This book is written not only toexpose the Rich biodiversity of Sikkim but also to encourage the future workersto carry out the detail inventorisation of orchid repository of Sikkim, Horizontal andvertical Trans-boundary migration of the species and its limitation. Thefactors controlling the species speciation. Orchid species as an indicatorplant for a particular forest type.

The world’s most highly used,abused and indiscriminately exploited plant for its’ exquisite beauty,brilliant myriad colours, fragrance and medicinal value is non other than theplant Orchid and has outlived the other flowering plants. This extraordinaryplant belongs to the family, the Orchidaceae in the plant group theMonocotyledon of Angiosperm. Perhaps the largest family of the flowering plantsknown to science. Very sensitive having ingeniously built biological mechanismto adapt themselves to ever-changing environment have placed them in leadingrank of evolutionary process of plant kingdom. Perhaps primus inter pares andforms an excellent group for studying evolution. The natural distribution of orchids in the world is so diverse andwide that no single country, no single habitat of growth and no single shape andcolour of flower can emagine to confine them within their limit of ecological boundary.With an almost worldwide distribution that encludes every conceivableecological nitch; orchid stands supreme in the plant kingdom for their beautyand diversity. The total estimated number of native orchid species in the worldranges between 17000 – 35000. But recent studies estimate that the distinctspecies fall close to 19500, derived from 750 genera, grouped in various tribesand subtribes.But still careful survey is going on, on the humid tropical,sub-tropical and temperate forest and new species are still being added. Mostof them are the species of established genera but occasionally even new generaare also being added.

HABIT and HABITAT

Orchid are perennial, mostly,partial shade loving and a few total shade loving plants and thus found growingin a wide range of situations and with different growth habit. A large numberof orchids are found growing near the surface of the ground and are known asterrestrial. Some are found growing high above the ground level in air, asepiphyte on branches and stem of shrubs and on tall trees. The epiphyticorchids do not draw food from their host plant but merely hold on to them foranchorage. They draw moisture and nourishment from the air and from humuscollected in the bifurcated pits of branches or crevices of the bark. Someepiphytic orchids are found growing completely on bare rocks or moss coveredrocks and are termed as Lithophytes. Most orchids are autotrophic but somespecies occurs as saprophytes growing on dead and decaying organic matters andthey lack chlorophyll but they are not parasite. Keeping aside the abovementioned fact the orchid plants exhibit an amazing diversity of form and rangein length between a few millimeters as in some Pleurothallidine andEurostylis to more than 30 meters as in liana like Vanilla. Some orchidsare known for its solitary flower, as inPaphiopedilum and some of which have many flowered inflorescence like thatof giant Gramatophyttum speciosum ofSouth America which bears thousands of flowers on a single inflorescenceweighing many tens of kilograms.