Task 2 (Internal vs External Fertilisation)

Include task 2 post here. Note: American English spell fertilisation as fertilization.

External Fertilisation

If the female frog is also ready to breed, she will approach the male frog. He will climb onto her back (this embrace is called amplexus) and she will carry him to the egg-laying site. As she releases her eggs, the male frog releases sperm over the eggs. Fertilisation occurs outside of the female's body, usually in pond water.

The fertilised eggs develop and hatch to produce tiny tadpoles, which grow and develop into frogs over a period of months. This change from tadpole to frog is called metamorphosis. The process involves not only the loss of the tail and the growth of legs and arms, but also the replacemnet of the tadpole's internal gills and lungs. Its digestive system also changes, to cope with a diet of insects rather than algae and decaying animal matter. The aquatic tadpole metamorphoses to become a land-dwelling frog. 

Internal Fertilisation

Pollination and Fertilisation in Flowers

The anther holds pollen. The anther swells when the pollen are ripe and splits open, releasing the pollen in response to sunny, dry conditions. The ovary is located at the base of the petals. Stigma is a sticky tip of a stalk-like style. 

 

A long pollen tube grows from the pollen landing on the stigma down the style towards the ovule, which is located in the ovary. The ovule is fertilized by the males nucleus of the pollen after passing down the tube. Once fertilisation occurs, a fruit usually formas after the petals fall. A swollen ovary wall forms flesh of the fruit.

By Lydia

 Internal Fertilisation

 

Mammals and birds have internal fertilisation to allow sperm to have a fluid in which to swim to the egg. They also require fewer sperms and eggs as internal fertilisation increases the probability of sperm and egg meeting for successful fertilisation. The development of internal fertilisation was essential for the evolution of terrestrial species. Terrestrial species have fewer young as they have better survival chances due to parental care. An example of internal fertilisation is a platypus.

External Fertilisation

 

Aquatic animals, such as fish and amphibians which have to return to water to reproduce, have internal fertilisation where sperm and eggs are released in huge numbers into the water.

They must produce much greater numbers of gametes due to less likelihood of fusion of gametes due to tides etc and fewer young survive due to predation before maturity since they do not have parental care.

By Abbas

External fertilisation:

Some flowers with both stamens and a pistil are capable of self-fertilization, which does increase the chance of producing seeds but limits genetic variation. The extreme case of self-fertilization occurs in flowers that always self-fertilize, such as many dandelions. Conversely, many species of plants have ways of preventing self-fertilization. Unisexual male and female flowers on the same plant may not appear or mature at the same time, or pollen from the same plant may be incapable of fertilizing its ovules. The latter flower types, which have chemical barriers to their own pollen, are referred to as self-sterile or self-incompatible. Reproduction is by seed and perennial rootstock.

Internal fertilization:
 

The kangaroo carries out both internal fertilisation and internal development of the young after birth. In good conditions, it can have three offspring at different stages of development. The female may have an older young out of the pouch but still being suckled, a newborn young in the pouch, and an embryo in diapauses in the uterus. At this stage, the mother will be producing two different types of milk simultaneously. The newborn young will get low fat/ high carbohydrate milk from nipples of the mammary glands, while the young outside the pouch will feed from a different nipple (or teat) to get large volumes of high fat/low carbohydrate milk. In times of drought the mother may be unable to produce sufficient milk to sustain a growing young in the pouch. If the young dies, a new young will enter the pouch a month later. Since the newborn young is so small, it only needs a small quantity of milk for the first few weeks in the pouch. This ensures there is always a young ready when the drought ends. This ‘production line approach allows very rapid population growth when conditions are good. However, under prolonged drought conditions, breeding stops and only begins again when rain triggers a hormonal response in the female. This very effective mechanism controls the rate of reproduction depending on the favourability of the environmental conditions at the time.
 
By Hala & Sharlyene