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SEED PLANTS - Coggle Diagram
SEED PLANTS
Evolution History of Seed Plants
Seed plants are the Gymnosperms and Angiosperms
Seed plants first appeared in 305-465 million years ago
Angiosperms(flowering plants) are the most recently evolved plants
Seeds and Pollen as an Evolutionary Adaptation to Dry Land
Embryo- produced from the diploid zygote, grows into the sporophyte when seed germinates
seed- offers the embryo protection, nourishment, and a mechanism to maintain dormancy, allow plants to disperse the next generation through both space and time
Female gametophyte
Develop within an ovule [egg and endosperm-producing cell (supports the growth of embryo)]
Enclosed within diploid sporophyte tissue in angiosperms
Male gametophytes
Within pollen grains (sperm of plant)
Protected from desiccation
Not dependent on water to reach female organs
Reproductive Adaptations of Seed Plants
Reduced & Retained Gametophytes
Seed plants are heterosporous
Megaspores (female gametophytes)
Microspores (grow into male gametophytes)
both gametophytes mature within sporangia
highly reduced in size
protected from drying & UV light
Seed
Develops from fertilized ovule
Ovule = integument + mega sporangium + megaspore
includes: embryo, food supply, seed coat
Megaspore germinated within ovule, develops into tiny female gametophyte - produces egg which will eventually be fertilized
Pollen grains
Male gametophyte surrounded by pollen wall (partly secreted by sporophyte
capable of long dispersal, tough, & resistant/ Sperm nuclei doesn't require external H2O for fertilization
Sporopollenin in the pollen wall protects pollent
Pollination: transfer of a pollen to a part of the seed plant that contains the ovule
Pollen grain germinates -> pollen tube -> sperm discharged into female gametophytes
Major group of seed plants
Gymnosperms
lack flowers & fruits
Naked seeds: all have ovule exposed on a scale (partially sheltered by sporophylls)
Pollination by wind
Reduced male/female gametophytes
Heterosporous
Monoecious or Dioecious
Dominant Sporophytes
Female and male reproductive organs can form in cones/strobili
4 phyla:
Coniferophyta
most diverse gymnosperms today
"cone-bearers"
male pollen cone (sperm not motile)
female ovulate cone
Dominate high altitudes & latitudes in N. Hemisphere
Many are evergreen with needle-like leaves
Cycadophyta (Cycads)
fern-like or palm-like fronds radiate from central stem
central female & male cones
with flagellated sperm
beetles are involved in pollination of some cycads
Ginkgophyta (ginkgos)
one extant tree: Ginkgo biloba
herbal medicine
with flagellated sperm
Gnetophyta
3 distinct genera:
Tropics: Gnetum
Sw Africa Deserts: Welwitschia
Us Deserts: Ephedra
female/male strobili (sperm not motile)
Ephedra - herbal medicine (ephedrine)
Angiosperms
Key innovations
Flower - structure specialized to facilitate sexual reproduction
success mainly due to coevolution with pollinators
Stamen (male):
Filament- stalk that holds up pollen-producing sac
Anther- terminal sac that produces pollen
Carpel (female):
Stigma- sticky tip of carpel that receives pollen
style- long tube leading from stigma to ovary
Ovary - structure at base of the carpel that produces ovules (develop into seeds if fertilized)
Life cycle
Microsporangia -> Male sporocytes(2n) -> (meiosis)-> microspores(n) ->(mitosis)-> pollen grains
Pollen grains: 1 generative cell/ 1 tube cell
Ovule -> Mega sporangium -> mageasporocyte (2n) ->(meiosis)-> 4 megaspores(n) -> only large megaspores survives ->(mitosis x3)-> 8 nuclei distrubuted among 7 cells of female gametophytes -> 3 cells at one pole become egg and 2 synergids(remaining 3 cells become antipodal cells)
Center cell -> 2 polar nuclei
Double fertilization with 2 sperm nuclei
one fuses with egg, forming zygote (2n)
one fuses with 2 central nuclei (polar nuclei), forming endosperm (3n) food supply in seed
Fruit - mature ovary of a flower, thickens around seeds
protects seeds
main function is enhancing seed dispersal
2 groups of Angiosperms:
Monocots
one cotyledon
parallel veins
scattered vascular tissue
no main root/ fibrous root system
pollen grain with one opening
petals in multiples of three
Eudicots/Dicots
two cotyledons
net-like veins
ringed vascular tissue
taproot (main root)
Pollen grain with 3 openings
petals in multiples of four or five