Download evolution and diversity of woody and seed plants

‘OLUTION AND DIVERSITY OF VASCULAR PLANTS
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I
EVOLUTION AND DIVERSITY OF
WOODY AND SEED PLANTS
LlGNOPHYTESWOODY PLANTS
129
SPERMATOPHYTES—SEED PLANTS
Seed Evolution
Pollination Droplet
Pollen Grains
Pollen Tube
Ovule and Seed Development
Seed Adaptations
Eustele
131
131
135
135
136
136
139
139
DIVERSITY OF WOODYAND SEED PLANTS
139
Archeopteris
“Pteridosperms”—”Seed Ferns”
Gymnospermae—Gymnosperms
Cycadophyta—Cycads
Cycadaceae
Zamiaceae
139
139
140
140
141
142
LIGNOPHYTIES—WOODY PLANTS
The lignophytes, or woody plants (also called Lignophyta),
are a monophyletic lineage of euphyllous vascular plants that
share the derived features of a vascular cambium, which
gives rise to wood, and a cork cambium, which produces
cork (Figures 5.1, 5.2). Growth of the vascular and cork
cambia is called secondary growth because it initiates after
the vertical extension of stems and roots due to cell expansion
(primary growth). A vascular cambium is a sheath, or hollow
cylinder, of cells that develops within the stems and roots as a
continuous layer, between the xylem and phloem in extant,
eustelic spermatophytes (see later discussion). The cells of
the vascular cambium divide mostly tangentially (parallel to
a tangential plane), resulting initially in two concentric layers
of cells (Figure 5.3A). One of these layers remains as the vas
cular cambium and continues to divide indefinitely; the other
layer eventually differentiates into either secondary xylem
wood, if produced to the inside of the cambium, or secondary
phloem, if produced to the outside (Figure 5.3A,B). Because
Ginkgophyta
Ginkgoaceae
Coniferae—Conifers
Pinopsida
Pinaceae
Cupressopsida
Araucariaceae
Cupressaceae
Podocarpaceae
Taxaceae
Gnetales
Ephedraceae
REVIEW QJESTIONS
160
EXERCISES
160
REFERENCES FOR FURTHER STUDY
161
WEB SITES
162
layers of cells are produced both to the inside and outside of
a continuously generated cambium, this type of growth is
termed bifacial. Generally, much more secondary xylem is
produced than secondary phloem. [Note that a secondary
cambium independently evolved in fossil lineages within the
lycophytes (e.g., Lepidodendron) and equisetophytes (e.g.,
Calamites), but this cambium was unifacial, producing sec
ondary xylem (wood) to the inside but no outer secondary
phloem, likely limiting in terms of an adaptive feature.]
Secondary growth results in an increase of the width or girth
of stems and roots (Figures 5.3B, 5.4). This occurs both by
expansion of the new cells generated by the cambium and by
accompanying radial divisions, increasing the number of cells
within a given growth ring. Many woody plants have regular
growth periods, e.g., forming annual rings of wood (Figure 5.4).
A cork cambium is similar to a vascular cambium, only it
differentiates near the periphery of the stem or root axis. The
cork cambium and its derivatives constitute the periderm
(referred to as the outer bark). The outermost layer of the
periderm is cork (Figure 5.3B). Cork cells contain a waxy
129
© 2010 Elsevier Inc. All rights reserved.
doi: 10.101 61B978-0- 12-374380-000005.2
144
145
145
148
148
151
151
151
154
154
156
157
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130
CHAPTER 5
UNIT II
EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS
-
EVOLUTION AND DIVERSITY OF PLANTS
classification and identification of woody plants. Wood ana
tomical features may also be used to study the past, a spe
cialty known as dendrochronolOgy (see Chapter 10).
Another feature of lignophytes is that they possess ances
shoot
trally monopodial growth, in which a single main
(see
develops branches from lateral (usually axillary) buds
to
presumed
is
growth
monopodial
Chapters 4, 9). Although
it
monilophyte—lignophyte
split,
have arisen prior to the
forming
of
enabled woody plants in particular the capability
extensive (sometimes massive) woody branching systems,
permitting them to survive and reproduce more effectively.
Lignophyta (Woody Plants)
Spermatophyta (Seed Plants)
Gymnospermae (Gymnosperms)
Coniferae (Conifers)
Cupressopsida
t
C
a
a)
4.,
a
C:
a)
S
o
a)
a)
S
o
0
o
S
a)
C
is
.sç
C)
4.
a)
a)
S
S
—
0
a)
a)
S
t
:t
a)
a)
oO
— (/,
a)
—,
I
0-I.
C
a)
r Gnetales
S
a)
C
o
—
-5c
C
-
a)
S
Ct
Ct
0
rj
•0
.
‘1
5
0
(/D
+
epimatium
SPERMATOPHYTESSEED PLANTS
F
receptacle
porose
-
iiiiifiiiii 1 ovule/scale
/
-
pollen tube—sperm nonmotile (siphonogamy)
iiiiiiiifiiii leaves simple
I
c
V
-,
1g.,
W
I
-
:14
— s eustele
— — pollen tube—sperm motile (zooidogamy)
— — endosporic, male gametophyte = pollen grain
— — pollination droplet
— — integument with micropyle
— — retention of megaspore within megasporangium
— — reduction to 1 megaspore per megasporangium
— — endosporic female gametophyte
=extincttaxon
= extinct lineage
— — heterospory
— — cork cambium (periderm)
—
vascular cambium (secondary vascular tissue,
SEED
(embryo
SEED EVOI.,UTION
The
The evolution of the seed involved several steps.
or more
exact sequence of these is not certain, and two
concomitantly
occurred
“steps” in seed evolution may have
in seed
and be functionally correlated. The probable steps
evolution are as follows (Figure 5.6):
a,,
-
+
nutntive tissue
+
integuments)
lIst.
r
pa-.
