. Bulletin. this range is stiffi-cient evidence that the occurrence of prairie vegetation is not limitedby climate alone. Edaphic factors and the difficulty with which a forestvegetation invades and succeeds that of the prairie must also be taken intoaccount. As the complex of climatic factors approaches the limitingconditions for forest development, the weight of the edaphic factors be- * Papers from the Department of Botany, Ohio State No. For non-technical summary, see page 569. , 524 comes more and more evident and may tip the balance either in favor offorest or of prair
. Bulletin. this range is stiffi-cient evidence that the occurrence of prairie vegetation is not limitedby climate alone. Edaphic factors and the difficulty with which a forestvegetation invades and succeeds that of the prairie must also be taken intoaccount. As the complex of climatic factors approaches the limitingconditions for forest development, the weight of the edaphic factors be- * Papers from the Department of Botany, Ohio State No. For non-technical summary, see page 569. , 524 comes more and more evident and may tip the balance either in favor offorest or of prairie. Similar conditions exist between prairie and plainsvegetation on the western edge of the prairie. But as we approach thecenter of the prairie region edaphic factors become less prominent, andit is here that we find prairie on the greatest number of edaphic (25,2()) was the first to point out the characteristics ofthe centers of distribution of the vegetation of bogs and forests of North. Map showing ratio of rainfall to evaporation in percentages.(After Transeau.) America and to call attention to the meaning and use of the term centerof distribution. As he used it the term does not imply that the plantshave necessarily spread from the present centers, but that the complexof climatic factors most favorable to the development of the type ofvegetation characteristic of each center is localized there, and that as wedepart from such centers we find conditions more and more unfavorableand resulting in the elimination of such species as are most rigidly 525 dependent upon climatic conditions. He also showed that the speciescomprising the vegetation of any center are more abundant, attain agreater size, and have a wider range of habitats within the limits of thecenter of distribution than elsewhere. These same conditions apply tothe prairie. In addition to the climatic, edaphic, and biotic factors men-tioned above, prairie fires are also important local
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