Evolution by Seed Plants
• cyanobacteria on land – 1.2 billion years ago
• 500 MYA – colonization by plants
• plant share characteristics with other more primitive organisms
– 1. multicellular, eukaryotic
– 2. photosynthetic autotrophs – brown, red, green algae
– 3. cell walls made of cellulose – green algae, dinoflagellates, brown
algae
– 4. chloroplasts with chlorophyll a and b – green algae, euglenids and a
few dinoflagellates
Defining the Plant Kingdom
• divided into two clades: non-vascular and vascular
• vascular plants form a single clade – 93% of all plant
species
– categorized into three smaller groups
• 1. lycophytes – club mosses and relatives
• 2. pterophytes – ferns and relatives
• 3. seed vascular plants
– A. gymnosperms - “naked seed” plants
– B. angiosperms – flowering plants
Seed plants
– three key reproductive adaptations
evolved in seed plants:
– 1. dominance of the sporophyte generation –
reduced gametophyte
– 2. the seed – ovules and eggs
– 3. pollen
Reduced
Gametophytes
• gametophytes of non-vascular
mosses are the dominant stage
• gametophytes of vascular ferns
are significantly smaller in size
• gametophytes of seed plants are
microscopic
• develop within the sporangium of
the parental sporophyte
Bryophytes
Seedless Vascular
Sporophyte
(2n)
Gametophyte
(n)
Sporophyte dependent
on gametophyte
(mosses and other
bryophytes)
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Sporophyte
(2n)
Gametophyte
(n)
Large sporophyte and
small, independent
game-tophyte (ferns and
other seedless vascular
plants)
Sporophyte (2n),
the flowering plant
(independent)
Microscopic male
gametophytes (n) in
inside these parts
of flowers
(dependent)
– protects the gametophyte
– gives it nourishment
Microscopic male
gametophytes (n)
in pollen cones
(dependent)
Sporophyte (2n),
(independent)
Microscopic female
gametophytes (n) in
inside these parts
of flowers
(dependent)
Reduced gametophyte dependent on sporophyte
(seed plants: gymnosperms and angiosperms)
Seed
Vascular
Ovaries & Seeds
• seed plants are unique in the presence of an ovary that will
develop an egg
• most plant ovaries are made up of smaller ovules
• inside each ovule is the female gametophyte
• meiosis produces an egg
female
gametophyte
Ovaries & Seeds
• development of the fertilized egg 
Seed
• seed = ovule after fertilization –
contains the embryo
– seed = embryo (2n) + food supply (left
over female gametophyte) + seed coat
(from the parental sporophyte)
– allows for the developing embryo to resist
harsh conditions
– multicellular structure - in contrast to the
spore
• evolutionary advantage of seeds:
– seeds carry their own food supply
– a seed can remain dormant for years
following its release
Seed coat
(derived from
integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
Pollen
• the male part of the sporophyte produces
microspores that develop into pollen
grains
• a pollen grain contains the male
gametophyte that will produce sperm via
meiosis
• transfer of pollen to the ovule =
pollination
• pollen grains are carried away from the
parent plant by wind, insects
• or they can travel to the female
reproductive structures within the same
sporophyte
• in order to fertilize - the pollen
grain must germinate (grow)
Pollen
– it produces a pollen tube
– pollen tube allows for the discharge of
two sperm (gametes) into the ovule
containing the egg
• in mosses and ferns – the sperm is
flagellated and swims to the female
gametophyte in order to fertilize the
egg which is also free living
• in vascular seed plants – the female
gametophyte produces an egg which
never leaves the sporophyte ovule
Gymnosperms
• “naked seed” – seeds are not enclosed in
ovaries
• seeds are exposed on modified leaves
that form cones
– in the ferns – development of fronds that bear
the sporangium (sori)
– in gymnosperms – development of modified
leaves that cluster together to form cones or
strobili
Gymnosperms
• drier environment favored gymnosperms
over the bryophytes and ferns
• gymnosperms with their thick cuticles
and reduced leaves as needles – adapted
well to the dry climates
• first seed plant in the fossil record – 360
MYA
– now extinct
• earliest fossils of gymnosperms – 305
MYA
• most common existing gymnosperms are
the conifers – spruce, pin, fir and redwood
Ponderosa pine
Gymnosperms
Cycas revoluta
•
gymnosperms are considered a division with 4 phyla:
Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta
– Phylum Cycadophyta – cycads
• 130 species survive
– Phylum Ginkgophyta - ginkos
Ginko biloba
Welwitschia mirabilis.