•‘
,41
t
wood)
.—
-‘-
.
-a
-
-
-
FIGURE 5.1 Cladogram of the woody and seed plants. Major apomorphies are indicated beside a thick hash mark. Families in bold are
described in detail. Modified from Bowe et al. (2000); Chaw et al. (2000); Frohlich et al. (2000); and Samigullin et al. (1999).
polymer called suberin (similar to cutin) that is quite resistant to water loss (see Chapter 10).
The vascular cambium and cork cambium constituted major
evolutionary novelties. Secondary xylem, or wood, functions
in structural support, enabling the plant to grow tall and acquire
massive systems of lateral branches. Thus, the vascular cambium was a precursor to the formation of intricately branched
shrubs or trees with tall overstory canopies (e.g., Figure 5.2),
a significant ecological adaptation. Cork produced by the cork
cambium functions as a thick layer of cells that protects
the delicate vascular cambium and secondary phloem from
mechanical damage, predation, and desiccation.
Wood anatomy can be quite complex. The details of
cellular structure are important characters used in the
—
or
The spermatophyta, commonly called spermatophytes
lignophytes
the
within
seed plants, are a monophyletic lineage
this
(Figure 5.1). The major evolutionary novelty that unites
is an
group is the seed. A seed is defined as an embryo, which
sur
immature diploid sporophyte developing from the zygote,
coat
seed
a
by
enveloped
rounded by nutritive tissue and
immature
(Figure 5.5). The embryo generally consists of an
called the
root called the radicle, a shoot apical meristem
cotyledons;
epicotyl, and one or more young seed leaves, the
called the
is
stem
the transition region between root and
prior to
hypocotyl (Figures 5.5, 5.10). An immature seed,
fertilization, is known as an ovule.
1.
!%:
md.
%H
•4L
I’
t
131
Et :rLct
Composite
massive, nonclonal
giant sequoia, a woody conifer that is the most
FIGURE 5.2
organism on Earth, and among the tallest of trees.
HeterospOry. Heterospory is the formation of two types
large,
of haploid spores within two types of sporangia:
meiosis
via
fewer-numbered megaspores, which develop
numerous
in the megasporangium, and small, more
microsporafl
the
in
meiosis
microsporeS, the products of
which a
gium (Figures 5.6, 5.7). The ancestral condition, in
Each
single spore type forms, is called “homosporyY
megaspore develops into a female gametophyte that bears
game
only archegonia; a microspore develops into a male
heterospory
Although
tophyte, bearing only antheridia.
plants
was prerequisite to seed evolution, there are fossil
among
that were heterosporous but had not evolved seeds,
5.1 3A;
5.1,
(Figures
these being species of Archeopteris
evolved
has
see later discussion). Note that heterospory
extant
independently in other, nonseed plants, e.g., in the
ferns
lycophytes Selaginella and Isoetes and in the water
(Chapter 4).
AND DIVERSITY OF WOODY AND SEED PLANTS
—
Lignophyta (Woody Plants)
—
Conjferae (Conifers)
1
1
r
Gnetales
-,
a:
a.)
C.)
a:
a:
—
a:
a.)
C.)
a:
a:
C.?
.
—
1
—
—
Cupressopsida
a.)
a.)
a.)
a.)
—
a:
a:
a
C
—
EVOLUTION AND DIVERSITY OF PLANTS
131
classification and identification of woody plants. Wood ana
tomical features may also be used to study the past, a spe
cialty known as dendrochronology (see Chapter 10).
Another feature of lignophytes is that they possess ances
trally monopodial growth, in which a single main shoot
develops branches from lateral (usually axillary) buds (see
Chapters 4, 9). Although monopodial growth is presumed to
have arisen prior to the monilophyte—lignophyte split, it
enabled woody plants in particular the capability of forming
extensive (sometimes massive) woody branching systems,
permitting them to survive and reproduce more effectively.
—
—
Spermatophyta (Seed Plants)
Gymnospermae (Gymnosperms)
-
UNIT II
a:
C
-a:
C.)
a.,)
a:
c_)
SPERMATOPHYTES—SEED PLANTS
aril
The Spermatophyta, commonly called spermatophytes or
seed plants, are a monophyletic lineage within the lignophytes
(Figure 5.1). The major evolutionary novelty that unites this
group is the seed. A seed is defined as an embryo, which is an
immature diploid sporophyte developing from the zygote, sur
rounded by nutritive tissue and enveloped by a seed coat
(Figure 5.5). The embryo generally consists of an immature
root called the radicle, a shoot apical meristem called the
epicotyl, and one or more young seed leaves, the cotyledons;
the transition region between root and stem is called the
hypocotyl (Figures 5.5, 5.10). An immature seed, prior to
fertilization, is known as an ovule.
S
pollen tube—sperm nonmotjie (siphonogamy)
—
—
—
I
eustele
SEED EVOLUTION
The evolution of the seed involved several steps. The
exact sequence of these is not certain, and two or more
“steps” in seed evolution may have occurred concomitantly
and be functionally correlated. The probable steps in seed
evolution are as follows (Figure 5.6):
pollen tube—sperm motile (zooidogamy)
endosporic, male gametophyte pollen grain
pollination droplet
integument with micropyle
retention of megaspore within megasporangjum
reduction to 1 megaspore per megasporangjum
endosporic female gametophyte
heterospory
1.
