LEAF-STRUCTURE AS RELATED TO ENVIRONMENT 537 
plained by the vapor cap which is thicker about the larger leaves. If 
the air were absolutely quiet Renner believed the thickness of the 
vapor cap would vary with the diameter of the leaves. 
Adamson (i) found the xerophytic structures in leaves of certain 
species of Veronica to consist in reduced leaf surfaces, reduction in 
intercellular spaces, and an increase in the thickness of the cuticle. 
Livingston and Brown (24) in their study of the daily march of 
transpiration showed that the water content of leaves falls during the 
day and rises during the night. 
Starr (30) compiled the structures of stems and leaves of plants 
on dunes and on flood plains. She discusses the ecological factors of 
the dunes, but no n.easurements are given. The leaves of the dune 
plants owe their greater thickness to increased palisade tissue chiefly. 
Ganong (16) states that one of his students found that the petioles 
from the exposed part of a tree were larger than those from more 
sheltered positions. 
Haberlandt (17) says that a comparison of the vigorously trans- 
piring sun leaves and the feebly transpiring shade leaves of the same 
plant shows an increase in the linear dimensions of the vascular system 
in the sun leaves. 
Hasselbring (21), working on tobacco plants growing in the sun 
and in the shade, showed that the proportions of dry matter, and the 
production of plant substance for equal areas of leaf surface were 
greater in the sun plants. The shade plants transpired 186.99 cc, of 
water in producing one gram of plant substance, while the sun plants 
transpired 241.72 cc. He found the water content of leaves from sun 
plants to be 81.39 percent and of leaves from shade plants to be 83.68 
percent. The sun plants transpired .412 cc. while the shade plants 
transpired .224 cc. per square decimeter of leaf surface per hour. 
Methods 
The readings of the environmental factors were made in the sun 
among the leaves on the south periphery of untrimmed isolated trees 
and at the apex of trees growing in the forest. At a height corres- 
ponding to the sun readings on isolated trees, readings were taken in 
representative positions within the crowns; and for the forest trees 
readings were taken among the lowest leaves. For a given species the 
readings were taken upon the same individual in the forest or in iso- 
lated positions. Care was taken so that the various factors were 
