Seed Plants: Angiosperms


•     Phylum Magnoliophyta

–    Gametophyte Development

•     Pollination

•     Fertilization and Seed Development

•     Specialization Trends in Flowering Plants

•     Pollination Ecology

•     Herbaria and Plant Preservation



•     Angiosperms are plants with seeds contained within a vessel (carpel).

–    Largest and most diverse phylum of the Plant Kingdom.

Phylum Magnoliophyta

•     Flowering Plants

–    Angiosperms are heterosporous.

•   Female gametophytes are wholly enclosed within sporophyte tissue and reduced to only a few cells.

•   At maturity, male gametophytes consist of a germinated pollen grain with three nuclei.

Phylum Magnoliophyta

•     Development of Gametophyte

–    Diploid megasporocyte cell differentiates from all other cells in the ovule.

•   Undergoes meiosis and produces four haploid megaspores.

–   Three degenerate

–    Integuments differentiate and eventually become the seed coat.

•   Leave micropyle at one end.

Gametophyte Development

•     Central Cell Nuclei may become a binucleate cell, or may fuse together, forming a single diploid nucleus.

–    One of the cells functions as the egg.

•     Female gametophyte (megagametophyte) consists of a large sac usually containing eight nuclei in seven cells.

Mature Megasporocyte

Gametophyte Development

•     While megagametophyte is developing, a parallel process that leads to the formation of male gametophytes occurs in the anthers.

Four patches of tissue differentiate from the main cell mass.

•     Contain diploid microsporocyte cells which undergo meiosis and produce a quartet of microspores.

Lily Anther Cross Section

Gametophyte Development

•     After meiosis, the haploid microspores in the pollen sac undergo several changes.

–    Nucleus in each microspore divides by mitosis.

–    Members of each quartet of microspores separate from one another.

–    A two-layered wall develops around each microspore.

•     Microspores are now pollen grains.

–    Outer wall - exine.


•     Pollination is the transfer of pollen grains from an anther to a stigma.

•     Fertilization involves the union of an egg and a sperm.

–    May not occur until days or weeks or even months after pollination has taken place.

Fertilization and Seed Development

•     After pollination, further development of the male gametophyte may not take place unless the pollen grain is:

–    From a different plant of the same species.

–    From a variety different from that of the receiving flower.

•   Under suitable conditions, a pollen tube grows down between the cells of the stigma and style until it reaches the ovule micropyle.

Fertilization and Seed Development

•     When the pollen tube reaches the micropyle, it continues to the female gametophyte and discharges its contents.

–    Double Fertilization (Fusion) Occurs

•   One sperm migrates from the synergid to the egg and forms a zygote.

•   Other sperm cell migrates from the synergid and unites with the central cell nuclei producing a triploid endosperm nucleus.

Mature Male Gametophyte

Apomixis and Parthenocarpy

•     Apomixis is development or fusion of gametes but with the normal structures otherwise being involved.

–    Fruits that develop from ovaries with unfertilized eggs are parthenocarpic.

•   Seedless (Navel oranges)

–    Seedless watermelon are produced by crossing varieties with different numbers of chromosomes. (Not parthenocarpic)

•   Fertilization and seed formation don’t occur.

Specialization Trends in Flowering Plants

•     Fossil record suggests flowering plants first appeared about 160 million years ago during the late Jurassic period.

–    Most botanists hypothesize primitive flowers had numerous spirally-arranged parts that were not fused together and were variable in number.

•   Flowers were regular and contained both stamens and pistils.

Specialization Trends in Flowering Plants

•     In advanced flowers, the receptacle has fused to the ovary.

–    When the ovary is embedded in the receptacle, it is said to be inferior.

•   Flower parts attached to to the top of the ovary are epigynous.

–    When the ovary is produced on top of the receptacle, it is said to be superior.

•   Flower parts attached around the base are hypogynous.



Specialization Trends in Flowering Plants

•     Flowers have tended to become irregular.

•     Complete flowers contain a calyx, corolla, stamens, and a pistil.

–    Perfect flowers have stamens and a pistil.

•   Imperfect flowers are missing either stamens or a pistil.

•     Monoecious plants have both male and female imperfect flowers.

–    Dioecious plants only have only male or female flowers.

Pollination Ecology

•     Throughout the evolutionary history of flowering plants, the pollinators have evidently coevolved with plants.

–    Twenty thousand bee species are included among current-day pollinators.

•   Many bee-pollinated flowers are delicately sweet and fragrant.

–   Flowers pollinated by beetles tend to have different, stronger odors.

Pollination Ecology

•     Moth and butterfly-pollinated flowers also often have sweet fragrances.

–    Night-flying moths tend to visit white or yellow flowers.

–    Butterflies tend to visit bright blue, yellow, or orange flowers.

•   Nectaries are at bottom of corolla tubes.

–   Situated for specialized mouth parts.

Pollination Ecology

•     Flowers visited by birds are often bright red or yellow, and usually have little, if any, odor.

