Phytogeography the science dealing with the geographical relationships of plants. It records and explains the pattern of plant distribution in the past and present and is closely related to Botany, Zoology and Anthropology. The subject includes almost all the aspects of the physical geography of the world including the continents and oceans, the islands, the mountains and the deserts.

The distribution of plant is controlled by the climatic and edaphic factors. With the geological ages the geography of the earth has greatly changed. After their origin, plants in different geological time have dispersed and found new homes where they are still continuing. The major segregation of the plant life on earth today is into three latitudinal zones - Polar, Temperate and Tropical. These are further divided into various sub-zones on the basis of the elevation of land and climate with latitude into Equatorial, Tropical, Sub-tropical, Warm temperate, Cold temperate, Sub-arctic, Arctic and Polar zones.

The floristic classification divides the land surface of the world into 37 zones. The major units are Boreal Kingdom; Palaeotropical Kingdom, Neo-tropical Kingdom; South-African Kingdom, Australian Kingdom, and Antarctic Kingdom.

Geographically Bangladesh is included partly in the Tropical zone and partly in the Sub-tropical zone. Most of its land forms the capstone of the arch formed by the bay of bengal. It is a comparatively narrow land bridge between the sub-continent of India and the sub-continent of Southeast Asia. Floristically, Bangladesh also lies in a very interesting juncture of two great regions. In the western side it is mostly in the Indian elements and eastern side lies in the continental south-east Asiatic region. At the same time it lies very near to the western side of Sino-Japanese region. Therefore, admixture of several great floristic regions are likely to be found in Bangladesh.

Bangladesh is a part of the Bengal Basin lying between the Himalaya, Rajmahal and Meghalaya Plateau. The Bengal Basin started to rise in the late Eocere (37-58 million years ago) which probably developed during the Pleistocene (0.1-2 million years ago) time. After post-Siwalik movements, the great Brahmaputra river passed through the western part of Sino-Japanese floristic kingdom. Many feeding canals of this river and the river itself bringing down some of these remote upland floristic elements and establishing at the brink of two great floristic composition in Bangladesh provided a high degree of complex community between the three great floristic regions. Though Bangladesh shares only about 0.01 percent of land, it sustains about 1.5 percent vascular plant species of the world. [Mostafa Kamal Pasha]