INTRODUCTION

Natural vegetation of India reflects a state of perfect harmony with the relief and climatic conditions of the subcontinent. In fact, this correspondence is so perfect that if one superimposes the two maps showing the annual rainfall and the altitude above the mean sea level, one can easily infer the types of vegetation that will be found in each major region of the country.

The present vegetal cover has a long history. According to Palaeo-botanists, most of our Himalayan and peninsular areas are covered with indigenous or endemic flora, while the Indo-Gangetic Plain and the Thar desert contain plant species that have come generally from outside. Here, the plant species are exotic and have migrated from the Trans Himalayan areas (Tibet and China). This natural vegetation is classified as boreal. Plants which have come from the adjacent tropical regions are known as palaeo-tropical. Those plants which came from north Africa have influenced the vegetation of the arid and semi-arid regions, such as the Thar, as well as a good deal of the Great Plains of India. Those immigrating from Indo-Malaysia have influenced the vegetal cover of the hilly regions of north-eastern India. This process of the immigration of uninvited plant species is not only continuous, but has actually become more marked with the increase in communication with other lands, both by sea and air. Some of the exotic varieties are troublesome weeds. They thrive under conditions of tropical sun with abundant moisture, multiply rapidly and spread out as there are no ‘natural’ enemies to curb them in the new habitat. In course of time, their eradication

becomes difficult; they invade the land and reduce the area for other uses, prevent the growth of plants which are economically important and become a hazard to public health by indirectly helping the spread of several diseases. We can cite two striking examples: lantana [Lantana camara varauyleata Mold} and water hyacinth [Eichhornia crassipes Solms).

Both were brought into India as decorative garden plants; the former having now spread out in forests and pasture lands, and the latter choking up our rivers, lakes and ponds so much so, as to earn its nickname ‘terror of Bengal’ because of its phenomenal growth in that region. It is spreading to almost all water sources, ponds and canals in the rest of the country.

It may be ascertained from the above description that much of our natural vegetal cover is not that natural, except perhaps in the inaccessible parts of the Himalayas and the interior of the

Thar desert. A considerable part of it has been replaced or destroyed as a result of human occupancy of the land. Much of the plant cover is in a degr aded condition, that is, low in quality and content. What we usually designate as ‘natural’ vegetation now refers to a plant community that has been left undisturbed over a long time, so as to allow individual species to adjust themselves to geo-climatic conditions, as far as possible.