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

• 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– protects the gametophyte– gives it nourishment

Reduced Gametophytes

Sporophyte(2n)

Gametophyte(n)

Sporophyte dependent on gametophyte (mosses and other bryophytes)

Gametophyte(n)

Sporophyte(2n)

Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants)

Microscopic femalegametophytes (n) inovulate cones(dependent)

Microscopic malegametophytes (n) ininside these partsof flowers(dependent)

Sporophyte (2n),the flowering plant(independent)

Microscopic malegametophytes (n)in pollen cones(dependent)

Microscopic femalegametophytes (n) ininside these partsof flowers(dependent)

Sporophyte (2n),(independent)

Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms)

Bryophytes Seedless Vascular

SeedVascular

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

femalegametophyte

Ovaries & Seeds

Seed coat(derived fromintegument)

Embryo (2n)(new sporophyte)

Gymnosperm seed

Food supply(femalegametophytetissue) (n)

• 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

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

Pollen • in order to fertilize - the pollen

grain must germinate (grow) – 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

Ginko biloba

Ephedra.

Welwitschia mirabilis.

• gymnosperms are considered a division with 4 phyla: Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta– Phylum Cycadophyta – cycads

• 130 species survive– Phylum Ginkgophyta - ginkos

• 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

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

Life Cycle: The Pine

• 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

• 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!!!

air cells

Female Pine Cone

ovule

Pollen grains

Male 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

Monocots

• 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

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

zucchini flower

Californiapoppy

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

zucchini flower

Californiapoppy

Flowers• 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

Flower Anatomy

Stamen

Filament

Anther

Stigma Carpel

Style

Ovary

Petal

Receptacle

Ovule

Sepal

• 1. sepals (sterile flower organ)– 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

Flower Anatomy

Stamen

Filament

Anther

Stigma Carpel

Style

Ovary

Petal

Receptacle

Ovule

Sepal

• 3. stamens (produce spores)– 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)

Flower Anatomy

Stamen

Filament

Anther

Stigma Carpel

Style

Ovary

Petal

Receptacle

Ovule

Sepal

• 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

• some flowers have a single carpel – others have multiple (separate or fused together)

• e.g. fused carpels = strawberry

Fruits• 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

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

Maize, a monocot

Radicle

Endosperm

Cotyledon

Seed coat

Seed coat

Radicle

Cotyledons

Common garden bean, a eudicot with thick cotyledons

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

Genetic Engineering in

Food

• 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”