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Although the longitudinal pattern of biological diversity may not be as striking as the latitudinal pattern, there are climatic gradients associated with habitat, topography and disturbance, which may generate variation in biological diversity along a longitudinal extent. We analysed the pattern of plant diversity using field data gathered from a national level ’biodiversity characterization at landscape level’ project, along the longest longitudinal extent of India, varying from desert to wet tropics. Across a 3 degrees latitudinal band (25-27 degrees N lat.), the aridity decreased drastically from west to east, and the vegetation varied from tropical thorn, tropical deciduous, tropical semi-evergreen to tropical evergreen forests. In general, the species diversity is maximum in the east, minimum in the centre and intermediate in the west. The proportion (with respect to total number of species of each corresponding grid) of woody trees increased by half and shrubs increased by one-third, whereas herbal proportion decreased by half from western to eastern grid. The major predictor of species richness was the number of forest vegetation types, followed by topography (ruggedness index); together these explained 63% of the variance as revealed from generalized linear modelling analysis. The study concludes that, from west to east, (i) the vegetation type varies from tropical thorn to tropical moist/wet evergreen forest corresponding to aridity; (ii) there is monotonic drastic increase of woody tree, moderate increase of shrub, and drastic decrease of herb species, and (iii) plant diversity has a nonlinear distribution pattern attributed mainly to variation in the number of forest vegetation types followed by abiotic topography and climate. The study was possible due to the availability of national plant diversity database that followed a uniform field sampling design.