gymnosperms Flashcards
what are some significant features of
the evolution of seeded plants
1) the evolution of an ovule and seed
2) the female gametophyte being retained within the mother sporophyte
3) the ovule developing into a seed
4) replacing a spore with a seed as a unit of dispersal
what is pollen
a coarse, powdery substance comprising of pollen grains (male microgametophytes) which produce male gametes
how do gymnosperms differ from angiosperms
- dont produce flowers
- dont produce true fruit
- no herbaceous forms
- xylems have no vessels
- phloem have no sieve tube elements
what is a coniferophyta
a classification of gymnosperm (the conifers) which are the largest and most widespread gymnosperm and dominate many ecosystems
what are some characteristics of a pine tree
- evergreen trees with needle leaves
- leaves are produced in bundles of up to 8
- leaves usually live 2-3 years
- leaves have thick cuticles and sunken stomata so it is suitable for growth in areas with low water availability
outline the life cycle of a pine tree
1) the tree is a mature sporophyte generation
2) there are seperate male and female cones on the same tree
3) male cones produce pollen grains (microgametophytes)
4) females produce ovules, megagametophytes and seeds
5) seed production can take up to 3 years from pollination
where are male cones usually found in pine trees
found in clusters and the lower part of the tree
outline the structure of a male pine cone and its gametophytes
- simple with central axis with microsporophyll’s which bear microsporangia
- microspore mother cells undergo meiosis to produce 4 haploid microspores
- each microspore develops into a microgametophyte (pollen grain) of 4 cells
- pollen grains have two air bladders, tube cell and generative cells
outline the structure of a male pine cone and its gametophytes
- simple with central axis with microsporophyll’s which bear microsporangia
- microspore mother cells undergo meiosis to produce 4 haploid microspores
- each microspore develops into a microgametophyte (pollen grain) of 4 cells
- pollen grains have two air bladders, tube cell and generative cells
what has happened to the antheridia in many gymnosperms
it has been reduced to a single generative cell within the pollen grain which divides and gives rise to 2 sperm cells
outline the life cycle of female cone gametophytes
1) megasporangium contains megaspore mothers cells which divides by meiosis to produce 4 haploid megapsores
2) only 1/4 megaspores survives
3) free nuclear division inside the megaspore fives rise to the coenocytic cell which eventually forms the megagametophyte
where are megasporangium found in the female pine cone
the ovuliferous scale
what odes the megagametophyte contain
1) 3 archegonia each with a single haploid egg
outline the structure of a female cone
the ovule is enveloped in an integument which has an opening called the micropyle at its apex which allows the entry of pollen
outline pollination in pine cones
1) in spring pollen grains are released from male cones and carried by the wind to female cones
at this point the female cone is small and it its first year
1) a pollination drop exudes from the micropyle of the ovule
2) pollen is drawn in towards the nucelluls as this evaporates
3) the pollen germinates to produce a pollination tube from the tube cell
4) pollen tube grows through the nucellus to the egg in the archegonia
outline fertilisation in pine cones
1) the genrative cell in the pollen divides produced two sperm cells
2) both sperm are released into the archegonia but only one fuses with the egg cell to produce a diploid zygote
3) each egg in each of the three archegonia may be fertilized but usually only one zygote developes into an embryo
outline the structure of a seed
each ovule on the ovuliferous scale of the female cone develops into a seed protected by a seed coat developed from diploid integuments
- the diploid embryo is within the seed
- the seed is embedded in the haploid megagemetophyte tissue
what does the embryo of a pine tree consist of
1) hypocotyl-root axis
2) apical meristem
3) cotyledons
4) root cap
what is a cycada
another classification of a gymnosperm found in the tropics and subtropics
outline the characteristics of a cycada
- dioecious
- females produce very large cones
- male produce pollen and motile sperm
- beeteles often involved in pollination
- many contain neurotoxins
what is a gingko
a classification of gymnosperm whihc only has one representative (maidenhair tree) which has survived in the temple gardens and holy places in china and japan
what are gnetophyta
an odd mixture of gymnsperms comprising of 3 living genera and are the closest living ancestors to angiosperms
