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Evolution of Land Plants
Artist’s rendering of Carboniferous forest in a tropical river delta. Most of the plants depicted here were nonseed tracheophytes 10 to 20 meters tall. In the distance, early seed plants up to 40 meters tall towered over the forest.
Prior to the origin and diversitication of green plants in the mid-Silurian (~450 mya), multicellular life was virtually entirely adapted to, and confined to, aquatic lifestyles
Continental land Masses virtually unoccupied by multicellular organism – multicellular-based ecosystems constituted tremendous potential for adaptive radiation
Terrestrial life in a gaseous medium required evolutionary solutions to structural, physiological and ecological challenges; Many of these innovations can be regarded as exaptations of pre-existing traits of green algae from which green plants diverged
Sources: Freeman (2002), Purves et al (2001)
Kingdom Plantae is monophyletic assemblage, descended from Green algae
Chlorophyll a and b are homogous in Chlorophyta, Charaphyta and Plantae
Currently includes over 250,000 described species classified in 12 monophyletic phyla
Most are terrestrial, some are secondarily aquatic
Diversification in plants involved successive adaptive radiations following evolution of key innovations that increased efficiency in a gaseous (air) and solid (Earth) environment
Major problems were posed by gravity and by water loss/availability
-maintain body structure in air (and with increasing body size) – resist gravity-obtain, transport and retain water-fertilize eggs and produce and protect embryos
Major evolutionary innovations included
-dimorphic body-waxy cuticle and stomata- true vascular tissue; -life history dominated by sporophyte generation -jacketed sex organs; antheridia and -archegonia-seeds; embryo with nutritive tissue in protective covering-flowers; vehicles for pollination strategies
Green algae of the lineage that includes Chara sp., are most likely ancestors of plants. Copyright BPS.
Ie, “true” vasular tissue
Sources: Freeman (2002), Purves et al (2001)
Sources: Freeman (2002), Purves et al (2001)
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
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Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
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pe r
ms
a bun
dan
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ymn
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a bun
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1st megafossils (small club mosses and other extinct groups 1st lycopod leave
1st evidence of land plants
Sources: Freeman (2002), Purves et al (2001)
A primer on terminology associated with vascular tissue
Vascular tissue; tissue involved in transport of materials (water, minerals…) in plants -- we’re focusing on water transport in this discussion
Tracheids specialized cells for water transport; first “true” vascular cells.
Tracheophytes are plants with Tracheids
Mosses have cells that have a vascular function, hence its not entirely correct to refer to them as “nonvascular”, but they are definitely “non-tracheophyes”, as are liverworts and hornworts
Tracheophytes have two vascular tissue types, each composed of several cell types
xylem; transports water and dissolved minerals; cell types are tracheids and, in flowering plants, vessel elements
phloem; transports sugars, ie, products of photosynthesis; minerals too; sieve-tube members and companion cells
Mature vessel elements in Zea mays; open perforation plate allows vessel to act as a water pipe. SEM. Copyright John N. A. Lott/BPS.
tracheids Vessel elements
Sources: Freeman (2002), Purves et al (2001)
A primer on reproductive cells, embryos, seeds, and life cycles
Alternation of generations occurs in all plant life cycles, but not all Charorphyceans; this trait evolved independently as a derived characteristic of land plants
In some algae with alternation of generations, sporophytes and gametophytes look similar; not so in land plants – in all species sporophytes and gametophytes are very different morphologically
I
Marchantia, a common liverwort. The sporophytes are born within the tissues of the umbrella shaped structures that arise from the surface of the flat, green, creeping gametophyte
A primer on reproductive cells, embryos, seeds, and life cycles
spores are 1n cells that can divide mitotically, with no syngamy, to produce individuals
gametes are 1n cells that undergo syngamy to produce a (2n) multicellular individual
embryo embryonic plant that develop from zygotes within tissues of the female parent (which provides nutrients)
seed Unique structure in “seed plants”; key structure in domination of terrestrial habitats by plants; plant embryo and its food supply, packaged in a protective coat. gymosperms (conifers and their relatives) produce “naked” seeds in that they are not born in protective chambers. angiosperms produce seeds
alternation of generations
Embryo of Marchantia, a liverwort
Embryo of shepherd’s purse an angiosperm
Three ways of expressing (the same!!) phylogenetic relationships among major plant lineages
Cladogram to the left is particularly useful in that it shows:
-extinct non-seed tracheophyte lineage Rhyniopsida
-origin of important innovations, including independent origins --homoplasies
Sources: Freeman (2002), Purves et al (2001)
Rhynie in the Grampian Region of Scotland has become famous as one of the most important palaeobotanical localities in the world. In 1912 the Scottish geologist William Mackie discovered an occurrence of Lower Devonian plant-bearing cherts near this small village. Recent radiometric datings of these rocks give an age of396 ± 12 million years
Rhynie Chert cut perpendicularly to the stratification showing successive horizons
Nature 389, 33 - 39 (1997) © Macmillan Publishers Ltd.
