Plant Evolution
Chapter 21
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
21.1 Speaking for the Trees
 Forests
• Release oxygen
• Absorb water, and slowly release it
• Hold soil in place
• prevent erosion, flooding, and sedimentation
• Fuel and lumber
 Tropical Forests – Biological treasure
• Hold 50% o fall land-dwelling species for 10,000 years
Speaking for the Trees
 Deforestation
• Affect evaporation rates, runoff, and regional
patterns of rainfall
• Annual rainfall declines  decrease fertility and
moisture content of the soil
 Tree –planting campaign 1977
• Green Belt Movements, Kenya
21.2 Adaptive Trends Among Plants
 Plants evolved about 475 million years ago from
charophytes (a group of green algae)
• Most modern plants are photoautotrophs on land
 Defining trait of land plants  multicelled embryo
• Clade of land plants are called embryophytes
Plant Adaptations to Land
 Most groups are adapted to dry and often cold
habitats through structural modifications
• Stomata across epidermal surfaces
• Stoma  opening in plant’s cuticle and epidermis
• opened for gas exchange
• OR closed to prevent water loss
• Waterproof cuticle  secreted covering
• Lignin-reinforced tissues
• Stiffens cell walls of vascular plants
• Xylem (water and dissolved ions) and phloem (sugars)
 vascular tissues
• Vascular plant  plant with a xylem and phloem
The Plant Life Cycle
 Land plants alternate between gametophyte
(haploid) and sporophyte (diploid) generations
mitosis
multicelled
sporophyte
(2n)
zygote
(2n)
fertilization
DIPLOID
meiosis
HAPLOID
gametes
(n)
mitosis
spores
(n)
multicelled
gametophyte
(n)
mitosis
Fig. 21.2, p.334
zygote is only
diploid phase
green algae bryophytes
ferns
gymnosperms
angiosperms
Fig. 21.2, p.334
From Haploid to Diploid Dominance
 Dominant stages
• Haploid body (algae and nonvascular plants)
• Diploid body (most modern plants)
 Complex sporophytes retain, nourish, and
protect new generations through seasons
 Production of two spore types allows evolution of
pollen grains and seeds in two lineages
flowering gnetophytes ginkgos conifers cycads
plants
ferns whisk ferns horsetails
seed
plants
plants
with
complex
leaves
vascular
plants
lycophytes
hornworts
liverworts mosses
charophytes
land
plants
plants and
close relatives
Fig. 21.3, p.335
Pollen and Seeds
 Seed plants  vascular plants
• Do not release spores
• Instead spores give rise to gametophytes inside
structures on the sporophyte body
• Pollen grain  Immature male gametophyte
• Released and transported by wind or animals
• Can allow for fertilization in the driest conditions
• Fertilization
• Seed  embryo sporophyte and nutritive tissue inside a
waterproof coat ( a mature ovule)
Key Concepts:
MILESTONES IN PLANT EVOLUTION
 Earliest known plants date from 475 million
years ago
 Since then, environmental changes have
triggered divergences, adaptive radiations, and
extinctions
 Structural and functional adaptations of lineages
are responses to some of the changes
21.3 Bryophytes
 Haploid Gametophyte-dominant life cycle
 Mosses, liverworts, and hornworts
• Nonvascular (no xylem or phloem)
• Rhizoids store moisture, anchor gametophyte
Life Cycle: Bryophytes
 Sperm swim through water droplets or film of water
to eggs
 Gametes form in a chamber (a gametangioum) that
develops in or on the gametophyte’s surface
 Diploid sporophyte remains attached to the
gametophyte and makes spores by meiosis
• Wind disperses the spores
 Some are drought tolerant – become dormant in
drought and resume growth during rain
 Most reproduce asexually  Fragmentation
Life Cycle: Bryophytes - Moss
Moss 
• nonvascular plant with a leafy green gametophyte
and an attached, dependent sporophyte consisting of
a capsule on a stalk
• Rhizoid: threadlike structure that anchors the
bryophyte
mature sporophyte (spore-producing
structure and stalk), still dependent
on gametophyte
Zygote grows,
develops into a
sporophyte
while still
zygote attached to
gametophyte.