SEED
(embryo
+ nutritive tissue
+ integuments)
cork cambjum (periderm)
vascular cambjum (secondary vascular tissue, mci. wood)
plants. Major apomorphies are indicated beside a
thick hash mark. Families in bold are
10); Chaw et al. (2000); Frohljch et al. (2000);
and Samigullin et al. (1999).
is quite resist
flstituted major
‘ood, functions
tall and acquire
vascular cam
ately branched
shrubs or trees with tall overstory canopies (e.g., Figure 5.2),
a significant ecological adaptation. Cork produced by the
cork
cambium functions as a thick layer of cells that protects
the delicate vascular cambium and secondary phloem from
mechanical damage, predation, and desiccation.
Wood anatomy can be quite complex. The details of
cellular structure are important characters used in
the
FIGURI 5.2 Composite photograph ofSequoiadendrongiganteum,
giant sequoia, a woody conifer that is the most massive, nonclonal
organism on Earth, and among the tallest of trees.
Heterospory. Heterospory is the formation of two types
of haploid spores within two types of sporangia: large,
fewer-numbered megaspores, which develop via meiosis
in the megasporangium, and small, more numerous
microspores, the products of meiosis in the microsporan
glum (Figures 5.6, 5.7). The ancestral condition, in which a
single spore type forms, is called “homospory.” Each
megaspore develops into a female gametophyte that bears
only archegonia; a microspore develops into a male game
tophyte, bearing only antheridia. Although heterospory
was prerequisite to seed evolution, there are fossil plants
that were heterosporous but had not evolved seeds, among
these being species of Archeopteris (Figures 5.1, 5.13A;
see later discussion). Note that heterospory has evolved
independently in other, nonseed plants, e.g., in the extant
lycophytes Selaginella and Isoetes and in the water ferns
(Chapter 4).
132
CHAPTER 5
UNIT II
EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS
NTS
EVOLUTION AND DIVERSITY OF PLA
133
I
I
I
A
vascular
cambjum
2 xylem
FIGURE 5.4
year’s growth. B. Four years’ growth.
Woody stem cross-section, Pinus. sp. A. One
the complete development of,
2. Endospory. Endospory is
hyte within the original
in this case, the female gametop
condition, in which
spore wall (Figure 5.6). The ancestral
an external gameto
the spore germinates and grows as
ution of endosporic
phyte, is called exospory. The evol
with that of
female gametophytes was correlated
grains); see later
endosporic male gametophytes (pollen
discussion.
ber to one. Reduction of
3. Reduction of megaspore num
. First, the number
megaspore number occurred in two ways
undergo meiosis
of cells within the megasporangium that
aspore mother
(each termed a megasporocyte or meg
(Figure 5.6). This
cell) was reduced, from several to one
to four haploid
single diploid megasporocyte gives rise
aspores pro
meg
oid
hapl
megaspores. Second, of the four
ng only
leavi
t,
duced by meiosis, three consistently abor
megaspore also
one functional megaspore. This single
lated with the
undergoes a great increase in size, corre
2 phloem
radicle
periderrn
embryo
fluthtive tissue
B
(female gametophyle
or endosperm)
cotyledons
cork
(epidermis sloughed off to outside)
FIGURE 5.3
eusteljc stem.
A. Development of the vascular cambium. B. Development of secondary vascular tissue in the stem, illustrated here for a
FIGURE 5.5
trated here.
Morphology of a seed. Pinus sp. illus
resources in the
increased availability of space and
megasporangium.
ad of the megaspore
4. Retention of the megaspore. Inste
ancestral condi
being released from the sporangium (the
eed plants),
tion, as occurs in all homosporous nons
megasporangium
in seed plants it is retained within the
a reduction in
by
(Figure 5.6). This was accompanied
thickness of the megaspore wall.
opyle. The final
5. Evolution of the integument & micr
lopment of the
event in seed evolution was the enve
d the integu
calle
e,
megasporangium by a layer of tissu
s from the base of
ment (Figure 5.6). The integument grow
called a nucellus
the megasporangium (which is often
envelopes it,
when surrounded by an integument) and
ests that the
sugg
except at the distal end. Fossil evidence
lobes derived
integument likely evolved from separate
surrounded the
from telomes (ancestral branches) that
ovules prior to
megasporangium. These “preovules”, i.e.,
rim or ring of
a
the evolution of integuments, possessed
the lagenos
tissue at the apex of the megasporangium,
n grains to a pol
tome, which functioned to funnel polle
Rothwell 1993
and
art
lination chamber. (See, e.g., Stew
occurred with
for details.) The epitome of seed evolution
es to form the
the evolutionary “fusion” of the telom
completely sur
integument, a continuous sheath that
all extant seed
of
nt
rounds the nucellus. The integume
called the microplants has a small pore at the distal end
stral lagenostome
pyle. The micropyle replaced the ance
in angiosperms, of
as the site of entry of pollen grains (or
functions in the
also
pollen tubes). The micropyle
ation and resorp
mechanics of pollination droplet form
ument represents
tion (see below). Note that a single integ
s; in angiosperms
the ancestral condition of spermatophyte
(Chapter 6).
later
a second integument layer evolved
AND DIVERSITY OF WOODYAND SEED PLANTS
UNIT II
EVOLUTION AND DIVERSITY OF PLANTS
133
1
FIGURE 5.4
2’ xylem
vascular
cambium
vascular
cambium 2 phloem
l’phloem
2’ xylem
Woody stem cross-section, Pinus. sp. A. One year’s growth. B. Four years’ growth.
2. Endospory. Endospory is the complete development of,
in this case, the female gametophyte within the original
spore wall (Figure 5.6). The ancestral condition, in which
the spore germinates and grows as an external gameto
phyte, is called exospory. The evolution of endosporic
female gametophytes was correlated with that of
endosporic male gametophytes (pollen grains); see later
discussion.
