Alternation of Generations: During the life cycle of a plant, the plant alternates between diploid cells (united cells or cells in the process of becoming united and have a full set of genes) and haploid cells. (Cells that are not united and only have a half set of genes. Simply put they are the male sperm or the female egg cell.)
In most lower plants, up to and including ferns, the gametophyte (haploid) and sporophyte (diploid) generations are separated from the parent plants as they unite in the prothalis. See Chapter on Fern reproduction.
In flowering plants the gametophyte generation is reduced to a few haploid cells. These haploid cells are enclosed within the flower and supported by the diploid parent, the plant. This is the germ cell within the ovary or egg.
In Angiosperms, the male gametophyte generation occurs in the anther, at times on a separate plant or separate flower resulting in the pollen grainand the plants sperm.
In Angiosperms the sperm cells form in the anthers of the flower.
There are 3 stages in completing the pollen process are:
- Meiosis occurs producing 4 haploid microspores from the diploid microspore of the mother cell.
- The nucleus in each of these microspores divides once to produce a pollen grain which has two haploid cells. The pollen grains are the male gametophyte.
- One of the cells of the pollen grain will grow into the pollen tube and grow down from the stigma through the style to the ovule. The other cell of the pollen grain is the generative cell which divides by mitosis to form 2 haploid sperm cells which will ultimately fertilize the female gametophyte or ovary.
Pollination is not fertilization; it is the meiosis and mitosis process of the diploid and haploid stage of growth with the transfer of pollen to the stigma for fertilization.
Actual pollination is when a pollen grain is released from the anther and lands on the stigma. Once this occurs, one part of the pollen grain is stimulated to germinate, producing a pollen tube that grows down the style to the egg in the ovule while the generative cell develops into a sperm to complete the process of fertilization. Chemical solutions on a stigma may only stimulate pollen grains of the same species to germinate which helps eliminate cross pollination from other specie.
Self Pollination & Cross Pollination
Cross pollination: While some plants do self pollinate, this is not the ideal situation as cross pollinating limits the genetic variation with in the species. This can prove a disadvantage in a changing environment where the widest genetic variation gives some members of the specie the greatest opportunity to survive a change in their environmental situation. Most plants have one or more mechanisms in place to prevent self pollination between individuals of the same species.
* Dioecious plants are trees that only have male or female flowers not both on the same tree which eliminates all chances of self pollination. They have achieved cross pollination by the very nature of having the male flowers with pollen and the female flowers with carpels on separate.
Monoecious plants have rectified this problem by having the male pollen mature at different times to the female carpels or vice versa on maturing perfect flowers.
* Protandrous Perfect Flowers are flowers that produce pollen prior to the carpel on the same flower before the stigma becomes receptive on the same plant.
* Protogynous Perfect flowers are flowers where a flower’s stigma is receptive before the anthers dehisce within the same flower on the same plant.
* The physical structure of the stamens and stigmas can help prevent self pollination when pollen and stigma mature at the same time.
* The physical attraction to certain insects with the release of certain pheromones or nectar can help prevent self pollination when the structures mature at the same time.
* Some species are genetically structured so that pollen from the same flower, or from other flowers on the same plant, cannot cause fertilization. This design is known as self incompatibility and ensures that seed production results only from cross-pollination. Often the pollen assumes different shapes or sizes to the core of the style. This is particularly noticeable between different specie and genre which also prevents cross pollination between different specie and genre.
<<Pollination Photos 10.3>>
Three different pollen grains. A grass sp. pollen grain on the left and an Euclyptus sp. grain on the right. Drawings copied from web photos.
The outer layer is mainly exine which isan environmental resistant substance that prevents damage while the Tube nucleus grows down to the ovule and fertilizes the germ cell in the Embryo sac.
have two or three morphological types of flowers within a given population. In each individual plant, all the flowers share the same morphology however the flower morphology differs in that the lengths of the pistil and stamens vary in size and these traits are not continuous. The morphological phenotype is genetically linked to its genes.
This method of self incompatibility is known as heteromorphic self incompatibility. That is, the pollen from a flower on one morphology cannot fertilize another flower of the same morphology. Heterostylous plants that have two flower morphs are known as distylous and those with three morphs are known as tristylous.
Basically it is where distylous plants have 2 different size stamens and styles. A short stamen is coupled with a long style or a long stamen is coupled with a short style. This method is known to work beautifully with plant specific insect pollinators.
The insects tend to collect and deposit pollen on the part of its body that comes in contact with the anther and stigma of a different morphological type of flower.
Self-pollination: In some species self-pollination is the norm despite the genetic benefits of cross-pollination. There are three common advantages:
* A specific genetic type may have adapted better to its environment displacing the need for variation so self pollination enhances this trait and keeps it in tact.
* Self pollinating plants are far less dependent on environmental pollinating agents or pollinators.
* Self pollination is an advantage when population densities are small or scattered.
<<Pollination Photos 10.4>>
<<Pollination Photos 10.5>>
Allogamy: From állos, which is Ancient Greek for the other and gámos, which is Ancient Greek for to marry or marriage. It refers to cross-fertilization, occurs when an ovum of a flower is fertilized by the spermatozoa of another flower.
Autogamy: From auto-/autós, which are Ancient Greek for self or oneself and gámos, which is Ancient Greek for to marry. It refers to where self fertilization, occurs when an ovum of a flower is fertilized by the spermatozoa from the same flower.
