The Evolution of Seed Plants

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Life Cycle of an Angiosperm

MEIOSIS

Key

Microsporangium

Microsporocytes (2n)

Generative cell

Anther

Tube cell

Pollen grains

Microspore (n)

Male gametophyte (in pollen grain) (n)

Mature flower on sporophyte plant (2n)

Haploid (n)

Diploid (2n)

MEIOSIS

Ovule (2n)

Ovary

Megasporangium (2n)

Megaspore (n)

Female gametophyte (embryo sac)

Antipodal cells Central cell Synergids Egg (n)

Pollen tube

Pollen tube

Stigma

Sperm (n)

Discharged sperm nuclei (n)

FERTILIZATION

Germinating seed

Embryo (2n) Endosperm (3n) Seed coat (2n)

Seed

Nucleus of developing endosperm (3n)

Zygote (2n)

Egg nucleus (n)

Style

Sperm

Slide 33

Angiosperm Phylogeny

The ancestors of angiosperms and gymnosperms diverged about 305 million years ago.

Angiosperms may be closely related to Bennettitales, extinct seed plants with flowerlike structures.

Amborella and water lilies are likely descended from two of the most ancient angiosperm lineages.

Slide 34

Angiosperm evolutionary history

Microsporangia (contain microspores)

Ovules

A possible ancestor of the angiosperms?

(a)

(b)

Angiosperm phylogeny

Most recent common ancestor of all living angiosperms

Millions of years ago

300 250 200 150 100 50 0

Living gymnosperms

Bennettitales

Amborella

Star anise and relatives

Water lilies

Monocots

Magnoliids

Eudicots

Slide 35

Angiosperm Diversity

The two main groups of angiosperms are: monocots - one cotyledon eudicots (“true” dicots) - two cotyledons.

More than one-quarter of angiosperm species are monocots.

More than two-thirds of angiosperm species are eudicots.

Slide 36

Angiosperms: Monocots and Eudicots

Monocot Characteristics

Eudicot Characteristics

Vascular tissue usually arranged in ring

Veins usually parallel

Veins usually netlike

Vascular tissue scattered

Leaf venation

One cotyledon

Embryos

Two cotyledons

Stems

Roots

Pollen

Root system usually fibrous (no main root)

Pollen grain with three openings

Taproot (main root) usually present

Pollen grain with one opening

Floral organs usually in multiples of three

Flowers

Floral organs usually in multiples of four or five

Slide 37

Evolutionary Links Between Angiosperms and Animals

Pollination of flowers and transport of seeds by animals are two important relationships in terrestrial ecosystems.