3. Reduction of megaspore number to one. Reduction of
megaspore number occurred in two ways. First, the number
of cells within the megasporangium that undergo meiosis
(each termed a megasporocyte or megaspore mother
cell) was reduced, from several to one (Figure 5.6). This
single diploid megasporocyte gives rise to four haploid
megaspores. Second, of the four haploid megaspores pro
duced by meiosis, three consistently abort, leaving only
one functional megaspore. This single megaspore also
undergoes a great increase in size, correlated with the
1’ xylem
2’ phloem
seed coat
radicle
periderm
cortex
embryo
nutritive tissue
(female gametophyte
or endosperm)
epicotyl
cotyledons
cork
(epidermis sloughed off to outside)
ambium. B. Development of second vascular tissue in
the stem, illustrated here for a
FIGURE 5.5
Morphology of a seed. Pinus sp. illustrated here.
increased availability of space and resources in the
megasporangium.
4. Retention of the megaspore. Instead of the megaspore
being released from the sporangium (the ancestral condi
tion, as occurs in all homosporous nonseed plants),
in seed plants it is retained within the megasporangium
(Figure 5.6). This was accompanied by a reduction in
thickness of the megaspore wall.
5. Evolution of the integument & micropyle. The final
event in seed evolution was the envelopment of the
megasporangium by a layer of tissue, called the integu
ment (Figure 5.6). The integument grows from the base of
the megasporangium (which is often called a nucellus
when surrounded by an integument) and envelopes it,
except at the distal end. Fossil evidence suggests that the
integument likely evolved from separate lobes derived
from telomes (ancestral branches) that surrounded the
megasporangium. These “preovules”, i.e., ovules prior to
the evolution of integuments, possessed a rim or ring of
tissue at the apex of the megasporangium, the lagenos
tome, which functioned to funnel pollen grains to a pol
lination chamber. (See, e.g., Stewart and Rothwell 1993
for details.) The epitome of seed evolution occurred with
the evolutionary “fusion” of the telomes to form the
integument, a continuous sheath that completely sur
rounds the nucellus. The integument of all extant seed
plants has a small pore at the distal end called the micropyle. The micropyle replaced the ancestral lagenostome
as the site of entry of pollen grains (or in angiosperms, of
pollen tubes). The micropyle also functions in the
mechanics of pollination droplet formation and resorp
tion (see below). Note that a single integument represents
the ancestral condition of spermatophytes; in angiosperms
a second integument layer evolved later (Chapter 6).
134
CHAPTER 5
UNIT II
EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS
,.._-‘
antheridia
EVOLUTION AND DIVERSITY OF PLANTS
135
Sporophyte Body
(2n)
mitosis, growth, & differentiation
mitosis, growth, & differentiation
7
Embryo
(2n)
/
/
male gametophyte
(n)
microsporangium
(2N)
t
rneiosis
——fertilization
/‘
I)
‘jN
Egg
(n)
SpOraflgium
female gametophyte
(lost in the Angiosperms
& some Gnetales)
{
k
1.
Sperm (sperm nonflagellate in
(n) j Conifers (mci. Gnetales)
and Angiosperms)
3’
0
megasporangium
megasporangiurn
2. Endospory
3. Reduction to 1 megaspore
megaspore
gametophyte
megasporangium
4. Retention of megaspore
FIGURE 5.6
meioszs
Microspores
(n)
(
——
Jn
Megaspores..)
(n)
1/
mitosis, growth, & differentiation
Male Gametophyte
(n)
archegonia
female gametophyte
(contained in megaspore)
——
mitosis, growth, & derentiaiion
megasporangium
wall
©
©
Archegonium Antheridium 1 (reduced to absent in
(n)
(n)
extant seed plants)
Female Gametophyte
(n)
archegonia
/
Megasporocyte
(2n)
GAMETOPHYTE GENERATION
(N)
)
1. Heterospory
‘\
Microsporocyte
(2n)
SPOROPHYTE GENERATION
Zygote
(2n)
\
\
mitosis, growth, & d(fferentianon
mitosis, growth, & differentiation
gametophyte
(n)
Microsporangium Megasporangium
(2n)
(2n)
5. Evolution of Integument & Micropyle
Ovule and seed evolution in the spermatophytes (hypothetical, for purpose of illustration).
FIGURE 5.7
Life cycle of heterosporous seed plants.
POLLINATION DROPLET
One possible evolutionary novelty associated with seed evo
lution is the pollination droplet. This is a droplet of liquid
that is secreted by the young ovule through the micropyle
(Figures 5.1OA, 5.171). This droplet is mostly water plus
some sugars or amino acids and is formed by the breakdown
of cells at the distal end of the megasporangium (nucellus).
The cavity formed by this breakdown of cells is called the
pollination chamber (Figure 5. bA). The pollination drop
let functions in transporting pollen grains through the micropyle. This occurs by resorption of the droplet, which “pulls”
pollen grains that have contacted the droplet into the pollina
tion chamber. It is unknown whether a pollination droplet
was present in the earliest seed plants. However, the presence
of a pollination droplet in many nonflowering seed plants
suggests that its occurrence may be apomorphic for at least
the extant seed plant lineages. Note that the ovules of
angiosperms lack pollination droplets or pollination cham
bers, as flowering plants have evolved a different mechanism
of pollen grain transfer (see Chapter 6).
POLLEN GRAINS
Concomitant with the evolution of the seed was the evolution of
pollen grains (Figure 5.8). A pollen grain is, technically, an
immature, endosporic male gametophyte. Endospory in pollen
grain evolution was similar to the same process in seed evolu
tion, involving the development of the male gametophyte within
the original spore wall. Pollen grains of seed plants are extremely
reduced male gametophytes, consisting of only a few cells.
They are termed “immature” male gametophytes because, at the
time of their release, they have not fully differentiated.
