LEAF-STRUCTURE AS RELATED TO ENVIRONMENT 
Herbert C. Hanson 
Introduction 
This investigation was begun during the summer of 1915 and 
carried on for about a year. PreHminary observation showed that 
the leaves growing in the sun at the south periphery were thicker than 
the leaves growing in the shade at the center of the same tree. The 
purpose of the investigation was to find out the exact differences in the 
structure of the leaves from the two positions, and to compare by 
measurements of the factors the environments in which the leaves were 
growing. The factors measured were light, evaporating power of the 
air, temperature, humidity and wind. A study of transpiration was 
also made during the summer of 1916. 
Historical 
The relation of leaf structure to factors of the environment has 
been studied by numerous investigators. Although very few works 
give factor measurements, some of the more recent investigations on 
leaf structure are here reviewed. One of the most detailed works is 
by Mrs. Clements (12), which shows the differences in the structure of 
numerous hydrophytic, mesophytic and xerophytic leaves, and of 
sun and shade leaves on different plants of the same species. The 
physical factors, light, water-content of the soil, humidity and tem- 
perature, were measured for most of the habitats in which the plants 
studied were growing. Good reviews of earlier articles on leaf struc- 
ture, and of explanations concerning the structure and formation of 
palisade cells, are given by Mrs. Clements. 
Experiments performed by Eberhardt (14) showed that humid air 
caused an increase in the size of the leaf, in the amount of chlorophyll 
and in root development ; while dry air caused an increase in the thick- 
ness of the cuticle, in the number of stomata, and in the amount of 
sclerenchyma, woody tissue and palisade. 
Brenner's (9) experiments on various succulent plants are interest- 
ing and important. Plants grown in moist air showed the following 
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