The origin and early evolution of plants on land
PAUL KENRICK AND PETER R. CRANE
the Swedish Museum of Natural History, Box 50007, S-10405, Stockholm, SwedenThe Field Museum, Roosevelt Road at Lake Shore Drive,
Chicago, Illinois60605, Department of the Geophysical Sciences, University of Chicago, USA.
The origin and early evolution of land plants in the mid-Palaeozoic era, between about 480 and 360 million years ago, was an important event in the history of life, with far-reaching consequences for the evolution of terrestrial organisms and global environments. A recent surge of interest, catalysed by palaeobotanical discoveries and advances in the systematics of living plants, provides a revised perspective on the evolution of early land plants and suggests new directions for future research.
Our current understanding of Plant phylogenetic relationships is based on analysis of both morphological and molecular characters
•SSU rRNA and Rubisco
•presence/absence of vascular tissue, leaves, seeds…
The broad picture of plant macroevolutionary patterns includes…
•Divergenge of entire clade from green algae
•All Plantae lineages; cellulose-based cell walls, chlorophyll a and b, starch as storage molecule in chloroplasts
•Two of the three earliest lineages (non-tracheophytes) lack water transport cells; mosses have have limited numbers of them
•Seedless vascular plants (nonseed tracheophytes) have vascular tissue and leaves, but reproduce by making spores; no seeds
Gymnosperms and Angiosperms have vascular tissue, they have complex leaves, and they produce seeds.
Early lineages dependent on wet habitats and more recent ones not – adaptive radiations into mesic and xeric conditions
Rubisco catalyzes a reaction in the Calvin-Benson cycle of photosynthesis in which carbon from CO2 is added to a five-carbon chain.
Leafy liverwort (Lophocolea bidentata), showing overlapping leaves and creeping growth form. Copyright BPS.
Gametophytes of Sphagnum moss growing in a bog in central Newfoundland. Copyright BPS.
Leafy liverwort (Lophocolea bidentata), showing overlapping leaves andcreeping growth form. Copyright BPS.
Dense matt of mosses colonizes old lava in Iceland. Copyright BPS
Marchantia, a common liverwort. The sporophytes are born within the tissues of the umbrella shaped structures that arise from the surface of the flat, green, creeping gametophyte
Hornwort sporophytes. Unlike other Bryophytes, hornwort sporophytes are photosynthetic
Club Moss Big leaf maples hung with epiphytes, most of which are club mosses; Hoh Rain Forest, WA. Copyright BPS.
Order Lycopodiales: club moss (Lycopodium annotinum); Denali National Park and Preserve, AK. Copyright BPS.
Fern (Blechnum magellanicum) in rain forest of Chile's Patagonian coast. Copyright Alejandro Frid/BPS.
Tree ferns (Dicksonia antarctica), in a forest of Eucalyptus regnans; Australia. Copyright BPS.
Family Pinaceae: Pinus ponderosa, ponderosa pine, is widespread in the American West. Copyright BPS.
Family Pinaceae: Colorado blue spruce (Picea pungens). Copyright Pollock/BPS.
Simple determinate inflorescence of shooting star (Dodecatheon meadia). Copyright J. Robert Stottlemyer/BPS.
Family Arecaceae: desert fan palm (Washingtonia filifera); Colorado Desert, CA. Copyright Jon Mark Stewart/BPS.
Sources: Freeman (2002), Purves et al (2001)
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups and 1st lycopod leave
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
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ms
a bun
dan
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ymn
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460 Fragment of plant cuticleFirst evidence of plants
The Transition to Land involved evolution of three important steps
Preventing cell dessication
Transporting water from cells in plant body with direct access to water, to cells without
Transporting gametes by a mechanism other than floating or swimming in water
The earliest interval of plant evolution on our time scale has yielded fosils of:
-spores surrounded by tough membrane made of sporopollenin-sheets of a waxy material – cuticle-small tubes
fossilized spores in tetrads
Sources: Freeman (2002), Purves et al (2001)
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups
1st lycopod leave
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
rms
a bun
dan
t
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Leaf epiderm of dayflower (Commelina sp.), a monocot. LM. Copyright Alfred Owczarzak/BPS.
