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

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

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 cross- fertilisation

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 cross- fertilisation and decrease chance of self- fertilisation.

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