Life cycles of early land plants

diploid nucleus becomes haploid daughter cells

haploid cells fuse to produce diploid nucleus

two gametes that fuse are not differentiated in size so cannot identify male or female gametes (have - or + strains instead)

two gametes that fuse are differentiated in size and the female gamete is the largest one

extreme anisogamy where the female gamete has lost motility and is much larger than the male gamete. Egg and sperm

haploid (gametophyte) and diploid (sporophyte) generations alternate (different to in animals which are zygote to zygote)

• Diploid zygotę which is sometimes a resting stage • Zygote becomes a zygospore (resting zygote) • When zygospore is reactivated it undergoes meiosis to make daughter cells with + or - strain cells • Daughter cells grow and divide until they are large enough to undergo mitosis and make gametes • Motile gametes fuze and form zygotes

• Diploid egg is retained on haploid parent • Can form a zygospore • Undergoes meiosis to make haploid spores • Spores germinate and grow to make gametes

• Zygote, instead of immediately making daughter cells, grow into a multicellular embryo and then a sporophyte (diploid individual) • Sporophyte makes haploid spores that grow into gametophytes that make gametes • Very similar to algae (just evolved a sporophyte multicellular phase)

• Spores are desiccation resistant and can be dispersed by wind • Algae zygotes need a watery medium • Sporophytes amplify the number of spores that can be produced from each zygote (algae only produce four spores from each zygote) - more chance of successful reproduction • Sporophyte longevity, photosynthetic ability and size become more complex as vascular and seed plants evolve • Gametophytes become less ecologically dominant

Sporophytes amplify number of spores from each zygote and so sporophytic dominance amplifies that further - can live through multiple reproductive seasons and produce many spores

• Could land plants have evolved the same morphological diversity with ancestral algal life cycle?Could we have had giant moss gametophytes? • A tree sized moss gametophytes would produce gametangia and the rain fall would disperse to archegonia (could have worked) • This life cycle would have been dependent on external factor (rain availability) - this would have limited ability to colonise all biomes (dry areas especially)

• animals • brown algae called fucus Fucus: Zygote is diploid and grows into an embryo Embryo grows into mature diploid individual Produce sperm and eggs which are released into the water and fuse to produce zygote • no, too dry on land