Chapter 18: Life cycles and
reproductive strategies
Reproductive strategies in plants
Reproductive strategies in plants
• Various modes of asexual reproduction
• Also flowering plants can
– Cross-fertilisation
– Self-fertilisation
– No fertilisation
Reproductive strategies in plants
Plant life cycles
• Plants spend more time in haploid stage
• Great variation between different types of
plants
Reproductive strategies in plants
• Gametes not the direct result of meiosis
• Meiosis produces haploid spores that
undergo two to many rounds of cell
division (mitosis) to produce multicellular
haploid stage – Gametophyte
Reproductive strategies in plants
• Gametophytes produce haploid gametes
by mitotic divisions
• Two gametes fuse to form diploid zygote
• Zygote undergoes mitosis to produce
sporophyte (diploid life stage)
Reproductive strategies in plants
Reproductive strategies in plants
• In some ferns and mosses (non-flowering
plants) a single type of spore produces
both male and female gametes
• In flowering plants, meiosis produces
produces two types of spores that can be
distinguished by their size
Reproductive strategies in plants
• Smaller spore (microspores) develop into
male gametophores which produce sperm
• Larger spore (megaspores) develop into
female mega-gametophytes which bear
eggs
Reproductive strategies in plants
• The development of gametes is another
feature that is different between animals and
plants.
• Animals – gametes form from direct line of
germ cells
• Plants – gametes form simply from the cells
that are centrally positioned within the
reproductive organs of the developing flower
Reproductive strategies in plants
• Plants can propagate themselves asexually
• Plants can produce new tissues throughout their
life
• Plant cells are totipotent (a single plant cell under
the right conditions can produce a complete new
plant)
• This allows plants to recover from major damage
• Humans utilise this ability to clone desirable plants
Reproductive strategies in plants
• Some plants switch between sexual and
asexual reproductive strategies.
• E.g. wattles and blackberries will move
into a new vacant environment by
vegetative suckers (shoots arising from
roots).
Reproductive strategies in plants
Fertilisation strategies
• The flowers of most plants are bisexual
(they produce male and female gametes)
Reproductive strategies in plants
Self-fertilisation (self-pollination)
• Pollen is transferred to the sigma of a flower
on the same plant
• Allows genetic continuity of successful plants
in a particular environment
• Loss of genetic variation due to lack of crossfertilisation
Reproductive strategies in plants
Self-fertilisation (self-pollination)
• Violets will cross-fertilise in conditions that
are good for pollination but self-fertilise in
less favourable conditions.
Reproductive strategies in plants
Cross-fertilisation
• Adaptations that promote crossfertilisation and decrease chance of selffertilisation.
Reproductive strategies in plants
• Physical separation of male and female
gametes. Maize and zucchini have male
and female flowers on plant. Willows and
cannabis have male and female flowers on
separate plants
Reproductive strategies in plants
• Maturation of male and female parts of
flower at different times
• Genetically controlled self-recognition
responses of stigma and style to prevent
self-pollination
Reproductive strategies in plants
No fertilisation (Apomixis)
• Some flowering plants produce an embryo
without fertilisation.
• Megaspore mother cell (2n) does not
undergo meiosis, but undergoes mitosis to
produce embryo
• Genetic clone of parent
Reproductive strategies in plants
Provision of resources to the embryo
• The amount of food stored in plant seeds
varies greatly
• Two extremes are orchids and avocados
Reproductive strategies in plants
Orchid
• Orchid seeds are minute, no endosperm or
food storage.
• Embryo lacks formed tissue
• Four million seeds produced per fruit
• Seeds dispersed by wind
Reproductive strategies in plants
Avocado
• Produce fewer seeds
• Supply large amounts of food
• Each fruit has only one seed
• Seed contains well developed embryo
Reproductive strategies in plants
Seed dormancy
• Mature seeds usually enter a period of dormancy
• Dormancy ensures seeds germinate when conditions are
favourable
• Seeds can remain dormant for days to thousands of years
• Germination triggered by environmental cues (light,
temperature, etc.)
• Requires water and oxygen to reach embryo
• Seed coat may need to be broken to allow this