–    Typically large flowers.

•     Birds are highly active pollinators and tend to burn energy rapidly.        

–    Many bird-pollinated flowers produce copious amounts of nectar to assure repeated visits.

•     Bats tend to visit flowers that open only at night.

Herbaria and Plant Preservation

•     Herbaria are essentially libraries of dried, pressed plants, algae, and fungi, arranged and labeled.

–    Properly prepared and maintained specimens may remain in excellent condition for 300 or more years.



•     Phylum Magnoliophyta

–    Gametophyte Development

•     Pollination

•     Fertilization and Seed Development

•     Specialization Trends in Flowering Plants

•     Pollination Ecology

•     Herbaria and Plant Preservation



•     The reproductive adaptations of angiosperms include flowers and fruits

•     Angiosperms

–    Are commonly known as flowering plants

–    Are seed plants that produce the reproductive structures called flowers and fruits

–    Are the most widespread and diverse of all plants

Characteristics of Angiosperms

•     The key adaptations in the evolution of angiosperms

–    Are flowers and fruits


•     The flower

–    Is an angiosperm structure specialized for sexual reproduction


•     The flower

–    Is an angiosperm structure specialized for sexual reproduction


•     A flower is a specialized shoot with modified leaves

–    Sepals, which enclose the flower

–    Petals, which are brightly colored and attract pollinators

–    Stamens, which produce pollen

–    Carpels, which produce ovules


•     Fruits

–    Typically consist of a mature ovary


•     Can be carried by wind, water, or animals to new locations, enhancing seed dispersal

The Angiosperm Life Cycle

•     In the angiosperm life cycle

–    Double fertilization occurs when a pollen tube discharges two sperm into the female gametophyte within an ovule

–    One sperm fertilizes the egg, while the other combines with two nuclei in the center cell of the female gametophyte and initiates development of food-storing endosperm

•     The endosperm

–    Nourishes the developing embryo


•                    The life cycle of an angiosperm

Angiosperm Evolution

•     Clarifying the origin and diversification of angiosperms

–    Poses fascinating challenges to evolutionary biologists

•     Angiosperms originated at least 140 million years ago

–    And during the late Mesozoic, the major branches of the clade diverged from their common ancestor

Fossil Angiosperms

•     Primitive fossils of 125-million-year-old angiosperms

–    Display both derived and primitive traits

An “Evo-Devo” Hypothesis of Flower Origins

•     In hypothesizing how pollen-producing and ovule-producing structures were combined into a single flower

–    Scientist Michael Frohlich proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures

Angiosperm Diversity

•     The two main groups of angiosperms

–    Are monocots and eudicots

•     Basal angiosperms

–    Are less derived and include the flowering plants belonging to the oldest lineages

•     Magnoliids

–    Share some traits with basal angiosperms but are more closely related to monocots and eudicots



•     Exploring Angiosperm Diversity


•     Exploring Angiosperm Diversity


•     Exploring Angiosperm Diversity


•     Exploring Angiosperm Diversity

Evolutionary Links Between Angiosperms and Animals

•     Pollination of flowers by animals and transport of seeds by animals

–    Are two important relationships in terrestrial ecosystems


•     : Human welfare depends greatly on seed plants

•     No group is more important to human survival than seed plants

Products from Seed Plants

•     Humans depend on seed plants for

–    Food

–    Wood

–    Many medicines


Seed Dormancy of Trees and Shrubs

by Jill Barbour

Germination Specialist



Organic Seed Dormancy

•       Endogenous

•     embryo characteristic prevents germination-epicotyl, hypocotyl, radical


•        Exogenous

•     characteristic of structures - seed coats, fruit walls, including endosperm or perisperm prevents germination

Types of Seed Dormancy

•       Physiological

•       Morphological

•       Morphophysiological

•       Physical

•       Physical & physiological

•       Chemical

•       Mechanical

Physiological Dormancy

•       Nondeep

•       Intermediate

•       Deep

Causes of Physiological Dormancy

•       Covering restricts oxygen

•       Inhibitors in coverings

•       Embryo cannot break through physical barriers

•       Endosperm restrict embryo growth

•       Interaction between embryo and covering

•       Abies alba, Castanea sativa, Corylus avellana, Euonymus europaeus, Juglans nigra, Juglans regia,  Juniperus, Prunus avium,Rhamnus frangula, Vaccinium myrtillus

•       Carpinus requires warm followed by cold stratification

•       Elaeagnus umbellata- chemicals shortened prechilling & increase germination

Nondeep Physiological Dormancy

•       Germinate over a narrow range of temperatures

•       Excised embryos usually grow

•       Broken by short periods of prechilling

•       Require germination temperature above 15°C

•       Broken by chemicals- potassium nitrate, thiourea, kinetin,  ethylene, gibberellins

