURE 3.8
The cuticle, an apombrphy for the land plants.
FIGURE 3.9 Equisetum shoot apex, showing parenchymatous
growth form, from an apical meristem.
61
metabolic activities such as respiration, photosynthesis, lat
eral transport, storage, and regeneration/wound healing.
Parenchyma cells may further differentiate into other special
ized cell types. It is not clear if the evolution of both apical
growth and true parenchyma is an apomorphy for the land
plants alone, as shown here (Figure 3.6). Both may be inter
preted to occur in certain closely related green plants, includ
ing the Charales.
Correlated with the evolution of parenchyma may have
been the evolution of a middle lamella in land plants. The
middle lamella is a pectic-rich layer that develops between
the primary cell walls of adjacent cells (Figure 3.5A).
Its function is to bind adjacent cells together, perhaps a
prerequisite to the evolution of solid masses of parenchyma
tissue.
Another evolutionary innovation for the land plants was
the antheridium (Figure 3. bA). The antheridium is a type
of specialized gametangium of the haploid (n) gametophyte,
one that contains the sperm-producing cells. It is distin
guished from similar structures in the Viridiplantae in being
surrounded by a layer of sterile cells, the antheridial wall.
The evolution of the surrounding layer of sterile wall cells,
which is often called a sterile “jacket” layer, was probably
adaptive in protecting the developing sperm cells from desic
cation. In all of the nonseed land plants, the sperm cells are
released from the antheridium into the external environment
and must swim to the egg in a thin film of water. Thus, a wet
environment is needed for fertilization to be effected in the
nonseed plants, a vestige of their aquatic ancestry. Members
of the Charales also have a structure termed an antheridium,
which has an outer layer of sterile cells (Figure 3.5C,D).
However, because of its differing anatomy, the Charales
antheridium may not be homologous with that of the land
plants, and thus may have evolved independently.
Another land plant innovation was the evolution of
the archegonium, a specialized female gametangium
(Figure 3. lOB). The archegonium consists of an outer layer
of sterile cells, termed the venter, that immediately surround
the egg, plus others that extend outward as a tube-like neck.
The archegonium is stalked in some taxa; in others the egg
is rather deeply embedded in the parent gametophyte. The
egg cell is located inside and at the base of the archegonium.
Immediately above the egg is a second cell, called the
ventral canal cell, and above this and within the neck region
there may be several neck canal cells. The archegonium may
have several adaptive functions. It may serve to protect the
developing egg. It may also function in fertilization. Before
fertilization occurs, the neck canal cells and ventral canal cell
break down and are secreted from the terminal pore of the
neck itself; the chemical compounds released function as an
60
CHAPTER 3
UNIT II
HAPLODIPLONTIC
LIFE CYCLE
(“Alternation of Generations”)
mechanical protection of inner tissue and to inhibit water
loss. The cuticle consists of a thin, homogeneous, transparent
layer of cutin, a polymer of fatty acids, and functions as a
sealant, preventing excess water loss. Cutin also impregnates
the outer cellulosic cell walls of epidermal cells; these are
known as a “cutinized” cell wall. The adaptive advantage of
cutin and the cuticle is obvious: prevention of desiccation
outside the ancestral water medium. In fact, plants that are
adapted to very dry environments will often have a particu
larly thick cuticle (as in Figure 3.8) to inhibit water loss.
A third apomorphy for the land plants was the evolution of
parenchyma tissue (Figure 3.9). All land plants grow by
means of rapid cell divisions at the apex of the stem, shoot,
and thallus or (in most vascular plants) of the root. This region
of actively dividing cells is the apical meristem. The apical
meristem of liverworts, hornworts, and mosses (discussed
later), and of the monilophytes (see Chapter 4) have a single
apical cell (Figure 3.9), probably the ancestral condition for
the land plants. In all land plants the cells derived from the
apical meristem region form a solid mass of tissue known as
parenchyma (Gr. para, “beside” + enchyma, “an infusion”;
in reference to a concept that parenchyma infuses or fills up
space beside and between the other cells). Parenchyma tissue
consists of cells that most resemble the unspecialized, undif
ferentiated cells of actively dividing meristematic tissue.
Structurally, parenchyma cells (1) are elongate to isodiamethc;
(2) have a primary (1°) cell wall only (rarely a secondary wall);
and (3) are living at maturity and potentially capable of
continued cell divisions. Parenchyma cells function in
Sporophyte Body
z
Embryo
Sporangium
/%
mitosis, growth, & dfferenuation
initosis, growth, & differentiation
SPOROPHYTE GENERATION
(2N)
Zygote
Sporocyte
——fertilization
rneiosis——
)
GAMETOPHYTE GENERATION
(N)
(Sperm nonflagellate in Conifers,
Gnetales, and Angiosperms)
Egg
Sperm
Spores©
/
lost by reduction and modificationf Archegonium Antheridium
in the Angiosperms
and some Gnetales
Gametophyte Body
FIGURE 3.7
Haplodiplontic “alternation of generations” in the land plants (embryophytes).
seed plants, the embryo will remain dormant for a period of
time and will begin growth only after the proper environmen
tal conditions are met. As the embryo grows into a mature
sporophyte, a portion of the sporophyte differentiates as the
spore-producing region. This spore-producing region of the
sporophyte is called the sporangium. The sporangium is
enveloped by a sporangial wall, which consists of one or
more layers of sterile, non-spore-producing cells. A sporan
gium contains sporogenous tissue, which matures into sporo
cytes, the cells that undergo meiosis. Each sporocyte produces,
by meiosis, four haploid spores (Figure 3.7).
