Sexual Reproduction in Plants
Alternation of Generations
Alternation of generations is when you have life history that alternates between free-living multi-cellular diploid stage (sporophyte) and the free-living multi-cellular haploid stage (gametophyte).
Alternation of generation can be applied to green algae. In the case of Ulva, the gametophytes and the sporophytes look exactly the same. This is call isometric alternation of generation. (Iso = same)
The general trend is to have sporophytes be the more common stage of reproduction. However, with moss this has not happened.
Alternation of generation can be applied to mosses. The sporophyte grow above the female moss.
Alternation of generation can be applied to ferns. The spores are located under the fern leaf. Each spore becomes a heart shaped gametophytes. Some ferns are hetero- or homo-sporophytes. They create male and female or just female ferns.
Alternation of generation can be applied to gymnosperms. The tree is the sporophytes. The gametophyes are located in the cones of the trees. The male cones are only on the tree for 3-4 weeks before completing the reproductive cycle. The female cones can stay on the tree for 3-4 years.
Alternation of generation can be applied to angiosperms.
“A diagram of the parts of flowers” Photo credit to Science & Plants for Schools retrieved from saps.org.uk
Ovule contains the embryo sac surrounded by integuments. Three cells are located at the bottom, the center one is the egg and the two that surround it are synergids. The three cells located at the top are antipodals. Two haploid polar nuclei are located in the middle. Ovule is the female gametophyte.
Embryo Sac = Female Gametophyte
Pollen Grain = Male Gametophyte
Double Fertilization is unique to angiospersms. A double fertilization occurs when a female gametophyte (embryo sac) and two male gametophytes (sperm) encounter each other. The sperm make its way down the pollen tube . One sperm produces endosperm, and one produce embryo. The endosperm is a nutritive tissue in seeds to feed the embryo.
Plant reproduction is complex.
There are sexual and asexual reproduction, but there are also apomictic seeds, agamospermy. The agamospermy produce seeds that are clones of themselves. This occurs in dandelions.
Perfect Flowers vs. Stamenate or Pistillate Flowers
Monoecious vs. Dioecious Plants
Androdiecy vs. Gynodiecy
Some individuals produce perfect flowers and some produce only male or female flowers.
Some of these species are obligate outcrossers. These outcrosser needs pollen from another plant and cannot self pollinate
Some of these species are obligate selfers. These selfers can only self pollinate.
A lot are flexible enough for both outcrossing and selfing to happen.
How does mating system influence geographic range size for selfers vs. outcrossers?
Selfers have a bigger geographic range because they can spread even with low densities. They have reproductive assurance.