Plant Diversity IIThe evolution of seed plants 

By Selam Ahmed, Megan Foley, and Anahis Kechejian 

Gametophyte and Sporophyte Relationships 

1. Gametophyte Dominated Life Cycleo Sporophytes are dependent on

gametophytes for nutritiono Examples: mosses and bryophytes   

2. Sporophyte Dominated Life Cycleo  There are large sporophytes which are the

dominant generation. Gametephotyes are independent and small.

o Examples: ferns and seedless vascular plants

3.  Reduced Gametophyte Dependent on Sporophyteo  Gametophytes are microscopic. They are

dependent on sporophytes for nutritiono Examples: Gymnosperms and Angiosperms 

Seedless Plants vs. Seed Plants 

Seedless Plants •  Homosporous: 1 kind of

spore. All gametophytes are bisexual

• Adaptations of Seedless Plantso  Spores serve as

protection from the outside world

o Spores allow dispersal • First 100 million years of

plant life

Seed Plants• Heterosporous 

o Female gametophytes 1 megaspore produced by

megasporangia o Male gametophytes

 multiple microspores produced by microsporangia

•  Evolutionary Advantages of Seedso  *gametophytes evolved to be

smaller → greater success 1. food supply in the seed2. seeds can be dispersed 3. more resistance and protection from

the outside environment

Reproduction in Seed Plants 

Ovules- where the female gametophyte develops, produces egg cells       Made of megaprorangium, megaspore, integuments (protective sporophyte tissue)    Pollen Grains- male gametophytes, can be carried away by the plant       Pollination- the transfer of pollen to the ovules     

Gymnosperms

Gymnosperms have seeds that are not protected in an ovule (naked seeds)

Progymnosperms may be the evolutionary link between seedless plants and seed plants

Example:  flagellated sperm                  cells

Four out of Ten Plant Phyla are Gymnosperms 

1.Phylum Cycadophyta2.Phylum Ginkgophyta3.Phylum Gnetophyta4.Phylum Coniferophyta

Gymnosperm Reproductive Cycle Note:Gametophyte generation smaller and more dependent on sporophyte generation 

Heteroporous: There are both male and female gametes

Angiosperms

The most diverse phylum

Includes 90% of all modern species (approximately 250,000)

The Angiosperm Life Cycle

1. The flower of the sporophyte produces:

•  Microspores (forms male gametophyte)o Are in the pollen grains, each pollen grain has TWO haploid cells:

1 Cell=Generative cell that divides (mitosis) forming two cells 1 Cell= Tube cell that produces a pollen tube

•  Megaspores (forms female gametophyte)o AKA embryo saco Contained in an ovuleo Ovule develops in ovary

The Angiosperm Life Cycle (continued)

2. Pollen is released from the anther3. Pollen is carried to the stigma (located at top of carpel)• Usually of another plant (cross pollination) but some plants to

self-pollinate•  Cross pollination enhances genetic variation•  Mechanisms to ensure cross pollination• Development of stamen and carpels at different times making self

pollination impossible

Mechanisms for Seed Dispersal Wings- to be carried by the wind

Barbs- to attach to animals 

Fruits are edible- digested by animals Seeds are dispersed in feces

4. Pollen Grain sticks to the stigma of the carpel and germinates

5. Matured male gametophyte (tube cell) extends a pollen tube that grows  down within the style of the carpel

6. When it finally reaches the ovary, the pollen tube penetrates through the micropyle (a pore or opening on the surface of the ovary)

The Angiosperm Life Cycle (continued)

The Angiosperm Lifecycle (continued)7. Two sperm cells are discharged into the female gametophyte (embryo sac)• One sperm fertilizes the egg forming a diploid zygote• The other sperm cell fuses with the two nuclei in the large central

cell of female gametophyte• This is known as double fertilization- a process unique to

angiosperms!

The Angiosperm Lifecycle (continued)

8. Ovule matures into a seed, and then the zygote develops into a sporophyte embryo (alternating generations) with rudimentary roots and one or two seed leafs know as cotelydons.

9. Nucleus of the central cell of female gametophyte divides repeatedly and develops into endosperms (tissue rich in strarch and other food reserves)

The Angiosperm Lifecycle (continued)

Double Fertilization

Evolved independently in Angiosperms and Gymnosperms

• Gymnosperm Double Fertilization fives rise to 2 embryos whereas Angiosperm Double fertilization gives rise to 1 embryo and 1 endosperm

Hypothesis' on the function of double fertilization:

• Adaptation to prevent plants from squandering the nutrients in fertile ovuleso Synchronizes development of embryo with development of

food storage

The Evolution of Angiosperms

• 125 million year old angiosperm fossils

• Archaefructus lioaningensis and Archaefructus sinensis

• Archaefructus: most closely realted known fossil to all living angiosperm

• Suggests that ancestors of Angiosperms were herbaceous rather than woody

•  Discovered along with fish fossils suggesting angiosperms originated as aquatic plants

"Mostly Male" Hypothesis

• Asserts that the flower evolved mainly from the pollen-producing (male) reproductive structure of a gymnosperm ancestor

• Proposed by Michael Frohlich

• “Evo-Devo” approach: synthesis of evolutionary and developmental biology

• Botanists test evo-devo models for origin of flowers by comparing angiosperm and gymnosperm genes

Biological DiversityCotyledon: seed leavesMonocots: species with one cotyledon Dicots: Species with two cotyledons Eudicots: “true” or “modern” dicots

30.4Human Welfare

•Humans depend on seed plants•food•Herbal remedies

Angiosperm-Animal Co-evolution

• •Selective pressure on both plants and animals

• Co-evolution results in diversity•Example: Plant-pollinator relationships

Threats to Plant Diversity

•Diversity is NOT a renewable source•Destruction of Habitat•Fast rate extinctions

Works Citedhttp://www.google.com/imgres?q=tree+seeds+with+wings&hl=en&gbv=2&tbm=isch&tbnid=N3v2JlMCqyw4cM:&imgrefurl=http://www.sciencephoto.com/media/33789/enlarge&docid=nvftpw2LENxvBM&imgurl=http://www.sciencephoto.com/image/33789/large/B7870346-Sycamore_tree_seeds-SPL.jpg&w=415&h=530&ei=oO66ToTIEcrq0gHm7aXeCQ&zoom=1&iact=hc&vpx=700&vpy=118&dur=4&hovh=254&hovw=199&tx=89&ty=120&sig=102685288777232855691&page=1&tbnh=132&tbnw=103&start=0&ndsp=61&ved=1t:429,r:3,s:0&biw=1826&bih=1024

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http://www.rbgsyd.nsw.gov.au

http://brp-journal.blogspot.com/2008/11/traveling-seeds-or-how-seeds-get-to-new.html 

http://mrswolfgang.wikispaces.com/Monocots+and+Dicots+team+twix 

http://mshallarvadahs.pbworks.com/w/page/38617351/Coevolution%20and%20Pollinators 

http://health.howstuffworks.com/wellness/natural-medicine/herbal-remedies/slippery-elm-herbal-remedies.htm 

http://www.fsd2010.org/program/muller-landau.htm