Although India’s flora can be broadly grouped under ‘tropical monsoonal’ category but due to complex physiographic, climatic and pedagogical conditions it is divided into 174 natural orders and over 30,000 plant species ranging from thorny bushes (Rajasthan etc.) to evergreen forests (Assam etc.) belonging to dominant plant families like Orchidaceae, Gramineae, Euphorbiaceae, Aeanthaceae, Compositeae, Cyperaceae, Labiatae and Urticaceae.
According to Palaeo-botanists most of our Himalayan and the Peninsular areas are covered with indigenous or endemic flora, while the Ganga Plain and the Thar Desert contain plant species that have come generally from outside, that is, these are exotic. Nearly 40 per cent of the plant species found in India are exotic, and these have mostly come from the Sino-Tibetan area, and are called boreal.
Those coming from the neighboring tropical regions are known as palaeo-tropical. Those received from the North African region have their impact on the vegetation of the arid and semi-arid regions of the Thar and western part of the Indian plain. Those in-migrating from the Indo-Malay region have influenced the vegetal cover of the hilly regions of North-Eastern India.
This process of in- migration is still going on. Examples include Lantana (Lantana camara vary aculeate Mold) and water Hyacinth (Eichhoria crassipes Solms) which were brought as decorative garden plants but have become undesirable weeds. Recently partheniumgrass due to its ill effect on human health is causing respiratory and skin diseases.
India’s 30,000 plant species are broadly classified under ten broad plant families which include Orchidaceae, Leguminosae, Graminaceae, Rubiaceae, Euphorbiaceae, Acanthaceae, Compositae, Cyperaceae, Labiatae, and Urticaceae. Except Compositae and Labiatae, which belong to the temperate group, others belong to the tropical group.
Compositae, which is the largest plant family of flowering plants in the world, is not very well developed in the country. On the contrary Orchidaceae family alone contains more than 1600 species of plants covering the parts of the eastern Himalayas. According to D. Chatterji (1939) about 61.5 per cent of the Indian flora is endemic.
The Himalayan region contains 3,165 such indigenous plant species while another 2,045 species are found in the peninsula area (Maheshwari et al., 1965). About 570 plant species belong to the European group.