•       Light required for germination

•       Arbutus unedo –can germinate in dark

•       Ulmus glabra- no prechill

•       Vaccinium- long period of light required, GA reduces length of light

Intermediate Physiological Dormancy

•       Excised embryos will grow

•       As much as 6 months prechilling needed

•       Gibberellins, kinetin, thiourea can shorten prechilling requirement

•       Acer negundo, Acer pseudoplatanus, Acer saccharum, Corylus avellana, Fraxinus americana, Fraxinus pennsylvanica

•       Fagus sylvatica – ethylene accelerated and increased germination at 15°C, at 5°C chemicals no better than water soak on germination, GA3 increased germination of unchilled seeds at 15°C, 10 weeks prechill negate chemical effect (Seed Sci 2004, p21-33)

Deep Physiological Dormancy

•       Excised embryos do not grow or produce abnormal seedlings (Prunus will)

•       Long prechill requirement

•       Chemicals do not affect germination of intact seeds

•       Sorbus aucuparis – secondary dormancy induced above 20°C,  germinates best at 1-3°C

•       Acer platanoides, Acer tartaricum, Malus domestica,

•       Prunus persica – 90 days prechill

•       Prunus mahaleb – 100 days prechill

•       3 to 5°C best germination temperature for Prunus mahaleb, Prunus padus

Morphological Dormancy

•       Morphology of embryo not developed

•       Temperate families- Apiaceae, Ranunculaceae

•       Tropical families – Annonacease, Arecaceae, Degeneriaceae, Lactoridaceae, Monimiaceae, Myrsticaceae, Winteraceae


Morphophysiological Dormancy

•       Underdeveloped embryos

•       Embryo growth and dormancy break required

•       Embryo grows first then dormancy broken or both at same time

•       Vary warm, moist and cold stratification periods

•       Viburnum- epicotyl dormancy, warm for radical then cold for epicotyl

•        Fraxinus excelsior, Magnolia acuminata

Physical Dormancy

•       Present in 15 angiosperm families

•       Large embryos with food reserve in embryo not endosperm

•       Hilum impermeable in Cercis siliquastrum

•       Impermeable in seed coats- micropyle, hilum, chalazal area, impermeable palisade cells

•       Embryo is not dormant

•       Air drying during development intensifies hardness

•       Cytisus scoparius – dry heat(65°C) for 2 minutes, or acid for 30 minutes

•       Crataegus in warm climates only endocarp dormant

•       Robinia pseudoacacia, Laburnum anagroides


Physical & Physiological Dormancy

•       Embryo dormancy usually broken first

•       Germinate at low temperatures (5, 10, 15°C)

•       Prechilling breaks physiological dormancy

•       Hot water, acid,  or mechanical scarification effective before prechilling

•       Cercis siliquastrum – 16 weeks prechilling = 77% germination(Jordan source)(2004 Seed Sci p 255-260)

•       Cersis canadensis, Cotinus coggygria, Cotinus obovatus, Sambuscus 

•       Tilia- endosperm is inhibitor, excised embryos grow

•       Crataegus – 3 month periods of cold-warm-cold-warm-cold=55% germination, apomixis common

Chemical Dormancy

•       Inhibitors in embryo, endosperm, seed coat

•       Leaching or seed coat removal

•       Seed may have physiological dormancy too so need prechilling

•       Abscisic acid inhibits germination when applied exogenously

•       Nickel (20 mg/liter) increased germination of Picea abies

Mechanical Dormancy

•       Stony endocarps

•       Embryos with deep physiological dormancy -require long prechilling

•       Anacardiaceae, Cornaceae, Juglandaceae, Nyssaceae, Oleaceae

•       Cornus sanguinea – 94% germination at 12 weeks prechilling, 81% germination at 12 weeks warm + 12 weeks cold stratification(2004 Seed Sci p 1-4)

•       Cornus mas- 18 week warm + 15-18 weeks cold stratification (Tylkowski 1991)

•       Cornaceae  not morphologically dormant

•       Elaeagnus angustifolia – snip both ends

•       Rosaceae -  warm maturation temperature prior to collection reduced dormancy

Mattoral Germination Conditions

•       Mean optimum germination temperature for trees  about  21°C – during cool season when soil is moist

•       Mean optimum germination temperature for shrubs about 19°C

•       Shrub seed germinate in light and dark

•       No shrub seed has morphological dormancy (underdeveloped embyros)


Boreal & North Temperate Subalpine

•       Pinus cembra- 90-270 days of prechilling

•       No morphological, morphophysiological, physical dormancy in species

•       Pinus mugo, Picea abies  – nondormant


•       Small seeds usually from mesic areas,  sometimes no endosperm(Ulmus), Sequoia, Sequoiadendron, Picea, Populus

•       Large seeds from xeric areas – Pinus edulis

•       Embryo size indicator of evolutionary development of Angiosperms, Magnolia precursor for Angiosperms with primitive embryo

•       Alpine seeds not very dormant, but may have undeveloped embryos, poor pollination = many empty seeds, short time for development of reproductive structures

•       Dry summers, cold winters – many dormancy mechanisms to prevent germination, Juniperus