Diploid Stage
Haploid Stage
fertilization
meiosis
Spores form by
way of meiosis
and are released.
Sperm reach eggs
by moving through
raindrops or film of
water on the plant
surface.
rhizoids
sperm-producing
structure at shoot
tip of male
gametophyte
egg-producing
structure at shoot
tip of female
gametophyte
Spores germinate.
Some grow and
develop into male
gametophytes.
Other germinating spores
grow and develop into
female gametophytes.
Fig. 21.5, p.336
Bryophyte Structures
Sporophyte
Gametophyte
Peat Bogs: Sphagnum
• Peat  carbon rich plant remains
• can be dried for fuel
Key Concepts:
NONVASCULAR PLANTS
 Bryophytes are nonvascular, with no internal
pipelines to conduct water and solutes through
the plant body
 A gamete-producing stage dominates their life
cycle, and sperm reach the eggs by swimming
through droplets or films of water
21.4 Seedless Vascular Plants
 Lycophytes, horsetails, whisk ferns, true ferns
Life Cycle: Seedless Vascular Plants
 Vascular tissue that produces spores
• Dominated by the sporophyte
• Spore-bearing structures
• Strobili of horsetails
• Sori of ferns
• Sorus  cluster of spore-producing
capsules on a fern leaf
• Sperm swim through water to reach eggs
Life Cycle: Fern
Ferns
• Tropical ferns are epiphytes, plants that attach to
and grow on a trunk or branch of another plant but do
not withdraw any nutrients
The sporophyte
(still attached to
the gametophyte)
grows, develops.
zygote
rhizome
Diploid Stage
fertilization
sorus
meiosis
Haploid Stage
Spores develop.
egg
sperm
eggproducing
structure
spermproducing
structure
Spores are
released.
mature
gametophyte
(underside)
A spore
germinates,
grows into a
gametophyte.
Fig. 21.9, p.339
Fern Diversity
Key Concepts:
SEEDLESS VASCULAR PLANTS
 Lycophytes, whisk ferns, horsetails, and ferns
have vascular tissues but do not produce seeds
 A large spore-producing body that has internal
vascular tissues dominates the life cycle
 As with bryophytes, sperm swim through water
to reach eggs
21.5 History of the Vascular Plants
 Coal
• One the our premier fossil fuels
• Fossil fuel was formed over millions of years by
compaction and heating of plant remains
• Took millions of years of
• Photosynthesis
• Burial by layers of sediment which
protected them from decomposers
• Compaction to form coal
• Nonrenewable source of energy
stem of a giant lycophyte ( Lepidodendron),
which could grow 40 meters (131 feet) tall
stem of a giant horsetail ( Calamites),
which was almost 20 meters (66 feet) tall
seed fern ( Medullosa); its seeds
were about the size of walnuts
Fig. 21.12, p.340
Rise of the Seed Plants –
Gymnosperms and Angiosperms
 Gametophytes of a seed plants form inside
reproductive parts on a sporophyte body
• In contrast to gametophytes of seedless vascular
plants which develop from spores that were released
into the environment
Rise of the Seed Plants
 Microspores
• Haploid spore formed in pollen sacs of seed plants
• Pollen sac  reproductive structure develops
sperm-bearing gametophytes (pollen grains)
• Develop into sperm-producing male gametophytes
 Megaspores
• Haploid spore formed in ovule of seed plants
• Ovule  reproductive structure develops eggbearing gametophytes
• After fertilization gametophyte matures  seed
• Develop into egg producing female gametophytes
Rise of the Seed Plants
 Seed: A mature ovule
• Part of ovule forms nutritive tissue and seed coat
(protects embryo sporophyte)
21.6 Gymnosperms: Naked Seeds
 Conifers, cycads, ginkgos, and gnetophytes
• Many are well adapted to dry climates
• Conifers  nonmotile sperm and woody cones
• Ex. Pine
• Cycads  tropical or subtropical gymnosperm
with flagellated sperm, palm like leaves, and
fleshy seeds
• Ginkgos  flagellated sperm, fan shaped leaves,
and fleshy seeds
• Gnetophytes  vineline with nonmotile sperm
Gymnosperms
Conifer
Ginkgo’s
Cycad
Gnetophyte
Gymnosperms: Naked Seeds
 Gymnosperms
• Seed plants that does not make flowers or fruits
 Life cycle: No ovaries
• Ovules form on exposed surfaces of strobili or (in
conifers) female cones
• “Naked” seed because unlike angiosperms
they are not inside a fruit
Life Cycle: Conifer
section through one
ovule (the red “cut”
in the diagram
to the left)
surface view of a
female cone scale
(houses two ovules)
ovule
section through
a pollen sac
(red cut)
mature
sporophyte
seedling
pollen tube
spermproducing
cell
Germinating pollen grain (the
male gametophyte). Sperm
nuclei form as the pollen tube
grows toward the egg.