• only one species left – Ginkgo biloba
– Phylum Gnetophyta – three genera alive today
• tropical and desert species
• Gnetum – 35 species of tropical trees, shrubs and vines
(Africa and Asia)
• Welwitschia – one species, Welswitchia (Africa)
• Ephedra – 40 species, desert shrubs
– Phylum Coniferophyta – largest group
•
•
•
•
“cone-bearing”
600 species of conifers
many are large trees
most are evergreens – retain their leaves throughout the
year
Ephedra.
Phylum Coniferophyta
• 575 species
• largest genus – Pinus
• leaves of conifers are always simple needles or
scales
• pine leaves – needles or needle-like
– arranged in clusters or bundles of two to five
leaves each bundle
– cluster = fascicle
– needles are covered with a waxy cuticle to
minimize water loss
Pine fascicle
• pine tree is the sporophyte
• sporangia are located on scale-like
leaves packed into cones
• two types of cones produce two
types of spores
– small pollen cones produce microspores 
pollen
– larger ovulate cones produce megaspores 
egg
Life Cycle: The
Pine
Life Cycle: The Pine
• pollen cones produce pollen
which contains the male
gametophyte
• ovulate cones have ovules
containing the female
gametophytes making eggs
• pollen lands on ovulate cones
and begins to germinate
• pollen tube delivers sperm to
egg = Fertilization
• fertilized eggs develop into
seeds
• seeds are released from
ovulate cones
• seeds land on new habitat and
develop into new sporophyte
in a conifer - more than
a year may pass
between pollination &
fertilization!!!
Male Pine Cone
Pollen
grains
ovule
air cells
Female Pine Cone
Angiosperms
• commonly known as the flowering plants
– angion = “container”
– angio – refers to seeds contained in fruits and
mature ovaries
• are seed plants that produce reproductive
structures called flowers and fruits
Angiosperm Diversity
• only about 1,000 species divided into three
groups:
• 1. magnoliids
• 2. monocots – embryo with one cotyledon
• 3. eudicots (dicots) – embryo with two
cotyledons
• embryo with one cotyledon (embryonic leaf)
• other traits:
–
–
–
–
1. veins in leaves are usually parallel
2. vascular bundles scattered in stems
3. root system is usually fibrous
4. most cannot undergo secondary (i.e. woody) growth
Monocots
Dicots (Eudicots)
• former classification known as dicots
has been abandoned (too polyphyletic)
• using DNA analysis – clade was created
of “true” dicots
– cotyledons: store food absorbed from the
endosperm
California
poppy
zucchini flower
Dicots (Eudicots)
• embryo with two cotyledons
• other traits:
– 1. veins in leaves are usually netlike
– 2. vascular bundles arranged in a ring in
stems
– 3. root system is usually a taproot
– 4. many are perennial and undergo
secondary
(i.e. woody) growth
California
poppy
zucchini flower
• flower = angiosperm structure that is
specialized for sexual reproduction
• structure of a flower – 4 rings of modified
leaves called flower organs:
– 1. sepals
– 2. petals
– 3. stamens
– 4. carpels
Flowers
• 1. sepals (sterile flower organ)
Flower
Anatomy
– usually green and enclose the
flower before it opens
• 2. petals (sterile flower organ)
– interior to the sepals
– many are brightly colored – to
attract pollinators like insects
– wind pollinated have leaves that
are less colorful
Stigma
Stamen Anther
Carpel
Style
Filament
Ovary
Petal
Sepal
Ovule
Receptacle
• 3. stamens (produce spores)
Flower
Anatomy
– contain chambers pollen sacs (male
sporangia)
– pollen sacs produce pollen grains
containing the male gametophyte
– consists of a stalk called the filament
and a terminal end called the anther
(pollen sacs)
Stigma
Stamen Anther
Carpel
Style
Filament
Ovary
Petal
Sepal
Ovule
Receptacle
• 4. carpels (produce spores)
– comprised of the stigma, style and ovary
– end of the carpel is a sticky stigma that
receives pollen
– the stigma leads to a style which leads to the
ovary at the base of the carpel
– the ovary contains one or more ovules – site
of the female gametophyte & the egg
– these ovules when fertilized develop into
seeds within a fruit
Flower
Anatomy
Stigma
Stamen Anther
Style
Filament
• some flowers have a single carpel
– others have multiple (separate or
fused together)
• e.g. fused carpels = strawberry
Carpel
Ovary
Petal
Sepal
Ovule
Receptacle
• fruits contain the mature ovary
– but can also contain other flower
parts
• the egg is fertilized within the
ovule - the embryo begins to
develop within the seed
• as seeds develop – the ovary wall
(pericarp) thickens = fruit
development
• fruits protect seeds and aid in their
dispersal
Fruits
• fruits can be either fleshy or dry
– fleshy = tomatoes, plums, grapes