Stomata in leaf of corn, a monocot. Copyright John N. A. Lott/BPS
Cross-section of a modern leaf (eudicot)
Waxy cuticle solved one problem and caused another – it impeded uptake of carbon dioxide
modern plants have specialized cells that solve this problem; two specialized guard cells form a stoma (pore) that can be open or closed.
Stomata occur in some extant basal lineages -- hornworts and mosses
Stomata are known from 385 million years ago, from a fossil Rhyniopsid
Sources: Freeman (2002), Purves et al (2001)
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups
1st lycopod leave
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
rms
a bun
dan
t
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Stomata in leaf of corn, a monocot. Copyright John N. A. Lott/BPS
Early Devonian rhyniophyte fossils. f. Stomate with tworeniform guard cells (stippled
Key evolutionary innovations solved the problems of structural support and internal water transport
First land plants probably were low, sprawling
before evolution of rigid body support, algal-like tissue could not grow erect, resist gravity
Before evolution of internal water transport system, virtually entire plant had to communicate directly with water
Among communities of low, sprawling plant – growing horizontally in a sense, competition for space and light was probably intense –intense selection for erect growth forms
Transition to terrestrial life required
-internal water transport to high and dry parts of plant, against the force of gravity
-rigidity of body in order to resist forces of gravity and wind
Dense matt of mosses colonizes old lava in Iceland may be remniscent of early plant life forms – low and sprawling. Copyright BPS
Artist’s rendering of Carboniferous forest in a tropical river delta. Most of the plants depicted here were nonseed tracheophytes 10 to 20 meters tall. In the distance, earl seed plants up to 40 meters tall towered over the forest.
Sources: Freeman (2002), Purves et al (2001)
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups
1st lycopod leave
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
rms
a bun
dan
t
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Water-conducting xylem vessels in a cucumber. LM. Copyright J. Robert Waaland/BPS.
Mature vessel elements in Zea mays; open perforation plate allows vessel to act as a water pipe. SEM. Copyright John N. A. Lott/BPS.
Cross section of vascular bundles from a eudicot stem; nutrient-conducting phloem, water-conducting xylem. LM. Copyright James Solliday/BPS.
Modern day tracheids (left) and vessel elements (right)
Water transport and structural support
Rhynie chert (~400 mya) bears fossil plants in an erect, upright postion – fossils suggest many or most plants grew erect
Kenrick and Crane identifed four types of water-conducting cells in the Rhynie chert (not tracheids or vessel elements) – some with lignified rings that may have allowed plants to assume an upright life form (lignin; strong, lightweight 6-carbon polymer
By about 380 mya, fossil record bears tracheids; advanced water conducting cells found in all modern phyla of vascular plants. Later tracheids with extensive lignification and thickened secondary walls
Wood develops from “secondary xylem”, and arose independently in several lineages by about 380 mya. First trees appear in fossil record some 370 mya
Sources: Freeman (2002), Purves et al (2001)
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups
1st lycopod leave
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an -
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
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a bun
dan
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tracheids
vessels
pits
top; picture of some of the first tracheids 380 mya from the fossil record; diagram shows tracheids from modern plantbottom; photo of 50 my old vessels and diagram of modern vessels. Vessels are shorter and fatter than tracheids, and stacked end to end. Water flows up vessels directly, and laterally, from vessel to vessl, as well, through perforations in primary and secondary cell walls
above water-conducting cells from rhynie chert
Evolution of heterospory and male and female gametophytes
The earliest tracheophytes were homosporous.
Heterospory evolved multiple times in tracheophytes in early evolution of land plants (occurs in Rhyniophyte lineage)
Multiple evolutionary origins suggests heterospory affords selective advantages;
Later tracheophytes, especially seed plants, show ever greater specialiation of the heterosporous condition Homosporous plants have a single type of
spore that gives rise to a single type of gametophyte, which bears two types of sex organs
Heterosporous plants have two types of spores, each of which develops into male or female gametophytes, each of which produces eggs or sperm
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups and 1st lycopod leaves
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
rms
a bun
dan
t20
410
360
270
140
460First evidence of land plants
Protecting and transporting Embryos: Evolution of seed and pollination
All land plants have life cycles with alternation of generations.