After being released from the microsporangium, pollen must
be transported to the micropyle of the ovule (or, in angio
sperms, to the stigmatic tissue of the carpel; see Chapter 6) in
order to ultimately effect fertilization. Wind dispersal, in com
bination with an ovule pollination droplet (see later discus
sion), was probably the ancestral means of pollen transport.
After being transported to the ovule (or stigmatic tissue), the
male gametophyte completes development by undergoing
additional mitotic divisions and differentiation. The male
gametophyte grows an exosporic pollen tube, which functions
134
CHAPTER 5
UNIT II
EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS
,.._-‘
antheridia
EVOLUTION AND DIVERSITY OF PLANTS
135
Sporophyte Body
(2n)
mitosis, growth, & differentiation
mitosis, growth, & differentiation
7
Embryo
(2n)
/
/
male gametophyte
(n)
microsporangium
(2N)
t
rneiosis
——fertilization
/‘
I)
‘jN
Egg
(n)
SpOraflgium
female gametophyte
(lost in the Angiosperms
& some Gnetales)
{
k
1.
Sperm (sperm nonflagellate in
(n) j Conifers (mci. Gnetales)
and Angiosperms)
3’
0
megasporangium
megasporangiurn
2. Endospory
3. Reduction to 1 megaspore
megaspore
gametophyte
megasporangium
4. Retention of megaspore
FIGURE 5.6
meioszs
Microspores
(n)
(
——
Jn
Megaspores..)
(n)
1/
mitosis, growth, & differentiation
Male Gametophyte
(n)
archegonia
female gametophyte
(contained in megaspore)
——
mitosis, growth, & derentiaiion
megasporangium
wall
©
©
Archegonium Antheridium 1 (reduced to absent in
(n)
(n)
extant seed plants)
Female Gametophyte
(n)
archegonia
/
Megasporocyte
(2n)
GAMETOPHYTE GENERATION
(N)
)
1. Heterospory
‘\
Microsporocyte
(2n)
SPOROPHYTE GENERATION
Zygote
(2n)
\
\
mitosis, growth, & d(fferentianon
mitosis, growth, & differentiation
gametophyte
(n)
Microsporangium Megasporangium
(2n)
(2n)
5. Evolution of Integument & Micropyle
Ovule and seed evolution in the spermatophytes (hypothetical, for purpose of illustration).
FIGURE 5.7
Life cycle of heterosporous seed plants.
POLLINATION DROPLET
One possible evolutionary novelty associated with seed evo
lution is the pollination droplet. This is a droplet of liquid
that is secreted by the young ovule through the micropyle
(Figures 5.1OA, 5.171). This droplet is mostly water plus
some sugars or amino acids and is formed by the breakdown
of cells at the distal end of the megasporangium (nucellus).
The cavity formed by this breakdown of cells is called the
pollination chamber (Figure 5. bA). The pollination drop
let functions in transporting pollen grains through the micropyle. This occurs by resorption of the droplet, which “pulls”
pollen grains that have contacted the droplet into the pollina
tion chamber. It is unknown whether a pollination droplet
was present in the earliest seed plants. However, the presence
of a pollination droplet in many nonflowering seed plants
suggests that its occurrence may be apomorphic for at least
the extant seed plant lineages. Note that the ovules of
angiosperms lack pollination droplets or pollination cham
bers, as flowering plants have evolved a different mechanism
of pollen grain transfer (see Chapter 6).
POLLEN GRAINS
Concomitant with the evolution of the seed was the evolution of
pollen grains (Figure 5.8). A pollen grain is, technically, an
immature, endosporic male gametophyte. Endospory in pollen
grain evolution was similar to the same process in seed evolu
tion, involving the development of the male gametophyte within
the original spore wall. Pollen grains of seed plants are extremely
reduced male gametophytes, consisting of only a few cells.
They are termed “immature” male gametophytes because, at the
time of their release, they have not fully differentiated.
After being released from the microsporangium, pollen must
be transported to the micropyle of the ovule (or, in angio
sperms, to the stigmatic tissue of the carpel; see Chapter 6) in
order to ultimately effect fertilization. Wind dispersal, in com
bination with an ovule pollination droplet (see later discus
sion), was probably the ancestral means of pollen transport.
After being transported to the ovule (or stigmatic tissue), the
male gametophyte completes development by undergoing
additional mitotic divisions and differentiation. The male
gametophyte grows an exosporic pollen tube, which functions
136
CHAPTER 5
EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS
UNIT II
137
EVOLUTION AND DIVERSITY OF PLANTS
micropyle
integument (2n)
pollen
grains
pollen
archegonial chamber
pollination droplet
integument
-
S
A
mitosiS and
differentiation
ICL
FIGURE 5.8
Pollen grains—immature male gametophytes of seed plants. A. Zamia sp., a cycad. B. Ginkgo biloba. C. Pinus sp., a conifer.
as a haustorial organ, obtaining nutrition by absorption from the
surrounding sporophytic tissue (Figure 5.9; see Pollen Tabe).
POLLEN TUBE
The male gametophytes of all extant seed plants form a pollen
tube (Figure 5.9) soon after the pollen grains make contact
with the megasporangial (nucellar) tissue of the ovule. In
extant seed plants the ancestral type of pollen type (found in
cycads and ginkgophytes) was haustorial, in which the male
gametophyte feeds (like a parasite) off the tissues of the
nucellus. Motile sperm is delivered from this male gameto
phyte into a fertilization chamber, where the sperm swims to
the archegonium containing the egg, a process known as
zooidogamy (zoom, animal + gamos, marriage). In the coni
fers (including Gnetales), pollen tubes are also haustorial, but
deliver nonmotile sperm cells to the archegonium or egg, a
process known as siphonogamy (siphono, tube + gamos,
megasporangium
(nucellus)
(2n)
marriage). A type of siphonogamy evolved independently in
the angiosperms. In angiosperms, however, the pollen tubes
grow through stylar tissue prior to delivering the sperm to the
egg of a female gametophyte (see Chapter 6).
seed coat’
radicle
mitosis and
egg)
embryo
(new 2n)
epicotyl
(shoot apex)
cotyledons
female
gametophyte
(n)
female
gametophyte
(n)
B
megasporangium
megaspOrangiUm
(degenerate)
A. Ovule development in the nonflowering spermatophytes. B. Seed development.
these male gametophytes may live in the megasporangial tissue
for some time, generally several months to a year.