• the pericarp becomes soft during
ripening
– dry = beans, nuts and grains
• some can split open at maturity to
release seeds
• fruits have adapted for seed
dispersal in many ways
– many are eaten – seeds “pooped” out
– others cling to animals – “burrs”
– e.g. dandelions and maples – fruits
function as parachutes or propellers
– e.g. coconut – dispersal by water
Fruits
Life Cycle of Angiosperms
• parental cells inside pollen sacs inside anther undergo meiosis to make
pollen grains (male gametophyte)
• pollination results in distribution of pollen to the stigma
Anther
pollen grains
Life Cycle of Angiosperms
• pollen germinates and develops a pollen tube for delivery of sperm down the
style toward the ovary
• pollen tube stops at the ovule inside the ovary
• sperm enters into the ovule
• the sperm fertilizes the
egg inside the ovule to
produce the zygote
• the zygote grows into the
embryo
• the surrounding ovule
becomes the seed
• the surrounding ovary
becomes the fruit
Pollination
• by numerous methods
– abiotic: wind
– by bees – 65% of all angiosperms
– by moths & butterflies – detect odors (sweet
fragrance)
– by flies – many are reddish and fleshy with a rotten
odor
– by bats – light colored petals and aromatic
– by birds – very large and brightly colored (red or
yellow) – no scent required but they produce a
nectar
Seed Development
• the seed consists of:
– the embryo
– the endosperm
– the seed coat
• the endosperm – rich in starch
–
–
–
–
usually develops before the embryo
initially has a milky consistency
thickens as the seed develops
stores nutrients that is used by the seedling as it germinates
• the embryo
–
–
–
–
develops leaves for food storage = cotyledons
monocot – 1 cotyledon
dicot – 2 cotyledons
develops an embryonic root = radical
Seed coat
Radicle
Cotyledons
Common garden bean, a eudicot with thick cotyledons
Seed coat
Cotyledon
Endosperm
Radicle
Maize, a monocot
And now for some interesting stuff you
probably knew but don’t really know
• The dandelion – Taraxacum (Dandelion - lion’s tooth)
– asexually reproduces through apomixis: asexual
production of multiple seeds
– the dandelion produce seeds without pollination and
fertilization
– a diploid cell in the ovule gives rise to the embryo
– seed development results – dispersed by the wind or you
blowing
•
six crops – maize, rice, wheat, potatoes, cassava and sweet potatoes – yield 80% of all the
calories consumed by humans
– crops domesticated 12,000 years ago
– seeds of domesticated crops usually much larger than their wilder “cousins”
•
•
5-7 kg of grain required to produce 1 kg of beef
the outer covering of the grain is called the bran
– rich source of vitamin B
•
•
the embryo is located at the upper corner of the grain and is called the germ
polishing the grain (e.g. in white rice) removes the bran and germ and leaves the endosperm
• flowering plants provide many edible products
–
–
–
–
teas and coffee beans
cacao tree – chocolate
spices – cloves, saffron
fruits and seeds – vanilla, black pepper, mustard
• many seed plants are sources of wood
– wood – tough walled xylem cells
• seed plants also provide numerous medicines
–
–
–
–
belladonna – atropine (dilator)
foxglove – digitalis (heart medication)
eucalyptus – menthol
periwinkle – vinblastin (leukemia)
Cloning Happens
• used to improve crops and ornamental plants
• clones from cuttings:
–
–
–
–
plant fragments taken from the stem called a “cutting”
at the end of the cutting – development of a callous of undifferentiated cells
these cells form new roots
can also be done from leaves
• grafting:
– a twig or bud from one plant is grafted onto another – to join their genomes
– the plant that provides the root system = stock
– the grafted twig = scion
• test-tube cloning:
– lab-based methods for cloning
– cells taken from a plant and cultured on artificial media to form a callous and then a
new seedling
– can also introduce new genes = genetic engineered organism
• genetically modified cassava: taproot of
almost pure carbs
– transgenic strains with dramatically increased
protein levels, iron and vitamin A
• genetically modified wheat and rice:
Norman Borlaug: PhD in plant physiology
– “father of the green revolution”
– Nobel prize Laureate
– work in modifying wheat strains – high yield, but
too tall
– produced a “dwarf” version by selective breeding
– also developed dwarf rice strains
– his group was credited with saving millions of
people from starvation
– worked with triticale – wheat and rye
• given a “shout out” in the Star Trek episode – “The
trouble with tribbles”
Genetic
Engineering in
Food