surface view of a
scale of a male
strobilus (houses
two pollen sacs)
seed
coat
embryo
nutritive
tissue
seed formation
zygote
fertilization
(view inside
an ovule)
eggs
female gametophyte
Diploid Stage
Haploid Stage
pollination
(wind
deposits
pollen grain
near ovule)
meiosis meiosis
Microspores Megaspores
form; one
form, develop
into pollen develops into
the female
grains.
gametophyte.
Fig. 21.15, p.343
21.7 Angiosperms: Flowering Plants
 Only angiosperms can make flowers and fruit
• Many coevolved with birds, bees, bats, and other
animal pollinators
• Flower  selective advantage
• Reproductive shoot of a flowering plant
• Fruit  mature flowering plant ovary
• After fertilization, an ovule matures into a seed
and the ovary around it becomes the fruit
Flowering Plant Structures
21.7 Angiosperms: Flowering Plants
 Most widely distributed and diverse plant group
• Two largest classes: Eudicots and monocots
• Monocots 
• includes grasses, orchids and palms
• Eudicots 
• includes herbaceous plants, woody trees,
and cacti
Evolution of Flowering Plants
Life Cycle: Flowering Plants
 Monocot life cycle: An example of sexual
reproduction in flowering plants
• Formation of pollen and eggs
 Double fertilization produces an embryo
sporophyte and nutritive tissue (endosperm) that
supports it
• Protective seeds form in ovaries
• Outer ovary tissues later develop into fruits
Monocot Life Cycle: Lily
a flowering
stem of the
mature
sporophyte
(2n)
pollen sac,
where each
seed coat
one of many
embryo (2n)
will
endosperm (nutritive tissue) cells
give rise to
seed
microspores
Diploid Stage
double fertilization
meiosis
Haploid Stage
Pollination and pollen
Microspores
tube formation:
form, then
male
develop into
gametophyte
pollen grains.
pollen tube
sperm (n)
Pollen is
released.
The pollen
tube enters
an ovule.
(line of cut
of diagram
at left)
ovary
ovules
inside
ovary
cell in ovule
that will give
rise to a
megaspore
meiosis
Megaspore gives
rise to haploid cells
in ovule. In one of
the cells, mitosis
without cytoplasmic
division gives it two
nuclei; it will give
rise to endosperm.
cell from which
endosperm
will form
egg
female gametophyte
Fig. 21.19, p.346
Summary:
Comparison of Major Plant Groups
Key Concepts:
SEED-BEARING VASCULAR PLANTS
 Gymnosperms and, later, angiosperms radiated
into higher and drier environments
 The packaging of male gametes in pollen grains
and embryo sporophytes in seeds contributed to
the expansion of these groups into new habitats
Key Concepts: SEED-BEARING
VASCULAR PLANTS (cont.)
 Angiosperms alone make flowers, which wind,
water, and animals help pollinate
 In distribution and diversity, angiosperms are the
most successful group of plants
 Nearly all plant species that we rely upon for
food are angiosperms
Animation: Fern life cycle
Animation: Flower parts
Animation: Haploid to diploid dominance
Animation: Monocot life cycle
Animation: Moss life cycle
Animation: Pine life cycle
Animation: Pinus cones
Animation: Seedless vascular plants