PLANT WORLD FOR DECEMBER, 1919, WAS IS8UED MAY 24, 1920 



THE GEOLOGICAL HISTORY OF THE SWEET GUM 



AND WITCH HAZEL 



EDWARD W. BERRY 



Johns Hopkins University, Baltimore, Maryland 



No part of the Temperate Zone can compare with southeastern 

 North America in the brilliancy of autumnal foliar display and a 

 considerable part of this is due to the sweet gum, whose leaves 

 assume a variety of shades ranging from rich yellow through 

 carmine to wine red. 



The sweet gum belongs to a family, often called the Witch 

 Hazel family (Hamamelidaceae), whose present geographical dis- 

 tribution is of remarkable interest. The family comprises nine- 

 teen genera in all and about fifty living species, and no less than 

 nine of these genera are monotypic, that is to say, they are each 

 represented by but a single existing species. 



Monotypic genera are either geologically old or else very mod- 

 ern, that is, their single species may have been recently evolved 

 or it may represent the last remaining descendant of a long and 

 now extinct line, and it is usually possible to get some idea as to 

 which of these categories we are dealing with by a consideration 

 of the present geographical distribution of the different members 

 of a plant family. In the case of this family twelve of the nine- 

 teen genera are confined to Asia, one is prevailingly Australian, 

 three are African and three are confined to Asia and southeastern 

 North America. 



Both the sweet gum and the witch hazel belong to this last 

 category, their present range being shown approximately on the 

 accompanying sketch map (fig. 1). It is obvious that a distri- 

 bution such as this indicates that the family to which they be- 

 long had an extended geological history and that the particular 

 genera once flourished in regions that connect the present discon- 

 tinuous occurrences. The only alternative is to suppose that the 



345 



THE PLANT WORLD, VOL. 22, NO. 12 

 DECEMBER, 1919 



