The Pachamalais are covered by extensive forests and are situated along the north-western border of Tiruchirapally district in Madras State. The altitude of the hills ranges from 567 m. to 1000 m. The range is surrounded by steep slopes and forms a plateau at the top. This area remained botanically unexplored until three exploration trips were conducted in the year 1958-59 ; a total of 285 species representing 75 families and 226 genera were collected.
The forests of Pachamalais, present a vegetation of the South Indian tropical dry deciduous type and are characterized by populations of Acacia chundra, A. leucophloea, Albizia amara, Anogeissus latifolia, Bauhinia racemosa, Chloroxylon swietenia, Dendrocalamus strictus, Dichrostachys cinerea, Dodonaea viscosa, Mangifera indica, Santalum album, Strychnos nux-vomica, Terminalia arjuna, T. chebula, T. paniculata, Zizyphus mauritiana, Z. xylopyra and climbers like Cardiospermum halicacabum, Dioscorea hispida, D. oppositifolia and Pachygone ovata. The plateau at the top shows a thick vegetation consisting of Capparis zeylanica, Carissa carandas, Clematis gouriana, Cylista scariosa, Gnetum ula, Jasminum rigidum, Morinda umbellata and Zizyphus rugosa.
In the present enumeration the families with the largest number of species are Leguminosae 29, Gramineae 28, Euphorbiaceae 23, Acanthaceae 13, Cyperaceae 13, Verbenaceae 11 and Compositae 10. These seven families contain 42.8 per cent of the total flora of the forest.