Periodical Literature 503 



leaves with entire margins and non-entire margins among the trees, 

 shrubs and herbaceous plants. They found that leaves and leaflets 

 with entire margins are overwhelmingly predominant in lowland 

 tropical regions ; those with non-entire margins in the mesophytic 

 cold-temperature areas. In tropical zones non-entire margins are 

 favored by moist uplands, equable environments, and protected, 

 comparatively cool habitats. In cold-temperate zones, entire 

 margins are favored by arid environments and other physiologically 

 dry habitats. Transitional forms reach their best development in 

 the intermediate environments. In the case of those families which 

 possess both types of leaf margins, it is very significant that their 

 distribution corresponds to that outlined above. These facts are, 

 however, best exhibited by trees and less so by small shrubs and 

 herbaceous plants. This is to be expected when one considers 

 the greater exposure of trees, their longer life cycle and their rela- 

 tively smaller mobility in migration. The presence of a limited 

 number of non-entire-leaved types in lowland-tropical environ- 

 ments and the comparatively few entire-leaved species in meso- 

 phytic cold-temperate regions may be explained by the fact that 

 not all the species will have been subjected to the effects of the 

 prevailing climatic conditions for equal lengths of time or an equal 

 nimiber of generations; nor is it necessary to suppose that all 

 species or groups of plants will respond with equal rapidity or in an 

 exactly similar manner to the influences of environment. 



In view of these facts, the authors assert that it is highly probable 

 that the present distribution of entire and non-entire dicotyledonous 

 leaves and leaflets is largely due to factors of environment rather 

 than to those of heredity. It does not necessarily follow, because 

 a certain foliar character has remained unaltered through long 

 periods of geological time, or has varied greatly among closely 

 related forms, that the leaf is inherently "conservative" or "incon- 

 stant." The authors believe that the character of the leaf margin 

 of fossil plants may be legitimately used as a criterion for inter- 

 preting the climatic conditions in the different periods of the Tertiary 

 and Cretaceous dicotyledonous floras. The interpretation of the 

 physiological significance of the entire and non-entire leaf margins 

 is to be considered in a subsequent paper. 



C. D. H. 



The Climatic Distribution of Certain Angiosperm Leaves. American Journal 

 of Botany, January, 1916, pp. 24-39. 