The functional megaspore greatly expands, accompanied
by numerous mitotic divisions, to form the endosporic
female gametophyte (Figures 5.1OA, 5.11B,C). In the seeds
of gymnosperms, archegonia differentiate at the apex of the
female gametophyte (Figure 5.11C,D). As in the nonseed
land plants, each archegonium has a large egg cell and a
short line of neck cells (plus typically a ventral canal cell or
nucleus). Eventually, the male gametophytes either release
motile sperm cells (in cycads and Ginkgo) into a cavity
between the megasporangium and female gametophyte
(known as the archegonial chamber; Figure 5.1 DA), or the
pollen tube of the male gametophyte delivers sperm cells
directly into the archegonial neck (in conifers). (Note that
germination &
sperm
mature male
gametophytes, each
with pollen tube
hypocotyl
megaSporangiUm
(nucellus)
(2n)
pollen tube
(haustorial)
pollen grain
(immature endosporic
male gametophyte)
female
gametophyte
(n)
functional
megaspore
(n)
micropyle
FIGURE 5.10
differentiation
(2n)
A
OVULE AND SEED DEVELOPMENT
After pollination, the megasporocyte develops within the
megasporangium of the ovule (Figures 5.1OA, 5.11A). The
megasporocyte is a single cell that undergoes meiosis, producing
a tetrad of four haploid megaspores, which in most extant seed
plants are arranged in a straight line, or linearly (Figure 5.IOA).
The three megaspores that are distal (away from the ovule base)
abort; only the proximal megaspore (near the ovule base) con
tinues to develop. In the pollination chamber, the resorbed
pollen grains (Figures 5. iDA, 5.1 1A) develop into mature male
gametophytes and form pollen tubes, which grow into the tissue
of the megasporangium (Figures 5. iDA, 5.1 1B). In gymnosperms
micropyle
archegonium
(with egg)
pollination
chamber
motile
sperm cell
FIGURE 5.9 Male gametophyte morphology and development in the nonflowering spermatophytes; Cycas sp., illustrated. (Reproduced
and modified from Swamy, B. G. L. 1948. American Journal of Botany 35: 77—88, by permission.)
1
the ovules of some Gnetales and all angiosperms lack arche
gonia.) The end result is that a sperm cell from the male
gametophyte fertilizes the egg of the female gametophyte. A
long period of time (perhaps a year or more) may ensue
between pollination, which is delivery of the pollen grains
to the ovu)e, and fertilization, actual union of sperm and
egg. Note: This is not true for the flowering plants, in which
fertilization generally occurs very soon after pollination (see
Chapter 6).
The resulting diploid zygote, once formed, undergoes
considerable mitotic divisions and differentiation, eventually
maturing into the embryo, the immature sporophyte (Figures
5.1DB, ShE). The tissue of the female gametophyte contin
ues to surround the embryo (Figure 5.11E) and serves as
nutritive tissue for the embryo upon seed germination (except
136
CHAPTER 5
EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS
UNIT II
137
EVOLUTION AND DIVERSITY OF PLANTS
micropyle
integument (2n)
pollen
grains
pollen
archegonial chamber
pollination droplet
integument
-
S
A
mitosiS and
differentiation
ICL
FIGURE 5.8
Pollen grains—immature male gametophytes of seed plants. A. Zamia sp., a cycad. B. Ginkgo biloba. C. Pinus sp., a conifer.
as a haustorial organ, obtaining nutrition by absorption from the
surrounding sporophytic tissue (Figure 5.9; see Pollen Tabe).
POLLEN TUBE
The male gametophytes of all extant seed plants form a pollen
tube (Figure 5.9) soon after the pollen grains make contact
with the megasporangial (nucellar) tissue of the ovule. In
extant seed plants the ancestral type of pollen type (found in
cycads and ginkgophytes) was haustorial, in which the male
gametophyte feeds (like a parasite) off the tissues of the
nucellus. Motile sperm is delivered from this male gameto
phyte into a fertilization chamber, where the sperm swims to
the archegonium containing the egg, a process known as
zooidogamy (zoom, animal + gamos, marriage). In the coni
fers (including Gnetales), pollen tubes are also haustorial, but
deliver nonmotile sperm cells to the archegonium or egg, a
process known as siphonogamy (siphono, tube + gamos,
megasporangium
(nucellus)
(2n)
marriage). A type of siphonogamy evolved independently in
the angiosperms. In angiosperms, however, the pollen tubes
grow through stylar tissue prior to delivering the sperm to the
egg of a female gametophyte (see Chapter 6).
seed coat’
radicle
mitosis and
egg)
embryo
(new 2n)
epicotyl
(shoot apex)
cotyledons
female
gametophyte
(n)
female
gametophyte
(n)
B
megasporangium
megaspOrangiUm
(degenerate)
A. Ovule development in the nonflowering spermatophytes. B. Seed development.
these male gametophytes may live in the megasporangial tissue
for some time, generally several months to a year.