Cross Pollination: Is the same as allogamy – From állos, which is Ancient Greek for the other and gámos, which is Ancient Greek for to marry or marriage. It refers to cross-fertilization, occurs when an ovum of a flower is fertilized by the spermatozoa of another flower.
Embryo sac: Is the female gametophyte of a seed plant consisting of a thin-walled sac within the nucellus that contains the egg nucleus and other nuclei which give rise to endosperm on fertilization.
Geitongamy: From geítōn, which are Ancient Greek for a neighbour and gámos, which is Ancient Greek for to marry. It refers to where self fertilization from the same plant occurs. In flowering plants, pollen is transferred from a flower to another flower on the same plant, and in animal pollinated systems this is accomplished by a pollinator visiting multiple flowers on the same plant.
Funiculous: From fūniculus, which is Latin for a thin rope or cordage. It is any cord-like structure in flowering plants, especially the funiculus, which is the stalk that attaches an ovule to the placenta. Refered as the umbilical cord in animals.
Integument: From integumentum, which is Latin for a thin covering. It refers to the natural covering of an organism or an organ, such as its skin, husk, shell, or rind. There are two thin intergumentums layers around the ovum, the outer and inner.
Mycropyle: From mikrós, which are Ancient Greek for small or tiny and púlē, which is Ancient Greek for a gate or entrance. It refers to a minute opening in the integument of an ovule of a seed plant of which the sperm must enter to complete fertilisation of the ovum.
Pollination: Is the act of transferring pollen grains from the male anther of a flower to the female stigma. The goal of every living organism, including plants, is to create offspring for the next generation. Pollination is the exact moment the correct pollen of a species adheres to the stigma of the same species. The stigmas of flowering plants selectively recognize pollen from the same species in many ways. The physical shape of the pollen and times that the stigma is receptive and the pollen is released are recognition factors to fertilization. The physical shape is akin to the recognition between the pollen and the stigma is a geometric one. It is analogous of a lock only being locked or unlocked by the key which was designed for it. This is only part of the mechanism of recognition. Another part is through biochemical interactions, where proteins and ensymes play an important role.
Self Pollination: It refers to where self fertilization, that is occurs when an ovum of a flower is fertilized by the spermatozoa from the same flower.
The shape of the pollen is important for several reasons from ensuring cross pollination it serves as in the distribution of the pollen.
* Wind pollination is more efficient when plants grow in close communities. Most grasses and reeds rely on wind pollination. Wind pollination is a hit and miss method of pollination than other methods. Wind pollinated plants need to produce higher volumes small, light weight pollen with large surface ratio pollen grains with little nectar as pollen would stick to the nectar and become ineffective.
* Pollinators include animals and insects which transfer the pollen grains from one flower to another. Plants vary greatly depending upon which pollinator the plant wants to attract. Animal pollination is a far better strategy as it is highly efficient because nectar uses less energy to produce; being a simple sugar than pollen which are complex proteins. The plants attract the pollinator which in return is fed a small quantity of nectar ensuring the animal or insect needs to move to another plant to feed. The flowers are usually larger and showier in reds and oranges for birds, Yellow and blue hues often with pheromones and scents for insects and white and scented for nocturnal pollinators like moths and bats.
Many plants have developed symbiotic relationships with very specific insects or animals in mind. Many Persoonia species can only be serviced by a few native hive bee specie. Stylidium specie or trigger plants attract specific unsuspecting male wasps to copulate with them while many Drosera have a special relationship with a small fly or beetle to transport their pollen from one plant to another. Many flies are attracted to flowers that process a decaying odour usually or rotting meat or urinated faeces.
- Alternation of generations is the alternation between diploid (2n) and haploid (1n) cells from one generation to the next.
2. In Angionsperms, the male gametophyte generation is the Pollen Grain with the pollen tube nucleus and the generative nucleus.
3. Pollination is the transfer of pollen from an anther to a stigma.
4. Protandrous flowers produce pollen before the carpel of the becomes receptive.
5. Protogynous flowers have a receptive stigma before the pollen grains are released from the anthers.
6. Self pollination is an advantage where the number of individuals is sparse over a large area, the plants are not dependant of pollinators and their genes have a better advantage within the environment.
7. Wind pollinated flowers are usually small, produce copious quantities of pollen and produce little or no nectar.
8. Animals and insects are attracted to a flower which is larger,
usually with bright colours, have scented flowers or those specific pheromones.
9. Angiosperms form sperm cells in the pollen of the anthers of the flower.
10. Self pollination is not ideal because the in breeding limits the diversity in genetic variations within the species.
11. Heterostyly is a genetically determined combination of mechanisms promoting cross pollination.
12. Exine has weather resistant properties which protect the pollen grains during pollination and fertilization.
13. Bats and moths pollinate white nocturnal flowers which usually have strong scents.
14. Honeyeaters prefer flowers which have large, bright red or orange coloured flowers.
15. Insects prefer flowers that are yellow or blue and have distinctive pheromone scents or that of decaying matter or scents of urinated faeces.
16. The diploid (2n) cell in a pollen grain forms two haploid (1n) sperms one which grows into a pollen tube and one for fertilization.
17. Shape, size surface area and projections all have a function in the way pollen is distributed.
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