Evolution of Seed Plants
Extraordinary success of the seed plants can be
attributed the “seed habit”,
This encompasses evolutionary trends and innovations
in life history traits; in life cycle
The particularly important structures in this aspect of
plant macroevolution are
-gametophytes
-pollen
-seeds
Seed plant life life cycles in
evolutionary context
Recent research indicates that ferns, by far the
most speciose lineage of seedless vascular plants,
are closest living relatives to seed plants
Tree fern in a Malaysian forest.
In nontracheophyte
lineages, such as mosses,
gametophyte is larger,
persists longer, and
supports, the sporophyte
In non-seed tracheophyte lineages,
such as ferns, the gametophyte and
sporophytes are unattached; the
gametophyte is reduced in terms of
size
In seed plant lineages,
gametophyte is highly reduced -- to
a small population of cells
Evolutionary trend in Plantae from “ancient” to “recent” lineages: gametophyte
reduction.
“Reduction” variously refers to;
reduced size and conspicuousness of gametophyte
reduced independence of gametophyte (support, nutrition)
Moss gametophytes with
capsule-topped sporophytes
growing from them. Copyright
Claudia Mills/BPS.
Above: Heart-shaped gametophyte of a fern;
hairlike rhizoids extend from the midrib region of
lower surface. SEM. Copyright John N. A.
Lott/BPS.
Below: Oak ferns (Gymnocarpium dryopteris),
understory of temperate rain forest; coast of
British Columbia, Canada. Copyright Alejandro
Frid/BPS.
Family Iridaceae: blue flag (Iris
missouriensis); Front Range, CO.
Copyright Pollock/BPS.
Sori on underside of a frond of western sword fern
(Polystichum munitum). Each sori consists of a cluster
of sporangia (spore producing organs) Copyright John
N. A. Lott/BPS.
Ecological and Evolutionary importance of seeds
lie in their adaptations to dessication-resistance,
dispersal and dormancy
•In seed, embryo is protected by an extra layer of
sporophyte tissue creating the ovule
•during seed development, this tissue hardens
to produce the seed coat
•seed coat also enhances dispersal ability
•Seeds have the physiological capability for
dormancy; the seed introduces a dormant phase into
the plant life cycle that allows the organism to delay
growth and physiological activity until environmental
conditions are favorable
Seed plants are heterosporous; the two
specialized spores produce two - male and female
-- gametophytes
Adaptive radiation of seed plants is related to a suite of
adaptations that collectively can be called the “seed habit”,
which begins, evolutionarily, with reduction of
megasporangium to a single functional megaspore
male and female gametophytes are highly reduced;
each consists of only a few cells
The entire male gametophytes --pollen grains -- travel
to female gametophytes by wind or pollinator - in
seedless plants, sperm moves to female
Female gametophyte develops within the ovule
•in angiosperms (“enclosed seed”), ovules are
completely enclosed in, and following fertilization,
seeds develop in, chambers called ovaries (fruit =
“ripened ovary”
•in gymnosperms, ovules are not completely enclosed
in chambers (ovaries);
“gymno-sperm”=‘naked seed”
origin and diversification of
seed plants
Sources: Raven et al (2002),Freeman (2002), Purves et al (2001)
20
First seed plants probaby arose about
425 mya, perhaps from from sporebearing “progymnosperms”
First extensive
grassland
•progymnosperms and modern
gymnosperms share several characters;
secondary xylem and phloem; leaves (in
some)
140
First Flowering
Plant
Progymnosperms(?)
•Since the "seed habit" begins with the
reduction to a single functional
megaspore in each megasporangium,
heterospory seems like a logical
intermediate step.
Mesozoic Era;
gymnosperms
dominate throughout
eraFirst nectar-drinking
insects
Permian period
conifers and
First vessels
cycads flourish
www.ucmp.berkeley.edu/seedplants/progymn
osperms.html
Carboniferous
(lycopods, seed ferns,
and horsetails
abundant)
280
360
•Many paleobotanists believe that the
seed plants evolved from heterosporous
Middle Devonian plants like the
progymnosperms.
Carboniferous period;
diversification; origin of
several gymnosperm
lineages including seed
ferns
Devonian Period;
earliest gymnosperm
fossils;
410
425st mya - first
1 megafossils (small club
seed plants
mosses and other extinct
groups. 1st lycopod leave
460
fossilized stump of Archaeopteris
1st evidence of land plants
fossil leaf of Archaeopteris
major lineages of seed plants
gymosperms
mostly cone-bearing seed plants;
the ovules of gymnosperms are not completely
enclosed by sporophyte tissue at pollination
conifer = “cone-bearer”.
A cone is a tight cluster of
scales or leaves specialized
for reproduction.
Pollen-producing male (staminate)
cones of ponderosa pine (Pinus
ponderosa).
megaspores and microspores
are produced in separate
seed and pollen cones
Female (ovulate) cones of
ponderosa pine before seeds
have been released.
Cones of ponderosa pine opened by heat
in a fire; wings of seeds being released
are visible
Wind transfers pollen from male to female
strobili of larch (Larix decidua
Pollen grains (male gametophytes) of red pine
(Pinus resinosa), each with two large wings.
Cloud of conifer pollen in springtime wind (most
conifers are wind-pollinated).
Cycadophyta
An african cycad,
Encephalartos transverosus
(Raven et al 2002)
Sago palm (Metroxylon
sagus), an edible cycad; New
Guinea. Copyright BPS.
