Palaeotropical Region

one of the principal terrestrial botanico-geographical subdivisions. The Palaeotropical region includes the tropical continental regions of the Old World (except Australia) and the islands of the Indian and Pacific oceans. In the north it borders on the Holarctic region, approximately along the tropic of Cancer. Sometimes the Palaeotropical region is considered to include the subtropics of southwestern Africa and New Zealand and its adjacent islands.

The flora of the Palaeotropical region is rich and strongly differentiated. This is due to the considerable distances between land masses, the various climatic conditions, and the complex history of various parts of the region. Pantropic families of plants predominate, including Palmae, Moraceae, Euphorbiaceae, Rubiaceae, Mimosaceae, Cesalpiniaceae, Anonaceae, Myrta-ceae, Melastomaceae, Acanthaceae, and Araceae. These families are represented to a large extent by specific Palaeotropical genera and groups (for example, among the palms—the tribes Calameae, Borasseae, and Phoeniceae). Such cosmopolitan families as Gramineae, Cyperaceae, Papilionaceae, Compositae, and Or-chidaceae (in humid parts of the region) are widespread yet specific in generic composition. Endemic families, such as Dip-terocarpaceae, Pandanaceae, and Nepenthaceae, play a lesser but theoretically essential role in the formation of the flora. Many endemic families are represented only in a few isolated parts of the Palaeotropical region and do not characterize its flora as a whole.

The relationship between floras of different parts of the Palaeotropical region reflects their individual histories. Also reflected are the past relationships of areas that are isolated from one another today. The dependence of flora composition on the distribution of the principal types of vegetation is also determined by comparing the different floras of the region. The principal subdivisions of the Palaeotropical region are characterized predominantly by the development of humid forests, savannas, or deciduous forests.

There is no conventional division of the Palaeotropical region into subregions. Some botanists divide it into two subregions: Malesia (Southeast Asia, Malay Archipelago, Pacific islands) and Indo-Africa (tropical Africa, the tropical region of southwest Asia, Hindustan). Others divide it into ten to 15 subregions that differ greatly in size and in the uniqueness of their flora. An especially high level of endemism is noted in highly isolated island subregions, such as the Hawaiian, New Caledonian, and Madagascar subregions. The richness of the flora depends on the size of a given subregion and on the diversification of its conditions.