Relationship between gametophyte and sporophyte varies among lineages in terms of size, dependence, dominance in life cycle
Mosses: sporophyte is small and depends on gametophyte for nutrition
Ferns; sporophyte is larger than gametophyte and is autotrophic (photosynthesizes its own food)
Seed plants (gymnosperms and angiosperms); sporophyte is dominant;
-male gametophyte is reduced to pollen grain-female gametophyte to a small structure that holds the egg or eggs
Gametophytes (green) of the star moss produce non-photosynthetic stalked sporophytes with caplike sporangia at their tips (Campbell 2002)
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups and 1st lycopod leaves
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
rms
a bun
dan
t20
410
360
270
140
460First evidence of land plants
Protecting and transporting Embryos: Evolution of pollen, pollination and seed and seed dispersal
Seed plants (gymnosperms and angiosperms); sporophyte is dominant;
-male gametophyte is reduced to pollen grain
-female gametophyte to a small structure that holds the egg or eggs
Selection favored male gametophytes highly reduced in size and encased in sporopollenin; pollen grains are the consequence of that bout of mutation and selection
Pollen grains survive for prolonged periods in dry environments without becoming dessicated
Pollen grains can be transported by wind, gravity or animals; through pollen grains, seed plants lost dependence on water for fertilization
Pollination. Carrion plant (left) has foul odor that attracts carrion flies. Hummingbird pollinated flower (middle) produces nectar. Many arge, complex flowers (eg sunflower, right) attract a host of species that act as pollinators
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups and 1st lycopod leaves
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
rms
a bun
dan
t20
410
360
270
140
460First evidence of land plants
Evolution of seeds and flowers
Seeds enclose and protects embryo, and has a stockpile of nutrients
Seeds are often attached to a structure that facilitates dispersal by wind, water or animals
dispersal by water
dispersal by animals
dispersal by wind
Extra slides
Sources: Freeman (2002), Purves et al (2001)
First extensive grassland
Archaefructus, an early flower
First Flowering Plant
Cones from Araucaria mirabilis, an early gymnsperm
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups
1st lycopod leave
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Seed fern leaves
Fragment of plant cuticle
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
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n
e xp
l os i
o n
Car
bon
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ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
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ms
a bun
dan
tG
ymn
o spe
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a bun
dan
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Cells in fossil stem of Rhynia major (extinct Phylum Rhyniophyta); late Silurian and Devonian. LM. Copyright Phil
Gates/BPS.
Spores in fossil Rhynia major; late Silurian and Devonian. LM. Copyright Phil Gates/BPS
Fossil stem of oldest known lycopod genus (Bothrodendron minutifolium); late Devonian. LM. Copyright Phil Gates/BPS.
Fossil frond of fern (Asterotheca arborescens); late Carboniferous. Copyright Barbara J. Miller/BPS.
Overview of the evolutionary history of plants
Consider five intervals of evolutionary history, each defined by at least one major evolutionary innovation
220
410
360
270
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460
First extensive grassland
First Flowering Plant
First nectar-drinking insects
First vessels
1st megafossils (small club mosses and other extinct groups and 1st lycopod leaves
1st seeds1st vascular plant leaves1st wood1st roots1st vascular tissue1st stomata
Ori
gin
of
lan
d p
lant
sS
ilur
i an-
Dev
o nia
n
e xp
l os i
o n
Car
bon
ifer
ous
(lyc
opod
s, s
eed
fern
s,
and
hor
seta
ils
abun
dan
t)
An
g io s
pe r
ms
a bun
dan
tG
ymn
o spe
rms
a bun
dan
t
20
410
360
270
140
460First evidence of land plants
Key Evolutionary Innovations and Trends in the Transition of Plants to Land
-reducing water loss; cuticle and stomata
-transporting water; vascular tissue and wood
-transporting gametes and prtoecting embryos; pollination mechanisms and nutrititive, protective seeds
Key evolutionary innovations and trends for capturing energy from sunlight
-Photosynthetic Pathways C3 and C4
-Crassulacean acid metabolism
-growth habits and life forms
Minimizing water loss and regulating gas exchange
Nature 389, 33 - 39 (1997) © Macmillan Publishers Ltd.
The origin and early evolution of plants on land
PAUL KENRICK AND PETER R. CRANE
the Swedish Museum of Natural History, Box 50007, S-10405, Stockholm, SwedenThe Field Museum, Roosevelt Road at Lake Shore Drive,
Chicago, Illinois60605, Department of the Geophysical Sciences, University of Chicago, USA.
