3 nuclei are in the three antipodal cells o Two nuclei, polar nuclei, are in a single cell in the middle o The two synergids cells each have a nucleus o The egg cell contains one nucleus o The embryo sac is inside the ovule, which is in the ovary of the carpel Pollen grain o Develop a weather resistant wall which has surface features that are specific for the species o Pollen is often preserved in sediments of lakes and when sampled reveal clues to changes that have occurred in the terrestrial vegetation o Inside the pollen grain are the tube cell and the generative cell o The generative cell divides by mitosis to produce two sperm o The tube cell will grow a pollen tube through a pore or furrow in the wall o Pollen is the land plant innovation that allows sperm to travel to the egg without swimming in water Pollination Pollen transfer to the stigma is dfined as pollination Self-pollination - If the pollen comes from the same flower or a different flower on the same plant o In some cases, the flower never opens o Ecologically advantageous – in environments where pollinators are not reliably available o Produces offspring that are more genetically similar to the parent plant then those produced by cross pollination o The offspring are not clones because meiosis is involved
o In stable environments, offspring with similar traits as the parents, will likely be better adapted to that enviornment Outcrossing (cross pollination) – transfer of pollen between different individuals and promotes genetic variation in the offspring Several strategies promote outcrossing o Separation of male and female structures o Monoecious – separate male and female flowers on the same plant Means one house If they mature at different times, outcrossing is promoted o Dioecious – male and female flowers are produced on different plants (unisexual) Means two houses Outcrossing is required o Most flowers are bisexual o Dichogamous – if stamens and carpels reach maturity at different times Outcrossing promoted Pollinator visits the older flowers that are carpelate first, then moves upward to the younger staminate flowers When the pollinator moves to a different flower stalk, it starts at the bottom again depositing the pollen from the previous plant o In many bisexual plants, the stamens and carpels mature at the same time o Self-incompatibility – when pollen and stigma recognize each other as being genetic related and pollen tube growth is blocked Self incompatibility increases the likelihood of outcrossing Pollinators and Angiosperms Co-evolution
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- Spring '08
- Biology, Pollination, Pollen, embryo sac