The functional megaspore greatly expands, accompanied
by numerous mitotic divisions, to form the endosporic
female gametophyte (Figures 5.1OA, 5.11B,C). In the seeds
of gymnosperms, archegonia differentiate at the apex of the
female gametophyte (Figure 5.11C,D). As in the nonseed
land plants, each archegonium has a large egg cell and a
short line of neck cells (plus typically a ventral canal cell or
nucleus). Eventually, the male gametophytes either release
motile sperm cells (in cycads and Ginkgo) into a cavity
between the megasporangium and female gametophyte
(known as the archegonial chamber; Figure 5.1 DA), or the
pollen tube of the male gametophyte delivers sperm cells
directly into the archegonial neck (in conifers). (Note that
germination &
sperm
mature male
gametophytes, each
with pollen tube
hypocotyl
megaSporangiUm
(nucellus)
(2n)
pollen tube
(haustorial)
pollen grain
(immature endosporic
male gametophyte)
female
gametophyte
(n)
functional
megaspore
(n)
micropyle
FIGURE 5.10
differentiation
(2n)
A
OVULE AND SEED DEVELOPMENT
After pollination, the megasporocyte develops within the
megasporangium of the ovule (Figures 5.1OA, 5.11A). The
megasporocyte is a single cell that undergoes meiosis, producing
a tetrad of four haploid megaspores, which in most extant seed
plants are arranged in a straight line, or linearly (Figure 5.IOA).
The three megaspores that are distal (away from the ovule base)
abort; only the proximal megaspore (near the ovule base) con
tinues to develop. In the pollination chamber, the resorbed
pollen grains (Figures 5. iDA, 5.1 1A) develop into mature male
gametophytes and form pollen tubes, which grow into the tissue
of the megasporangium (Figures 5. iDA, 5.1 1B). In gymnosperms
micropyle
archegonium
(with egg)
pollination
chamber
motile
sperm cell
FIGURE 5.9 Male gametophyte morphology and development in the nonflowering spermatophytes; Cycas sp., illustrated. (Reproduced
and modified from Swamy, B. G. L. 1948. American Journal of Botany 35: 77—88, by permission.)
1
the ovules of some Gnetales and all angiosperms lack arche
gonia.) The end result is that a sperm cell from the male
gametophyte fertilizes the egg of the female gametophyte. A
long period of time (perhaps a year or more) may ensue
between pollination, which is delivery of the pollen grains
to the ovu)e, and fertilization, actual union of sperm and
egg. Note: This is not true for the flowering plants, in which
fertilization generally occurs very soon after pollination (see
Chapter 6).
The resulting diploid zygote, once formed, undergoes
considerable mitotic divisions and differentiation, eventually
maturing into the embryo, the immature sporophyte (Figures
5.1DB, ShE). The tissue of the female gametophyte contin
ues to surround the embryo (Figure 5.11E) and serves as
nutritive tissue for the embryo upon seed germination (except
138
CHAPTER 5
UNIT II
EVOLUTiON AND DIVERSITY OF WOODYAND SEED PLANTS
integument
integument
I
I’,
EVOLUTION AND DIVERSITY OF PLANTS
139
rchegonia
1, phloem
female
gametophyte
1xylern
gametophyte
.
V
/
4
cortex
7
a
pith
female
gametophyte
A
B
C
vascular
bundle
outside. B. Helianthus stem
FIGURE 5.12 Eustele. A. Diagram of eustele. Note single ring of vascular bundles, with xylem inside, phloem
associated fibers.
cross-section, an example of a eustele. C. Close-up of vascular bundle, showing xylem, phloem, and
/--
megasporocyte
female
gametophyte
r.
11
embryo
/
U-.
nuêleus
-1
-
•••
‘
.
‘\sIrile
‘.:;.
...
•
in the flowering plants; see Chapter 6). The megasporangium
(nucellus) eventually degenerates. The integument matures
into a peripheral seed coat, which may differentiate into
various hard andlor fleshy layers.
cells
E
FIGURE 5.11 Ovule and seed development, illustrated by Pinus sp. A. Young ovule, longitudinal-section, at time of pollination. Pollen
grains are pulled into micropyle by resorption of pollination droplet. Meiosis of the megasporocyte has yet to occur. B. Post-pollination,
showing development of the female gametophyte and haustorial pollen tube growth of the male gametophytes within tissue of megasporangium
(nucellus). C. Mature ovule, showing two functional archegonia within female gametophyte. D. Close-up of archegonia, each containing
a large egg cell with a surrounding layer of sterile cells and apical neck. E. Seed longitudinal-section, seed coat removed, showing embryo
and surrounding nutritive layer of female gametophytic tissue.
SEED ADAPTATIONS
The adaptive significance of the seed is unquestioned. First,
seeds provide protection, mostly by means of the seed coat,
from mechanical damage, desiccation, and often predation.
Second, seeds function as the dispersal unit of sexual repro
duction. In many plants the seed has become specially modi
fied for dispersal. For example, a fleshy outer seed coat layer
may function to aid in animal dispersal. In fact, in some plants
the seeds are eaten by animals, the outer fleshy layer is
digested, and the remainder of the seed (including the embryo
protected by an inner, hard seed coat layer) passes harmlessly
through the gut of the animal, ready to germinate with a built
in supply of fertilizer. In other plants, differentiation of the
seed coat into one or more wings functions in seed dispersal
by wind. Third, the seed coat may function in dormancy
mechanisms that ensure germination of the seed only under
ideal conditions of temperature, sunlight, or moisture. Fourth,
upon germination, the nutritive tissue surrounding the embryo
provides energy for the young seedling, aiding in successful
establishment.
Interestingly, in seed plants the female gametophyte (which
develops within the megaspore) remains attached to and
nutritionally dependent upon the sporophyte. This is exactly
the reverse condition as is found in the liverworts, homworts,
and mosses (Chapter 3).
