Major Themes in Land Plant Evolution

Progressive reduction of the gametophyte stage

Independent sporophyte

Evolution of pollen (replaces swimming sperm)

Evolution of the seed (replaces spores as dispersal agent)

Homospory Heterospory (separate male and female spores)

also... Improved vascular tissues

Increased height and structural strength

Anatomical + physiological adaptations to drier environments

Gymnosperms: Evolution of the Seed

Adaptations to Dry Living:Stomata and Cuticle

Cuticle = waxy protective layer on the outside of cells that prevents water loss

In stomata (singular = stoma), guard cells open to allow CO2 to enter open pores..

Reduction of Gametophyte

Bryophyte Fern

Gametophyte is visibleGametophyte is microscopic -

Gymnosperm Angiosperm

Homosporous vs. Heterosporous

Sporangium

Spores

Gametophyte

Archegonium Antheridium

Egg Sperm

Zygote

Megasporangium Microsporangium

Megaspores Microspores

Mega-gametophyte Micro-gametophyte

Archegonium Antheridium

Egg Sperm

Zygote

Bryophyte life cycle

--

Gametophyte is dominant

Seedless vascular plant life cycle

- Sperm must still swim to reach egg - Little protection for the embryo -

Sporophyte is dominant

Appeared ~360 million years ago

Seed plants

Gymnosperms - - -

4 phyla

GymnospermsSporophylls: modified leaves that contain sporangia (spore-producers)

FEMALEmegasporangium

megaspore

MALEmicrosporangium

meiosis1 of 4 cells

survives

N

2N

GymnospermsSporophylls: modified leaves that contain sporangia (spore-producers)

FEMALEmegasporangium

megaspore

female gametophyte

archegonia

egg

MALEmicrosporangium

microspore

male gametophyte growsinside pollen grain

sperm

N

2N

Development of the gymnosperm seed from an ovule in Pinus sp.

Gymnosperm ovules have 1 protective outer layer, or integument, composed of tissue from sporophyte (mom’s tissue)

Pollen grain grows a pollen tube of haploid cells to penetrate the micropyle, the opening through the integument; releases sperm

Fertilized egg grows into embryo, surrounded by female gametophyte tissue (its food) and the sporophyte’s integument (its protection)

Gymnosperms

Mature female pine cone

Strobilus (= cone) is a branch tip specialized for reproductionLeaves are modified into..

GymnospermsStrobilus (= cone) is a branch tip specialized for reproduction

Leaves are modified into sporophylls

Megasporangium

Gymnosperm life cycle

“ovule”(developing seed)

“seed”(after fertilization)

Life cycle of Pinus sp.

heterospory

Stern 1991

Pine ovule after formation of the megagametophyte, archegonium & egg(s)

Raven et al. 1986

egg

sperm

second sperm nucleus

zygote

Pine ovule at the fertilization stage

Raven et al. 1986

n

2n

embryo (2n)

Gametophyte (n)

Pine seed showing embryo (baby sporophyte) within nutritive tissue, surrounded by a seed coat

Phylum Ginkgophyta – Ginkgo biloba

Only 1 living species, almost identical to fossils 150 million years old

Phylum Cycadophyta – about 160 species

Phylum Gnetophyta (~ 70 species)

Phylum Coniferophyta Conifers – cone bearing

entirely wind-pollinatedfertilization can take a year to occur

Raven et al. 1986

Cross-section of a pine needle showing adaptations to drought

Dense packing of cells

Sunken stomata

Thick cuticle

Raven et al. 1986

Transport tissues (secondary xylem & phloem) in a pine tree provide improved long- distance transport and structural support wood

Height: coast redwoods (Sequoia sempervirens) >100 m tall

Mass: giant redwoods (Sequoiadendron gigantea) > 26 m. circumference

Age: bristlecone pine (Pinus aristata)4900 years old

Biggest + oldest

Economic importance of conifers

Pacific Yew (Taxus brevifolia)

Mackenzie 2003

Seedlessvascularplants

Gymnospermsdominate

Angiosperms

First gymnosperms

Evolution of Plant Biodiversity

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