A marsupial is a mammal that raises its newborn offspring inside an external pouch at the front or underside of their bodies. In contrast, a placental is a mammal that completes embryo development inside the mother, nourished by an organ called the placenta. Marsupials are certainly placental mammals. However marsupials have an additional trick in their pouches, with the physiologically sophisticated and extended lactation that has allowed them to exchange the umbilical cord for the teat.

Some flowers, however, contain only male or female parts and need the help of another flower to form seeds. The androecium includes all the stamens of a flower, which often occur in one or two whorls. The number of stamens in a flower is typically a multiple of the number of perianth members in a single whorl. For example, if a flower has 5 sepals and 5 petals, it might have either 5 stamens or 10 stamens.

The seminiferous tubules of the testis open into the vasa efferentia through rete testis.
The vasa efferentia leave the testis and open into epididymis located along the posterior surface of each testis.

Leydig cell are found adjacent to the seminiferous tubules in the testes. They synthesize and secrete testicular hormones called androgens (testosterone).
Sertoli cells are found in the seminiferous tubules. They act as nurse cell and provide nutrition to the male germ cells.

Apomixis is a mechanism to produce seeds without fertilization. This mechanism produces clones, hence can be considered as a form of asexual reproduction.

Apomixis can occur in a number of ways, some of which are mentioned below:

Nucellar cells which are diploid & located outside the embryo sac continuously divide and enter inside embryo sac and later develop into embryos. In this way, multiple embryos can exist inside one ovule. This is termed as Polyembryony. It is seen commonly in Citrus, mango etc..

Sometimes, the egg cell is not formed as a result of reduction division, hence diploid. This diploid egg cell later directly develops into an embryo.

Tuber : It is a modified stem of the plant.

Potato is considered as a stem because it is having the nodes and inter nodes and even buds. these three characteristic of the potato has made it a stem instead of a tuber.  

Biennials are plants that complete their life cycle in two years. They germinate, develop a root system, stem and leaves in the first year. Later in their second year, they yield flowers and bear fruit. A few herbs are also classified as biennials, including spinach. Along with spinach and other herbs, biennials also include carrot, cabbage, petunias radish, onions, etc.

The tapetum is a specialised layer of nutritive cells found within the anther, of flowering plants, where it is located between the sporangenous tissue and the anther wall. ... The unusually large nuclear constitution of the tapetum helps it in providing nutrients and regulatory molecules to the forming pollen grains. Usually the cells are larger and have more than one nucleus per cell. The nuclei of tapetal cells also split as the sporogenous cells undergo mitosis. Sometimes, this mitosis is not normal due to which many cells of mature tapetum become multinucleate. Tapetum nourishes the developing pollen grains. The layer of the anther wall that helps in anther dehiscence is endothecium. The cells of endothecium develop fibrous thickenings at maturity which radiate from the inner tangential walls. ... These thickenings are hygroscopic and thus, help in anther dehiscence.

Latitudinal gradients: Species diversity generally decreases as we move away from the equator towards the poles. Tropics have more species than temperate or polar areas. Many hypotheses have been proposed by scientists to explain greater diversity in the tropics. They are as follows:

1. Speciation is generally a function of time. Temperate regions had been subjected to frequent glaciations in the past. But tropical latitudes have remained relatively undisturbed for millions of years. This allowed a long evolutionary time for species diversification in the tropics.
2. Tropical environments are less seasonal, relatively more constant and predictable. A constant environment promotes niche specialization, and leads to a greater species diversity.
3. More solar energy is available in the tropics. It contributes to higher productivity. It may have indirectly contributed to greater diversity in the tropics.

Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. It is used in horticulture, animal breeding and in production of gene knockout organisms. When F1 individuals are crossed with one of the two parents from which they were derived, then such a cross is called a back cross.

Thalassemia is a hereditary condition that affects the blood which leads to an abnormal form of haemoglobin. Haemoglobin is an important protein molecule present in RBC that carries oxygen. This disorder leads to the destruction of RBC which leads to anaemia. These range from a mild illness without symptoms to a serious or life-threatening disease. The modern treatment offers a good outlook on severe causes of thalassemia, but it requires long-term care and treatment. Good treatment is important in order to prevent complications from emerging. Thalassemia is most common among people with Mediterranean or Asian family backgrounds.

The phenomenon of failure of segregation of two homologous chromosomes of a particular pair during cell division as a result of gain or loss of chromosome(s) is called aneuploidy.

Sewage treatment is the process of removing contaminants from wastewater and household sewage water. Click here for our range of sewage treatment plants. It includes physical, biological and sometimes chemical processes to remove pollutants. Sewage treatment is the process of removing contaminants from municipal wastewater, containing mainly household sewage plus some industrial wastewater. ... If the sewer system is a combined sewer then it will also carry urban runoff (stormwater) to the sewage treatment plant. The major aim of wastewater treatment is to remove as much of the suspended solids as possible before the remaining water, called effluent, is discharged back to the environment. As solid material decays, it uses up oxygen, which is needed by the plants and animals living in the water.

• Competition is struggling by the organisms for the same resources like food, shelter and ***.
• The competition is between two organisms belonging to same species, this is called intraspecific competition.
• The competition between organisms belonging to different species is called interspecific competition. For example- in some shallow South American lakes visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake.

Carcinogens are physical, chemical or biological cancer causing agents.

Two carcinogens are X-rays and aflatoxin.