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Evolution of the seed
Adaptations of seed plants
• Seeds
• Reduced gametophyte stage
– seed plants reduced to microscopic (spores develop in
sporangia of sporophyte)
• Heterospory
– Megaspores – make female gametophyte
– Microsporangia – male gametophyte
• Ovules = megaspore & integuments (protect)
• Pollen = micro (male gametophyte) & sporopollenin
Microsporangium: on anther
Megasporangium
• Ovary produces the ovule
• Ovule parts:
– INTEGUMENT
– MACROSPORAGIA
– FEMALE GAMETOPHYTE (egg)
Seed
• Seed :
– Embryo: immature plant
– Endosperm:food supply
– Seed coat of integument
• Ovule develops into seed
• Cotyledons: 1 or 2 seed leaves
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Seed parts
Embryo
Endosperm
Seed coat
Seed
• Double Fertilization
– The union of two
sperm cells with
different nuclei of the
embryo sac.
• Endosperm
– Food storing tissue of
the seed.
http://www.learner.org/channel/courses/essential/life/imag
es/show4.open_seed.jpg
Seed to Seedling
• Imbibition is that state when the seed wakes
up from dormancy and starts the intake of
water.
• It causes the seed to expand and ruptures,
releasing the shoot, the cotyledons and the
stalk.
• It is the first sign of life after the dormant
state.
Seed to Seedling
http://students.usm.maine.edu/deidre.rice/_ILLUS_ILT_T630888A.GIF
Spore survival
• Prior to seeds: spore was the only protected
stage for plants.
– Spore is single cell
– Could survive: cold hot dry
– Could be dispersed to another environment
Spore Release
Then, seeds
• Seeds are multicellular
• Has protective integument coat from ovule
• Can be dormant for days months years
Seed Release
Vascular Seed plants
• Gymnosperms: fir, pine
• Angiosperm: flowering plants
Pine tree
• The pine tree is the sporophyte (2N)
• Its sporangia are located on the pine cone
• Heterosporous
– Small pollen cones: carried by wind and critters
– Large ovule cones
Each tree has both types of cones
Female - Male
female
male
Angiosperm
• Flowering plants: vascular and seeds
• Most diverse and widespread plants
• Flower: specialized for reproduction
– Carried by wind: like gymnosperm
– Carried by insects: more directed pollination
• Fruit: protect seed and aid in dispersal
– From ovary. Ovary thickens after pollination.
Flower Structure
• Four main organs: sepals,
petals, stamens, and
carpels
• Stamens and carpels are
reproductive
• Carpel:
• Stamen: filament, anther
– Anther: stalk where pollen
made
• Sepals and petals: no
repro organs
– Stigma: collects pollen
– Style: to ovary with 1 or
more ovules
– ovary
– Pistil: multiple fused
carpels
http://images.google.com/imgres?imgurl=http://andromeda.cavehill.uwi.edu/
Gametophyte Development and
Pollination
• Sporangia- structure on
– anther and
– ovules where spores are produced
• Pollen grains are made up of mature male
gametophytes that are enclosed by a spore
wall
– Found in the microsporangia, or pollen sacs
Hormones
• Auxin
– Stem elongation, root growth, develop fruit
• Cytokinins: affect, cell division, cell differentiation, and apical
dominance.
• Gibberellins
– Seed and bud germination
• Abscisic acid
– Inhibit root growth, close stomata
• Ethylene
– Fruit ripening
Mechanisms that Prevent SelfFertilization
• Sexual reproduction ensures that there will be
genetic diversity among offspring
• “Selfing” refers to self-fertilization in plants
• Ensures that seed will develop
Pollination Vs. Fertilization
• Pollination is the transfer of pollen from
anther to stigma (of other plant)
– Results in formation of a pollen tube
– Purpose is to grow down to ovary and release
sperm within embryo sac
– This fertilizes the egg
– Embryo -> seed -> fruit containing seed
– The fruit disperses seeds which germinate and
develop into seedlings
Development
• Microsporocytes form four haploid
microspores
– These develop into haploid male gametophytes
– The microspore goes through mitosis and
cytokinesis
– This results in a generative cell and tube cell which
make up the pollen grain
• Megasporocyte grows resulting in four haploid
megaspores
– May form a multicellular female gametophyte
• Contains 3 cells: 1 egg and 2 synegrids
• Synegrids attract and guide pollen tubes to the embryo
sac
• there are also two nuclei at the other end of the cell
that are not separate and share cytoplasm embryo sacs
• These result in two integuments that form a seed
• Ensuring that the egg and sperm cells come
from different parents is a mechanism that
inhibits self fertilization
• Dioecious plants cannot self fertilize because
they have either staminate or carpellate
flowers
Avoid self fertilization
• Flowers with functional stamen and carpels
have organs that mature at different rates
– An animal pollinator would not transfer pollen
from the anther to a stigma of the same flower
Self Incompatibility
• The ability of a plant to reject its own pollen
• Also, in some cases, the pollen of similar
plants
• If pollen were to land on the stigma of a
flower on the same plant a biochemical would
prevent the pollen from developing and
fertilizing an egg
Special Circumstances
• Plant breeders will hybridize different crop
varieties to combine the best traits and to get
sufficient results
• This can result in inbreeding
• May cause plants that are able to self fertilize
to lose that characteristic.