The origin and early evolution of land plants in the mid-Palaeozoic era, between about 480 and 360
million years ago, was an important event in the history of life, with far-reaching consequences for
the evolution of terrestrial organisms and global environments. A recent surge of interest, catalysed
by palaeobotanical discoveries and advances in the systematics of living plants, provides a revised
perspective on the evolution of early land plants and suggests new directions for future research.
Figure 1 Morphological diversity among basal living land plants and potential land-plant sister groups. a,Coleochaete orbicularis (Charophyceae) gametophyte; magnification × 75 (photograph courtesy of L. E.Graham). b, Chara (Charophyoceae) gametophyte; magnification × 1.5 (photograph courtesy of M.Feist). c, Riccia (liverwort) gametophyte showing sporangia (black) embedded in the thallus; magnification× 5 (photograph courtesy of A. N. Drinnan). d, Anthoceros (hornwort) gametophyte showing nbranchedsporophytes; magnification × 2.5 (photograph courtesy of A. N. Drinnan). e, Mnium (moss) gametophyteshowing unbranched sporophytes with terminal sporangia (capsule); magnification × 4.5 (photographcourtesy of W. Burger). f, Huperzia (clubmoss) sporophyte with leaves showing sessile yellow sporangia;magnification × 0.8. g, Dicranopteris (fern) sporophyte showing leaves with circinate vernation;magnification × 0.08. h, Psilotum (whisk fern) sporophyte with reduced leaves and spherical synangia(three fused sporangia); magnification × 0.4. i, Equisetum (horsetail) sporophyte with whorled branches,reduced leaves, and a terminal cone; magnification × 0.4. j, Cycas (seed plant) sporophyte showing leavesand terminal cone with seeds; magnification × 0.05 (photograph courtesy of W. Burger).
Figure 2 a, Longitudinal section of part of a silicified early fossil gametophyte (Kidstonophyton discoides
from the Rhynie Chert). Antheridia (male sexual organs) are located on the upper surface of the branch;
magnification × 3.4. b, Longitudinal section of antheridium of Lyonophyton rhyniensis from the Rhynie
Chert; magnification × 40. c, Longitudinal section of archegonium (female sexual organ) of Langiophyton
mackiei from the Rhynie Chert; magnification × 80. a–c are from the Remy Collection (slides 200, 90 and
330), Abteilung Paläobotanik, Westfälische Wilhelms-Universität, Münster, Germany (photographs
courtesy of H. Hass and H. Kerp). d, Scanning electron micrograph of Tetrahedraletes medinensis
showing a spore tetrad of possible liverwort affinity from the Late Ordovician (photograph courtesy of W.
A. Taylor); magnification × 670.
Figure 3 Sporophyte diversity in Early Devonian rhyniophyte fossils. a, Cooksonia pertoniiapiculispora: sporophyte (incomplete proximally) with terminal sporangium15; magnification × 15. b,Tortilicaulis offaeus: sporophyte (incomplete proximally) with terminal sporangium81; magnification × 40.c. Tortilicaulis offaeus: sporophyte (incomplete proximally) with terminal bifurcating sporangium81;magnification × 30. d, Transverse section of sporangium showing thick wall and central spore mass;magnification × 70. e, Details of epidermis at rim of sporangium; magnification × 45. f, Stomate with tworeniform guard cells (stippled); magnification × 120.
Figure 5 Diversity of water-conducting cells (tracheids) in early land plants (median longitudinal sectionthrough cells, basal and proximal end wa. lls not shown; cells are 20–40 m diameter). a, Top,bryophyte hydroid; bottom, details of hydroid wall showing distribution of plasmodesmata-derivedmicropores (10–50 nm diameter; stipple)84. b, Top, S-type tracheid (fossil) of Rhyniopsida; bottom,details of S-type cell wall showing distribution of plasmodesmata-derived micropores (stipple) and'spongy' interior to thickenings19. c, Top, G-type tracheid (fossil) of basal extinct eutracheophytes, whichclosely resemble the tracheids of some living vascular plants; bottom, details of G-type cell wall showingpores distributed between thickenings19. d, Top, scalariform pitted P-type tracheid (fossil) typical oftrimerophyte grade plants (euphyllophytes); bottom, details of P-type cell wall showing pit chambers andsheet with pores that extends over pit apertures26.