Gynkophyta
Branch of female Ginkgo biloba with leaves and
ovules. Copyright Alfred Owczarzak/BPS.
Ginkgo biloba, the maidenhair
tree, with leaves ready to fall
Branch of male Ginkgo biloba with clusters
of pollen-producing microsporophylls.
Copyright BPS.
Welwitschia mirabilis, one of
the three genera of
gnetophytes (Raven et al
2002)
Gnetophyta
Ephedra viridis (Mormon tea or joint fir) with cones;
Arizona. Copyright Pollock/BPS.
Coniferophyta
Family Taxaceae: Pacific yew tree (Taxus
brevifolia); source of cancer-fighting drug
taxol.
Family Araucariaceae: the monkey
puzzle tree (Araucaria araucana); alpine
south-central Chile.
Family Pinaceae: Pinus
ponderosa, ponderosa pine, is
widespread in the American West.
Family Cupressaceae: California
juniper (Juniperus californica); Joshua
Tree National Park, CA.
Family Pinaceae: Colorado
blue spruce (Picea pungens).
Family Pinaceae:
Pinus aristata, the
bristlecone pine,
may live for
thousands of years.
Slash pines, Pinus
palustris, in
Florida, are
representativel of
the Coniferophyta,
the largest phylum
of gymnosperms
(Raven et al 2002)
Family Taxodiaceae: co astal
redwood (Sequoia sempervirens)
in a California national park.
the ovule is a
sporophytic structure; it
includes the
megasporangium, the
integument in which it
lies (integument will later
form seed coat) and the
tissue attaching
ingegument to
sporophyte
meiotic event in sporophytic
megasporangium gives rise to
megaspore which, through
mitosis,will give rise to female
gametophyte
female gametophyte forms a sex
organ - a female gametangium or
archegonium --in which it
produces eggs The archegonium
is enclosed in sporophytic tissue
that will later develop into seed
coat
The
the pollen grain is the male
gametophyte; as it
germinates, it produces pollen
tubes to reach the egg.
embryo in
protective seed
coat
life cycle of a pine,
illustrating
gymnosperm
reproduction and
alternation of
generations
Cones of ponderosa pine opened by heat in a fire;
wings of seeds being released are visible.
Fruit-like "berry" of yew (Taxus sp.).
20
Evolutionary perspective Angiosperm reproductive
biology
Decline of some woody plants; rise of herbaceous
plants
Extinction of some plant species
Expansion of extensive grasslands and deserts; decline of forests
140
Flowering plants continue to diversify
Spread of forests; flowering plant communities expand
Flowering plants dominant
Semi-tropical vegetation – flowering plants and conifers widespread
Rise of flowering plants
Carboniferous
(lycopods, seed ferns,
and horsetails
abundant)
270
360
410
460
Floral Organs: Structure and Function
•Flowers develop from compressed shoots, from
four whorls of modified leaves
•Flowers consist of four organs,
•Non-reproductive floral organs
•Sepals
•Petals
•Reproductive floral organs
•Stamens
•Carpels
•Reproductive organs contain sporangia –
chambers in which gametophytes develop:
•male gametophyte: pollen grain
•female gametophyte; embryo sac
Idealized Flower
Lily
Extra slides
Important trend across all intervals:
decreasing dependence over time on moist habitats. For example, sperm of mosses and ferns swim to accomplish fertilization, while gmnosperms and
angiosperms encase their sperm in pollen grains that are transported via the wind or insect.
Oldest interval
begins early in paleozoic some 475 million years ago and spans some 60 million years of evolutionary history. Origin of lant plants.
-fossils of sprores surrounded by a tought membratne
-fossils of sheets of wayx material called cuticle
-fossils of small tubes
Fossils of spores covered with the same tough material, sporopollenin membranes of modern plants. Sporopollenin membrantes help pollen and spores
resist damage from drying out; so this discovery sugest sthat the spores wer adapted to conditions o land.
Silurian-Devonian Explosion
410-360 mya. Macroscopic fossils from most of the majorplant lineages. Virtuall all of the adaptations that allow plants to coccupy dry, terrestrial
habitats are present, including water conducting cells, roots and wood. To undersand how plants diversified, then, botanists must deermine the
relationshyips among the major groups, which diverged from one another 410-360 mya. Only then can they unravel the relationships of speceis within
these groups, which diverged much more recently.
Carboniferous Period
350-290 mya. Extensive coal deposits in sediments from about 350 to 290 mya. Coal is a carbon-rich rock packed with fossil spores, branches, leaves
and tree-trunks. These fossils are frequently derived from platns called lycopods. Because coal formation is thought to start only in the presence of
water, the Carboniferous fossils indicate the presen of extensive, forested swamps.
Gymosperm-dominated communit1es
250-210 mya. Fossil record from this time is dominated by members of the lineage collectivley called gymnosperms (naked seeds). These lineages
include cycads, confiers such as pines and spruces, and ginkgos. Because gymnosperms growreadily in dry habitats, biolgists infer that both wet and
dry environments on the continent became blanketed with green plants fro the furst time during this interval.
Current Angiosperm dominated world
Age fo flowering plants, or angiosperms (encased seeds). First flowering plants in the fossil record appear about 125 mya. The woody plants that
produced the first flowers are the ancestors of teoday’s grasses, orchids, daisies, oaks maples, and roses.