Our planet earth is the only one in the solar system where life is known to exist. Despite its apparent vastness, life exists in a very thin layer enveloping the earth called biosphere where conditions for its sustenance are favorable.
Three main subdivisions of biosphere are lithosphere (solid matter), hydrosphere (liquid matter) and atmosphere or the gaseous envelope of the earth which extends up to a height of 22.5 kilometers depicts an idealized scheme for biosphere in relation to hydrosphere, atmosphere and lithosphere.
The area of contact and interactions between these components are important for life. The basic processes of life like respiration and photosynthesis occur in this layer.
Living organisms are mostly confined to the parts of biosphere that receive solar radiation during the day time. This includes atmosphere, the surface of the land, a few meters of the soil and upper layers of oceans, lakes and rivers.
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The light entering zone may be few centimeters in turbid water or up to 100 meters in the clearest water of the oceans. A variety of life forms also occur at the varying altitudes of the mountains.
Life forms or the organisms occurring at these variable habitats show high degree of numerical, structural and functional diversity. No one knows accurately the number of species of living organisms inhabiting this planet.
There are various estimates – the most common estimate puts the number at nearly 30 to 50 million. The World Conservation Union report of May, 2003 points that there are 1.5 to 1.8 million named species of living organisms.
There may be many more yet to be discovered, identified and named. Although the plants and animals comprise the life forms of this planet, the diversity of plants is only described in the following.