CLIMATIC DISTRIBUTION OF CERTAIN ANGIOSPERM LEAVES 
35 
Among herbaceous plants, historical factors appear to have had 
a more important effect upon the distribution of the two principal 
types of leaf-margins than they have among arborescent and shrubby 
Dicotyledons. This seems to have been due to the fact that, owing 
to the fundamental differences between these growth forms, herbaceous 
plants are less subject to or react differently toward prevailing environ- 
mental influences; and to the more recent origin and rapid dispersal 
of herbs. 
What then is the physiological significance of the entire and of 
the non-entire types of Dicotyledonous leaf -margins? Are they 
actually of vital functional importance or merely necessary con- 
comitants of certain types of foHar structure? This is clearly a 
problem for anatomical and experimental investigation, and will be 
considered separately in a subsequent paper. However, there are one 
or two suggestive facts that should be noted at this time. The 
serrations, dentations, etc., of non-entire leaves, which are often more 
conspicuous in the earlier than in the later stages of the ontogeny 
of the leaf, are frequently glandular or provided with hydathodes or 
water stomata. The structure and possible excretory function of 
the non-entire leaf-margin, therefore, deserve careful investigation. 
Among woody plants, well-developed non-entire margins occur 
commonly on comparatively thin, soft leaves with prominent veins. 
Entire margins, on the other hand, usually occur on thicker, stiffer, 
more leathery leaves which are provided with structures that seem to 
retard evaporation and transpiration. The possibility suggests itself, 
accordingly, that the form of the leaf -margin may be largely influenced, 
either directly or indirectly, by phenomena of evaporation and trans- 
piration. In those environments where non-entire margins reach 
their optimum development, the leaves can draw upon abundant 
soil moisture and transpire freely." In alpine and arctic regions, 
bogs, steppes, prairies, moors, arid, and saline habitats the leaves of 
most plants with persistent aerial stems and of many herbaceous forms 
are exposed to conditions of physiological drought, and are subject to 
the grave danger of excessive transpiration and evaporation. 
If this is the case, why are there such high percentages of entire- 
leaved plants in tropical rainforests and in other moist tropical 
^ It is interesting to note that in regions with marked alternating periods of 
heat and cold or dry and wet seasons, the evergreen foliage may be entire when the 
deciduous types are strikingly non-entire. 
