30 
IRVING W. BAILEY AND EDMUND W. SINNOTT 
interval between seed germination and seed production, can migrate 
less rapidly. Herbs, in marked contrast to trees, are the most adapt- 
able and variable type of vegetation. This is due, in part to their 
small size which enables them to take advantage of local variations 
in environment, in part to the brevity of their life cycle which increases 
their opportunity for migration and variation, and largely to the fact 
that they can avoid periods of unfavorable climatic conditions under- 
ground or as small resistant seeds. Thus, although the bulk of her- 
baceous Dicotyledons are in all probability of comparatively recent 
origin, they have migrated very rapidly and have established them- 
selves in most regions of the earth. ^ 
It is not at all suprising, therefore, that the correlations between 
leaf-margin and climate are somewhat less strikingly shown among 
small shrubs and herbs than they are among arborescent species. 
In tropical lowlands, the smaller plants can escape the full effects of 
intense heat and sunlight; and it is significant that non-entire species 
are usually small trees, shrubs, climbing plants, and herbs, many of 
which occur in protected, comparatively cool habitats. On the other 
hand, the leaves of the dominant Dicotyledons of tropical forests are 
almost always entire. Within the temperate zones, not only do 
small shrubs and herbs live in those arid and unfavorable environments 
where arborescent plants are of infrequent occurrence, but in meso- 
phytic situations may avoid the full effects of climatic influences to 
which large trees and shrubs are directly exposed. Of course, her- 
baceous types may appear above ground only during the warmer or 
moister seasons of the year. 
In view of these facts, it appears to be highly improbable that the 
present distribution of entire and non-entire Dicotyledonous leaves 
and leaflets is largely due to factors of heredity rather than those of 
environment. If leaf form is little subject to modification by 
environment and is veiy firmly held on by heredity, the existing ratios 
between the two types of leaf-margins must have been determined 
by the original location, subsequent migrations, etc., of those families 
or groups of Dicotyledons that developed entire and non-entire leaves 
and leaflets. But, as is shown in the next table, the majority of the 
famihes of the Dicotyledons possess both types of foliage. Further- 
more, in the distribution of the woody representatives of Dicoty- 
3 Sinnott, E. W. and Bailey, I. W. The origin and dispersal of herbaceous 
Angiosperms, Annals of Botany 28: 547-600. 1914. 