One adaptive advantage of a sporophyte generation as a
separate phase of the life cycle is the large increase in spore
production. In the absence of a sporophyte, a single zygote
(the result of fertilization of egg and sperm) will produce four
spores. The elaboration of the zygote into a sporophyte and
sporangium can result in the production of literally millions
of spores, a potentially tremendous advantage in reproductive
output and increased genetic variation.
Another possible adaptive value of the sporophyte is
associated with its diploid ploidy level. The fact that a sporo
phyte has two copies of each gene may give this diploid phase
an increased fitness in either of two ways: (1) by potentially pre
ventiiig the expression of recessive, deleterious alleles (which,
in the sporophyte, may be “shielded” by dominant alleles,
but which, in the gametophyte, would always be expressed);
and (2) by permitting increased genetic variability in the
sporophyte generation (via genetic recombination from two
“parents”) upon which natural selection acts, thus increasing
the potential for evolutionary change.
A second innovation in land plants was the evolution of
cutin and the cuticle (Figure 3.8). A cuticle is a protective
layer that is secreted to the outside of the cells of the epider
mis (Gr. epi, “upon” + derma, “skin”), the outermost layer
of land plant organs. The epidermis functions to provide
cuticle
cell wall
—
single apical cell
epidernial cell
I
FIGURE 3.8
The cuticle, an apombrphy for the land plants.
FIGURE 3.9 Equisetum shoot apex, showing parenchymatous
growth form, from an apical meristem.
61
metabolic activities such as respiration, photosynthesis, lat
eral transport, storage, and regeneration/wound healing.
Parenchyma cells may further differentiate into other special
ized cell types. It is not clear if the evolution of both apical
growth and true parenchyma is an apomorphy for the land
plants alone, as shown here (Figure 3.6). Both may be inter
preted to occur in certain closely related green plants, includ
ing the Charales.
Correlated with the evolution of parenchyma may have
been the evolution of a middle lamella in land plants. The
middle lamella is a pectic-rich layer that develops between
the primary cell walls of adjacent cells (Figure 3.5A).
Its function is to bind adjacent cells together, perhaps a
prerequisite to the evolution of solid masses of parenchyma
tissue.
Another evolutionary innovation for the land plants was
the antheridium (Figure 3. bA). The antheridium is a type
of specialized gametangium of the haploid (n) gametophyte,
one that contains the sperm-producing cells. It is distin
guished from similar structures in the Viridiplantae in being
surrounded by a layer of sterile cells, the antheridial wall.
The evolution of the surrounding layer of sterile wall cells,
which is often called a sterile “jacket” layer, was probably
adaptive in protecting the developing sperm cells from desic
cation. In all of the nonseed land plants, the sperm cells are
released from the antheridium into the external environment
and must swim to the egg in a thin film of water. Thus, a wet
environment is needed for fertilization to be effected in the
nonseed plants, a vestige of their aquatic ancestry. Members
of the Charales also have a structure termed an antheridium,
which has an outer layer of sterile cells (Figure 3.5C,D).
However, because of its differing anatomy, the Charales
antheridium may not be homologous with that of the land
plants, and thus may have evolved independently.
Another land plant innovation was the evolution of
the archegonium, a specialized female gametangium
(Figure 3. lOB). The archegonium consists of an outer layer
of sterile cells, termed the venter, that immediately surround
the egg, plus others that extend outward as a tube-like neck.
The archegonium is stalked in some taxa; in others the egg
is rather deeply embedded in the parent gametophyte. The
egg cell is located inside and at the base of the archegonium.
Immediately above the egg is a second cell, called the
ventral canal cell, and above this and within the neck region
there may be several neck canal cells. The archegonium may
have several adaptive functions. It may serve to protect the
developing egg. It may also function in fertilization. Before
fertilization occurs, the neck canal cells and ventral canal cell
break down and are secreted from the terminal pore of the
neck itself; the chemical compounds released function as an
r
62
CHAPTER 3
UNIT II
2 rows of
dorsal leaves
antheridial wall (sterile “jacket” layer)
neck
dorsal (upper)
view
B
/
FIGURE 3.10
1 row of
ventral leaves
neck
sperm cells
A
63
ventral (lower)
view
leafy liverwort
thalloid liverwort
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, flavonoid chemical compounds, and a proliferation of
heat shock proteins. These are not discussed here.
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.
archegonium
(n)
LIVERWORTS
Liverworts, also traditionally called the Hepaticae, are one of
the monophyletic groups that are descendents of some of the
archegoniophore (n)
(longitudinal-section)
archegoniophore (n)
(longitudinal-section)
first land plants. Today, liverworts are relatively minor com
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 hornworts 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
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
capsule
sporophyte
(2n)
elater
antheridiophore (n)
(longitudinal section)
antheridium
(n)
spore
C
germinating spore
FIGURE 3.11
Liverwort morphology and life cycle.

EVOLUTION AND DIVERSITY OF GREEN AND LAND PLANTS

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