PLANT STRUCTURE AND FUNCTIONS OF STEMS
Theme D – lecture 17
flowers
What is a flower?
An angiosperm’s way of having sex
• No matter our symbolic attachment to the diversity of flowers, from the plants perspective, the flower has evolved for sexual
reproduction
• Most (but not all!) aspects of flower evolution have been shaped by natural selection to optimise reproductive fitness
• Some questions to keep in mind when trying to explain a bizarre/spectacular feature of a flower is, why has this evolved the way
it has? How does this influence reproductive fitness? In other words, what is the cost/benefit to the plant or its genes?
Plant sex is weird:
Reason 1 Reason 2
• Plants have multicellular haploid and diploid phases in •Unlike animals, plants cannot move, so their sex typically
their life cycles involves a third party that mediates the movement of
• Haploid plant – gametophyte pollen (and male gametes) to embryo sac (and female
• Diploid plant – sporophyte gamete) – a pollinating agent/ pollinating vector
• The phases look different to one another, but both are • Biotic – insects (bees, moths, butterflies, flies,
necessary for the completion of the life cycle wasps, bugs etc), birds, mammals (rodents,
• In angiosperms, the gametophyte phase is further split shrews, bats)
into separate plants – a male gametophyte (pollen grain) • Abiotic – wind, water
and a female gametophyte (embryo sac) • Pollination =/= fertilization!
• male gametophytes can only produce sperm, and female
gametophytes can only produce eggs
• Pollination is the transfer of pollen from anther to
stigma (preferably of another plant of the same
• Unlike animals, plants produce entirely (genetically) species)
separate, multicellular individuals out of the end-products
of meiosis • Fertilisation is the fusion of male and female
gametes
• Meiosis results in spores, not gametes!
Reason 3
•Spores grow into multicellular, haploid plant
• Unlike most animals, flowers of most plants are
(gametophytes) and eventually produce sex cells
hermaphroditic – they have both male (sperm-producing,
(gametes) via mitosis
via pollen) and female (egg producing, via the embryo sac)
• In angiosperms, these gametophytes are the pollen grain
function
(male gametophyte), which produces two sperm cells, and
• Because of this, most plants have evolved ways of
the embryo sac (female gametophyte), which produces a
preventing sex with themselves i.e. inbreeding
single egg
• Both pollen (partly) and embryo sac are dependent on Reason 4
the parent, diploid plant for nutrition: • In angiosperms, both sperm cells are used in sexual
reproduction
• Pollen – during original development and growth • One sperm cell fuses with the egg to produce the diploid
of pollen tube in the style zygote, which will divide via mitosis to produce the embryo
• Embryo sac – during its entire existence • The other sperm cell fuses with two other nuclei in the
• In seed plants, particularly angiosperms, the pollen and embryo sac (called polar nuclei) to form a triploid
embryo sac are basically tiny, gamete-producing machines endosperm mother cell, which divides via mitosis to
– they are multicellular, if just barely! produce endosperm • This process is double fertilization –
• The embryo sac lives entirely inside the diploid mother unique to angiosperms • Endosperm serves a critical role
plant, in a structure called the ovule as a storage tissue in the early maturation of the seed
• Also arguably the most important plant tissue to
humankind! It is endosperm in rice/maize/wheat/barley
that feeds our global civilization
Theme D – lecture 17
flowers
What is a flower?
An angiosperm’s way of having sex
• No matter our symbolic attachment to the diversity of flowers, from the plants perspective, the flower has evolved for sexual
reproduction
• Most (but not all!) aspects of flower evolution have been shaped by natural selection to optimise reproductive fitness
• Some questions to keep in mind when trying to explain a bizarre/spectacular feature of a flower is, why has this evolved the way
it has? How does this influence reproductive fitness? In other words, what is the cost/benefit to the plant or its genes?
Plant sex is weird:
Reason 1 Reason 2
• Plants have multicellular haploid and diploid phases in •Unlike animals, plants cannot move, so their sex typically
their life cycles involves a third party that mediates the movement of
• Haploid plant – gametophyte pollen (and male gametes) to embryo sac (and female
• Diploid plant – sporophyte gamete) – a pollinating agent/ pollinating vector
• The phases look different to one another, but both are • Biotic – insects (bees, moths, butterflies, flies,
necessary for the completion of the life cycle wasps, bugs etc), birds, mammals (rodents,
• In angiosperms, the gametophyte phase is further split shrews, bats)
into separate plants – a male gametophyte (pollen grain) • Abiotic – wind, water
and a female gametophyte (embryo sac) • Pollination =/= fertilization!
• male gametophytes can only produce sperm, and female
gametophytes can only produce eggs
• Pollination is the transfer of pollen from anther to
stigma (preferably of another plant of the same
• Unlike animals, plants produce entirely (genetically) species)
separate, multicellular individuals out of the end-products
of meiosis • Fertilisation is the fusion of male and female
gametes
• Meiosis results in spores, not gametes!
Reason 3
•Spores grow into multicellular, haploid plant
• Unlike most animals, flowers of most plants are
(gametophytes) and eventually produce sex cells
hermaphroditic – they have both male (sperm-producing,
(gametes) via mitosis
via pollen) and female (egg producing, via the embryo sac)
• In angiosperms, these gametophytes are the pollen grain
function
(male gametophyte), which produces two sperm cells, and
• Because of this, most plants have evolved ways of
the embryo sac (female gametophyte), which produces a
preventing sex with themselves i.e. inbreeding
single egg
• Both pollen (partly) and embryo sac are dependent on Reason 4
the parent, diploid plant for nutrition: • In angiosperms, both sperm cells are used in sexual
reproduction
• Pollen – during original development and growth • One sperm cell fuses with the egg to produce the diploid
of pollen tube in the style zygote, which will divide via mitosis to produce the embryo
• Embryo sac – during its entire existence • The other sperm cell fuses with two other nuclei in the
• In seed plants, particularly angiosperms, the pollen and embryo sac (called polar nuclei) to form a triploid
embryo sac are basically tiny, gamete-producing machines endosperm mother cell, which divides via mitosis to
– they are multicellular, if just barely! produce endosperm • This process is double fertilization –
• The embryo sac lives entirely inside the diploid mother unique to angiosperms • Endosperm serves a critical role
plant, in a structure called the ovule as a storage tissue in the early maturation of the seed
• Also arguably the most important plant tissue to
humankind! It is endosperm in rice/maize/wheat/barley
that feeds our global civilization
