According to Trewartha 'climate repre sents a composite of day to day weather condi tions and of the atmospheric elements, within a specified area over a long period of time. According to Critchfield climate is more than a statistical average; it is the aggregate of the atmospheric conditions involving heat, moisture, and air movement. Extremes must always be considered in any climatic description in addition to means, trends, and probabilities'

There are three distinct branches of clima tology e.g. 

• Physical climatology, 

• Regional climatology, and 

• Applied climatology. 

deals with the interpretation of fac tors responsible for the spatial and temporal variations of exchange of air circulation, heat and humidity. It studies various elements of weather namely insolation temperature, air pressure, wind, evaporation and humidity, precipitation, fogs, visibility etc. Different climates are formed due to combinations of these weather elements. The occurences of different combinations of these weather elements are accomplished through dif ferent processes and mechanisms. Thus, these processes of exchange of heat, humidity and momentum between atmosphere and earth's sur face are also studied thoroughly. It is, thus, evident that physical climatology studies the factors and processes of regional variations of climatic conditions. 

 Rregional climatology includes the study of types, distribution, pattern and characteristic fea tures of world climates. Regional climatology is studied in three ways on the basis of spatial scales viz. 

1. macro-climatic regions,

2. meso-climatic regions, and 

3. micro-climatic regions. 

Applied climatology studies the application of climatic knowledge to solve various problems faced by human society e.g. global warming and climatic changes. It studies the interactions between climate and biosphere. control plants and animals including man and in turn how does man modify climate by introducing advertent and inadvertent changes in the physical environment and by making certain weather modifications e.g. cloud seeding and induced precipitation. Further, applied climatol ogy includes the study of variability of climate, climatic changes, air pollution, climate and com fort, climate and health, climate and society, extreme weather events (e.g. floods, drought etc.) and their impacts, climate and agriculture, climate and manufacturing industries, climate and recrea tion, climate and urban planning, weather fore casting, climate and transport and communica tion, weather modifications etc.

Biosphere is a life supporting layer which surrounds the earth and makes plants and animal life possible without any protective device. The organic world or biosphere is that part of the earth which contains living organisms-the biologically inhabited soil, air and water' (J. Tivy, 1982). According to A.N. and A.H. Strahler (1977) all the living organisms of the earth and the environment to which these organisms interact muke biosphere. Thus, the biosphere consists of all the in living organisms (the biotic component), energy (the energy component) and physical environ ment (abiotic component) and there are continu ous interactions between living organisms and physical environment and among the living or ganisms themselves,

The average thickness of biosphere or life supporting layer consisting of land, air, water, soil and rock is about 30 km. The upper limit of biosphere is determined by the availability of oxygen, moisture. temperature and air pressure which are necessary for the existence of organ isms. Decrease in oxygen, temperature, moisture and air pressure with increase in height in the atmosphere limits the upper boundary of the biosphere. Though the NASA has discovered the presence of bacteria upto the height of 15 km in the atmosphere but the lower layer of the atmos- phere upto a few hundred metres accounts for most of the living organisms because favourable environmental conditions are available for the growth and development of living organisms in the lower part of the atmosphere. The depth of the biosphere over the land is upto the depth of deepest roots of trees or the depth upto which can live the burrowing organisms or the depth at which lie the parent rocks. The biosphere extends upto greater depth in the oceans. The existence of life has been detected upto a depth of 9.000 m in the deep oceanic trenches and deep sea plains.

In the early stages of the development of biogeography two aspects were more emphasized viz. taxonomic aspect and ecological aspect. The classification (taxonomy) of plants, their nomen clature and distribution, dispersion and evolution of individual plants became the centre of investi gation and study by botanists whereas geogra phers attached more importance to the ecological aspects (relationships between plants and physi cal environmental factors) and geographical fac tors responsible for spatial var ons in plant species. In fact, 'plant geography has made and condinues to make important contributions to the elucidation and assessment of the relative impor tance of the factors which determine floristic (plants) distribution' (J. Tivy, 1982). Plant geog rapher, as influenced by the Huttonian theory of uniformitarianism based on two concepts of present is key to the past' and 'no vestige of a beginning and no prospect of an end' (as propounded by Scottish geologist James Hutton), also attempted to study the influences of past events and environ mental conditions on present distribution of plants.