Tuesday, July 29, 2008
Conclusion
Reproduction in plants
Well, firstly we must understand that the flowering plant has male and female organs too. Male organs are collectively called the androecium. The parts include the stamen, which consist of the filament which supports the anthers. The anther contains four microsporangia within which microspores (pollen) are produced by meiosis. Inside the pollen are two (or, at most, three) cells that comprise the male gametophyte. The tube cell (also referred to as the tube nucleus) develops into the pollen tube. The germ cell divides by mitosis to produce two sperm cells.
Female organs are collectively known as the Gynoecium. The gynoecium consists of the stigma, style, and ovary containing one or more ovules. These three structures are often termed a pistil. The stigma functions as a receptive surface on which pollen lands and germinates its pollen tube. The style serves to move the stigma some distance from the ovary. The ovary contains one or more ovules, which in turn contain one female gametophyte, also referred to as the embryo sac.
So for the stigma to even get pollens and reproduce, something known as pollination must occur. This is accomplished by a variety of methods. Entomophyly is the transfer of pollen by an insect. Anemophyly is the transfer of pollen by wind. Other pollinators include birds, bats, water, and humans. Some flowers (for example garden peas) develop in such a way as to pollinate themselves. Flower color is thought to indicate the nature of pollinator: red petals are thought to attract birds, yellow for bees, and white for moths. Wind pollinated flowers have reduced petals, such as oaks and grasses.
The process of pollination being accomplished, the pollen tube grows through the stigma and style toward the ovules in the ovary. The two sperm cells will move down the pollen tube. Once the tip of the tube reaches the micropyle end of the embryo sac, the tube grows through into the embryo sac through one of the synergids which flank the egg. One sperm cell fuses with the egg, producing the zygote which will later develope into the next-generation sporophyte. The second sperm fuses with the two polar bodies located in the center of the sac, producing the nutritive triploid endosperm tissue that will provide energy for the embryo's growth and development.
After fertilization, the zygote divides mitotically. The product of this repeated nuclear division and cell multiplication is an embryo. The seed will now develop. The development consists of a conversion of the integument of the ovule into a resistant seed coat, the development of the endosperm, and the development of the embryo. All these events take place within the original ovary.
A section through a nearly mature seed will reveal an embryo consisting of two large cotyledons with a small epicotyl between them attached to the hypocotyl. Most or all of the endosperm has been absorbed by the cotyledons and the integuments of the ovule have grown into a seed coat. The basal portion of the embryo is termed the radicle. The epicotyl develops into the above ground structures of the plant (stem, leaves, flowers). The radicle develops into the true root system while the hypocotyl develops into the transition zone between root and stem.
The seed will usually lay in the ovary wall which has been devloped into a fruite. The seed will then disperse by either wind, air, animals or water and germinate themselves.
I thank http://io.uwinnipeg.ca/~simmons/lb4pg12.htm and http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookflowers.html for the info
Monday, July 28, 2008
Amphibian reproduction
Now on reproduction of amphibians. For the purpose of reproduction most amphibians are bound to have fresh water. They need to be laid in damp places because of their jellylike characteristic and because there is no shell which means the eggs would quickly dry out and the young would die before they even had a chance to develop if not in water. When the eggs are laid, the male releases sperm over the female's eggs as they are laid. During this time, the male is in a posture called Amplexus which may be inguinal (male grasping the female in front of the her back legs) or axillary (male grasping the female in front of the her hands).
Several hundred frog species in adaptive radiations (e.g. Eleutherodactylus), however, do not need any water whatsoever. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg.
Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs. Symbiosis with single celled algae that lives in the jelly-like layer of the eggs has evolved several times. The larvae (tadpoles or polliwogs) breathe with exterior gills.
Other kinds of fertilisation include those by salamanders whereby the male deposits a packet of sperm called a spermatophore onto the ground. The female then pulls it into her cloaca where fertilization occurs internally. Some caecilians and tailed frogs use internal fertilization where the male deposits sperm directly into the female's cloaca via an intromittent organ.
Amphibians laying their eggs in water normally leave them alone, but those laying eggs on land usually watch over them to protect them from predators. A few have developed strange ways to guard their eggs until they have hatched. The Surinam toad (Pipa pipa) of South America carries its eggs in pockets of skin on its back until the froglets hatch three months later. The gastric brooder (Rheobatrachus silus) of Australia no sooner lays its eggs than it swallows them! The eggs incubate in the frog's stomach where acid production ceases until the froglets hatch and crawl out through their mother's mouth.
Once hatched, the larva feed on yolk from their egg until they are able to find food for themselves. Then, they start to transform gradually into the adult's appearance. This process is called metamorphosis. Typically, the animals then leave the water and become terrestrial adults.
I thank http://www.eoearth.org/article/Amphibian_morphology_and_reproduction and wikipedia for the information.
Sunday, July 27, 2008
Reproduction in fish
Saturday, July 26, 2008
Reptile reproduction
Using the green iguana as an example, even without a male present to fertilize eggs, a healthy adult female may begin developing eggs. The process begins with the ovaries, where eggs are stored. Follicles begin developing in the ovaries. Each follicle is composed of a tiny egg and a sac filled with yolk. The follicles then detach and move into the oviducts where the egg white is added, and then a shell is placed around the yolk and white. The female usually will not eat for three to six weeks prior to laying her eggs.
When it is time for the baby to hatch, the neonate uses its egg tooth (also called caruncle) to cut through the eggshell. The baby will usually remain in the egg for 12-48 hours after it pokes its head through the egg. During that time, any remaining yolk still attached to the hatchling will be absorbed.
Babies hatched from eggs, as well as those that are born alive from the female, are able to fend for themselves as soon as they are up and walking around/slithering.
Thursday, July 24, 2008
Animal reproduction
Once transferred, the sperm remain for a while in storage at the lower end of the oviduct, and then swim to the upper end of that duct to fertilize the egg. Consequently, there is considerable variation among species in the frequency of copulations, how long it takes for the egg to be laid and how many eggs are actually laid. But in order for an egg to form, the sperm must meet the egg in the oviduct before the layers of the egg form.
Wednesday, July 23, 2008
Abortion
Dilation and curettage (D&C), the second most common method of abortion. Curettage refers to cleaning the walls of the uterus and embryo with a curette. The World Health Organization recommends this procedure when MVA is unavailable.
Other techniques must be used to induce abortion in the second trimester. Premature delivery can be induced with prostaglandin; this can be coupled with injecting the amniotic fluid with caustic solutions containing saline or urea.
From the 20th to 23rd week of gestation, an injection to stop the fetal heart can be used as the first phase of the surgical abortion procedure to ensure that the fetus is not born alive.
Mifepristone binds to the progesterone receptor to block progesterone. Progesterone is necessary for pregnancy maintenance. Mifepristone also softens and dilates the cervix, and causes decidual necrosis (which leads to placental detachment).
Methotrexate blocks an enzyme necessary for DNA synthesis, thus inhibiting the growth of rapidly dividing placental trophoblastic cells.