Chapter 18: Life cycles and reproductive strategies

Chapter 18: Life cycles and reproductive strategies

Reproductive strategies in plants


• Various modes of asexual reproduction

• Also flowering plants can– Cross-fertilisation– Self-fertilisation– No fertilisation


Plant life cycles• Plants spend more time in haploid

stage• Great variation between different

types of 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


• Gametophytes produce haploid gametes by mitotic divisions

• Two gametes fuse to form diploid zygote

• Zygote undergoes mitosis to produce sporophyte (diploid life stage)



• 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


• Smaller spore (microspores) develop into male gametophores which produce sperm

• Larger spore (megaspores) develop into female mega-gametophytes which bear eggs


• 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


• 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


• 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).


Fertilisation strategies• The flowers of most plants are

bisexual (they produce male and female gametes)


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

cross-fertilisation


Self-fertilisation (self-pollination)• Violets will cross-fertilise in

conditions that are good for pollination but self-fertilise in less favourable conditions.


Cross-fertilisation• Adaptations that promote cross-

fertilisation and decrease chance of self-fertilisation.


• 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


• Maturation of male and female parts of flower at different times

• Genetically controlled self-recognition responses of stigma and style to prevent self-pollination


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


Provision of resources to the embryo• The amount of food stored in plant

seeds varies greatly• Two extremes are orchids and

avocados


Orchid• Orchid seeds are minute, no

endosperm or food storage. • Embryo lacks formed tissue• Four million seeds produced per fruit• Seeds dispersed by wind


Avocado• Produce fewer seeds• Supply large amounts of food• Each fruit has only one seed• Seed contains well developed

embryo


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

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Chapter 18: Life cycles and reproductive strategies