EUSTELE
In addition to the seed, an apomorphy for spermatophytes is
the eustele (Figure 5.12). A eustele is a primary stem vascu
lature (“primary” meaning prior to any secondary growth)
that consists of a single ring of discrete vascular bundles.
Each vascular bundle contains an internal strand of xylem
and an external strand of phloem that are radially oriented,
i.e., positioned along a radius (Figure 5.12).
The protoxylem of the vascular bundles of a eustele is
endarch in position, i.e., toward the center of the stem. This is
distinct from the exarch protoxylem of the lycophytes and the
mesarch protoxylem of most monilophytes (Chapter 4) and of
some fossil relatives that diverged prior to the seed plants.
DIVERSITY OF WOODY
AND SEED PLANTS
ARCHEOPTEPJS
A well-known lignophyte that lacked seeds was the fossil plant
Archeopteris (not to be confused with the very famous fossil,
reptilian bird Archeopteryx). Archeopteris was a large tree, with
wood like a conifer but leaves like a fern (Figure 5.13A,B).
Sporangia, producing spores, were born on fertile branch
systems. Some species of Archeopteris were heterosporous.
“PTERIDOSPERMS”—”SEED FERNS”
The “pteridosperms,” or “seed ferns:’ are almost certainly
a paraphyletic group of fossil plants that had femlike foliage, yet
bore seeds. Medullosa is a well-known example of a seed fern
138
CHAPTER 5
UNIT II
EVOLUTiON AND DIVERSITY OF WOODYAND SEED PLANTS
integument
integument
I
I’,
EVOLUTION AND DIVERSITY OF PLANTS
139
rchegonia
1, phloem
female
gametophyte
1xylern
gametophyte
.
V
/
4
cortex
7
a
pith
female
gametophyte
A
B
C
vascular
bundle
outside. B. Helianthus stem
FIGURE 5.12 Eustele. A. Diagram of eustele. Note single ring of vascular bundles, with xylem inside, phloem
associated fibers.
cross-section, an example of a eustele. C. Close-up of vascular bundle, showing xylem, phloem, and
/--
megasporocyte
female
gametophyte
r.
11
embryo
/
U-.
nuêleus
-1
-
•••
‘
.
‘\sIrile
‘.:;.
...
•
in the flowering plants; see Chapter 6). The megasporangium
(nucellus) eventually degenerates. The integument matures
into a peripheral seed coat, which may differentiate into
various hard andlor fleshy layers.
cells
E
FIGURE 5.11 Ovule and seed development, illustrated by Pinus sp. A. Young ovule, longitudinal-section, at time of pollination. Pollen
grains are pulled into micropyle by resorption of pollination droplet. Meiosis of the megasporocyte has yet to occur. B. Post-pollination,
showing development of the female gametophyte and haustorial pollen tube growth of the male gametophytes within tissue of megasporangium
(nucellus). C. Mature ovule, showing two functional archegonia within female gametophyte. D. Close-up of archegonia, each containing
a large egg cell with a surrounding layer of sterile cells and apical neck. E. Seed longitudinal-section, seed coat removed, showing embryo
and surrounding nutritive layer of female gametophytic tissue.
SEED ADAPTATIONS
The adaptive significance of the seed is unquestioned. First,
seeds provide protection, mostly by means of the seed coat,
from mechanical damage, desiccation, and often predation.
Second, seeds function as the dispersal unit of sexual repro
duction. In many plants the seed has become specially modi
fied for dispersal. For example, a fleshy outer seed coat layer
may function to aid in animal dispersal. In fact, in some plants
the seeds are eaten by animals, the outer fleshy layer is
digested, and the remainder of the seed (including the embryo
protected by an inner, hard seed coat layer) passes harmlessly
through the gut of the animal, ready to germinate with a built
in supply of fertilizer. In other plants, differentiation of the
seed coat into one or more wings functions in seed dispersal
by wind. Third, the seed coat may function in dormancy
mechanisms that ensure germination of the seed only under
ideal conditions of temperature, sunlight, or moisture. Fourth,
upon germination, the nutritive tissue surrounding the embryo
provides energy for the young seedling, aiding in successful
establishment.
Interestingly, in seed plants the female gametophyte (which
develops within the megaspore) remains attached to and
nutritionally dependent upon the sporophyte. This is exactly
the reverse condition as is found in the liverworts, homworts,
and mosses (Chapter 3).
EUSTELE
In addition to the seed, an apomorphy for spermatophytes is
the eustele (Figure 5.12). A eustele is a primary stem vascu
lature (“primary” meaning prior to any secondary growth)
that consists of a single ring of discrete vascular bundles.
Each vascular bundle contains an internal strand of xylem
and an external strand of phloem that are radially oriented,
i.e., positioned along a radius (Figure 5.12).
The protoxylem of the vascular bundles of a eustele is
endarch in position, i.e., toward the center of the stem. This is
distinct from the exarch protoxylem of the lycophytes and the
mesarch protoxylem of most monilophytes (Chapter 4) and of
some fossil relatives that diverged prior to the seed plants.
DIVERSITY OF WOODY
AND SEED PLANTS
ARCHEOPTEPJS
A well-known lignophyte that lacked seeds was the fossil plant
Archeopteris (not to be confused with the very famous fossil,
reptilian bird Archeopteryx). Archeopteris was a large tree, with
wood like a conifer but leaves like a fern (Figure 5.13A,B).
Sporangia, producing spores, were born on fertile branch
systems. Some species of Archeopteris were heterosporous.
“PTERIDOSPERMS”—”SEED FERNS”
The “pteridosperms,” or “seed ferns:’ are almost certainly
a paraphyletic group of fossil plants that had femlike foliage, yet
bore seeds. Medullosa is a well-known example of a seed fern