CLIMATIC DISTRIBUTION OF CERTAIN ANGIOSPERM LEAVES 39 
the persistence of Dicotyledonous foliar types from the Cretaceous 
to the present. 
The problem of the relative conservatism of the various organs 
or parts of plants is such an exceedingly complicated one, that it 
deserves careful experimental investigation. 
Summary 
There is a very clearly marked correlation between leaf-margin 
and environment in the distribution of Dicotyledons in the various 
regions of the earth. 
Leaves and leaflets with entire margins are overwhelmingly pre- 
dominant in lowland-tropical regions; those with non-entire margins 
in mesophytic cold-temperate areas. 
In the tropical zones, non-entire margins are favored by moist 
uplands, equable environments, and protected, comparatively cool 
habitats; in the cold-temperate zones, entire margins are favored 
by arid environments and other physiologically dry habitats. 
Correlations between leaf-margin and prevailing climatic influences 
are more strikingly shown among trees and large shrubs than among 
herbs, as might naturally be expected, when the fundamental dififerences 
between these important growth forms are taken into consideration. 
The determination of the percentages of entire and of non-entire 
leaves in Cretaceous and Tertiary Dicotyledonous floras, affords a 
simple and rapid means of gauging the general climatic conditions 
which existed in the regions where these plants flourished. 
There is grave danger in inferring, 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 in- 
herently "conservative" or "inconstant." 
The writers wish to express their sincere thanks to their colleagues 
in the Arnold Arboretum and Gray Herbarium for many courtesies 
during this investigation. To Dr. F. H. Knowlton of the United 
States Geological Survey and Dr. E. W. Berry, they are much indebted 
for valuable suggestions in regard to fossil floras. 
BussEY Institution, Harvard University. 
