Studies on the origin, evolution, and distribution of the Gramineae. II. The tribe Paniceae

Abstract

A world distribution map of the tribe Paniceae has been prepared, based on the percentage frequency of species of Paniceae in the total grass flora of each region. The map shows that relative specific differentiation is higher in the western hemisphere than in the eastern hemisphere, and that there is a zone of maximum concentration in north-eastern South America. The significance of this distribution pattern is discussed, and it is shown that the taxonomic evidence does not support a New World origin for the tribe. The distribution of the Paniceae in the United States shows a close relationship to winter temperature, and especially to annual rainfall. These climatic factors are also of major importance in other parts of the world, and in large measure explain the present distribution of the tribe. The Paniceae and Andropogoneae both appear to be "natural" tribes, which probably originated from a common panicoid stock in the warmer parts of the eastern hemisphere, possibly in the East Africa–Madagascar region. The Paniceae spread rapidly throughout the tropics and subtropics of both hemispheres, especially in the equatorial zone with high temperatures and well distributed rainfall. They show a pattern of climatic adaptation differing from that of the Andropogoneae. The latter, apparently formed more recently from the panicoid stock, reach their highest development in monsoonal climates with a much shorter season of heavy rainfall.